esper.codehaus.org and espertech.comDocumentation

Esper Reference

Version 4.11.0


Preface
1. Technology Overview
1.1. Introduction to CEP and event series analysis
1.2. CEP and relational databases
1.3. The Esper engine for CEP
1.4. Required 3rd Party Libraries
2. Event Representations
2.1. Event Underlying Java Objects
2.2. Event Properties
2.2.1. Escape Characters
2.2.2. Expression as Key or Index Value
2.3. Dynamic Event Properties
2.4. Fragment and Fragment Type
2.5. Plain-Old Java Object Events
2.5.1. Java Object Event Properties
2.5.2. Property Names
2.5.3. Parameterized Types
2.5.4. Setter Methods for Indexed and Mapped Properties
2.5.5. Known Limitations
2.6. java.util.Map Events
2.6.1. Overview
2.6.2. Map Properties
2.6.3. Map Supertypes
2.6.4. Advanced Map Property Types
2.7. Object-array (Object[]) Events
2.7.1. Overview
2.7.2. Object-Array Properties
2.7.3. Object-Array Supertype
2.7.4. Advanced Object-Array Property Types
2.8. org.w3c.dom.Node XML Events
2.8.1. Schema-Provided XML Events
2.8.2. No-Schema-Provided XML Events
2.8.3. Explicitly-Configured Properties
2.9. Additional Event Representations
2.10. Updating, Merging and Versioning Events
2.11. Coarse-Grained Events
2.12. Event Objects Instantiated and Populated by Insert Into
2.13. Comparing Event Representations
3. Processing Model
3.1. Introduction
3.2. Insert Stream
3.3. Insert and Remove Stream
3.4. Filters and Where-clauses
3.5. Time Windows
3.5.1. Time Window
3.5.2. Time Batch
3.6. Batch Windows
3.7. Aggregation and Grouping
3.7.1. Insert and Remove Stream
3.7.2. Output for Aggregation and Group-By
3.8. Event Visibility and Current Time
4. Context and Context Partitions
4.1. Introduction
4.2. Context Declaration
4.2.1. Context-Provided Properties
4.2.2. Keyed Segmented Context
4.2.3. Hash Segmented Context
4.2.4. Category Segmented Context
4.2.5. Non-Overlapping Context
4.2.6. Overlapping Context
4.2.7. Context Conditions
4.3. Context Nesting
4.3.1. Built-In Nested Context Properties
4.4. Partitioning Without Context Declaration
4.5. Output When Context Partition Ends
4.6. Context and Named Window
4.7. Operations on Specific Context Partitions
5. EPL Reference: Clauses
5.1. EPL Introduction
5.2. EPL Syntax
5.2.1. Specifying Time Periods
5.2.2. Using Comments
5.2.3. Reserved Keywords
5.2.4. Escaping Strings
5.2.5. Data Types
5.2.6. Using Constants and Enum Types
5.2.7. Annotation
5.2.8. Expression Declaration
5.2.9. Script Declaration
5.2.10. Referring to a Context
5.3. Choosing Event Properties And Events: the Select Clause
5.3.1. Choosing all event properties: select *
5.3.2. Choosing specific event properties
5.3.3. Expressions
5.3.4. Renaming event properties
5.3.5. Choosing event properties and events in a join
5.3.6. Choosing event properties and events from a pattern
5.3.7. Selecting insert and remove stream events
5.3.8. Qualifying property names and stream names
5.3.9. Select Distinct
5.3.10. Transposing an Expression Result to a Stream
5.3.11. Selecting EventBean instead of Underlying Event
5.4. Specifying Event Streams: the From Clause
5.4.1. Filter-based Event Streams
5.4.2. Pattern-based Event Streams
5.4.3. Specifying Views
5.4.4. Multiple Data Window Views
5.4.5. Using the Stream Name
5.5. Specifying Search Conditions: the Where Clause
5.6. Aggregates and grouping: the Group-by Clause and the Having Clause
5.6.1. Using aggregate functions
5.6.2. Organizing statement results into groups: the Group-by clause
5.6.3. Selecting groups of events: the Having clause
5.6.4. How the stream filter, Where, Group By and Having clauses interact
5.6.5. Comparing Keyed Segmented Context, the Group By clause and the std:groupwin view
5.7. Stabilizing and Controlling Output: the Output Clause
5.7.1. Output Clause Options
5.7.2. Aggregation, Group By, Having and Output clause interaction
5.7.3. Runtime Considerations
5.8. Sorting Output: the Order By Clause
5.9. Limiting Row Count: the Limit Clause
5.10. Merging Streams and Continuous Insertion: the Insert Into Clause
5.10.1. Transposing a Property To a Stream
5.10.2. Merging Streams By Event Type
5.10.3. Merging Disparate Types of Events: Variant Streams
5.10.4. Decorated Events
5.10.5. Event as a Property
5.10.6. Instantiating and Populating an Underlying Event Object
5.10.7. Transposing an Expression Result
5.10.8. Select-Clause Expression And Inserted-Into Column Event Type
5.11. Subqueries
5.11.1. The 'exists' Keyword
5.11.2. The 'in' and 'not in' Keywords
5.11.3. The 'any' and 'some' Keywords
5.11.4. The 'all' Keyword
5.11.5. Subquery With Group By Clause
5.11.6. Multi-Column Selection
5.11.7. Multi-Row Selection
5.11.8. Hints Related to Subqueries
5.12. Joining Event Streams
5.12.1. Introducing Joins
5.12.2. Inner (Default) Joins
5.12.3. Outer, Left and Right Joins
5.12.4. Unidirectional Joins
5.12.5. Hints Related to Joins
5.13. Accessing Relational Data via SQL
5.13.1. Joining SQL Query Results
5.13.2. SQL Query and the EPL Where Clause
5.13.3. Outer Joins With SQL Queries
5.13.4. Using Patterns to Request (Poll) Data
5.13.5. Polling SQL Queries via Iterator
5.13.6. JDBC Implementation Overview
5.13.7. Oracle Drivers and No-Metadata Workaround
5.13.8. SQL Input Parameter and Column Output Conversion
5.13.9. SQL Row POJO Conversion
5.14. Accessing Non-Relational Data via Method Invocation
5.14.1. Joining Method Invocation Results
5.14.2. Polling Method Invocation Results via Iterator
5.14.3. Providing the Method
5.14.4. Using a Map Return Type
5.15. Creating and Using Named Windows
5.15.1. Creating Named Windows: the Create Window clause
5.15.2. Inserting Into Named Windows
5.15.3. Inserting Into Named Windows Using Fire-And-Forget Queries
5.15.4. Selecting From Named Windows
5.15.5. Triggered Select on Named Windows: the On Select clause
5.15.6. Triggered Select+Delete on Named Windows: the On Select Delete clause
5.15.7. Populating a Named Window from an Existing Named Window
5.15.8. Updating Named Windows: the On Update clause
5.15.9. Updating Named Windows Using Fire-And-Forget Queries
5.15.10. Deleting From Named Windows: the On Delete clause
5.15.11. Deleting From Named Windows Using Fire-And-Forget Queries
5.15.12. Triggered Upsert using the On-Merge Clause
5.15.13. Explicitly Indexing Named Windows
5.15.14. Versioning and Revision Event Type Use with Named Windows
5.16. Declaring an Event Type: Create Schema
5.16.1. Declare an Event Type by Providing Names and Types
5.16.2. Declare an Event Type by Providing a Class Name
5.16.3. Declare a Variant Stream
5.17. Splitting and Duplicating Streams
5.18. Variables and Constants
5.18.1. Creating Variables: the Create Variable clause
5.18.2. Setting Variable Values: the On Set clause
5.18.3. Using Variables
5.18.4. Object-Type Variables
5.18.5. Class and Event-Type Variables
5.19. Declaring Global Expressions And Scripts: Create Expression
5.19.1. Declaring a Global Expression
5.19.2. Declaring a Global Script
5.20. Contained-Event Selection
5.20.1. Select-Clause in a Contained-Event Selection
5.20.2. Where Clause in a Contained-Event Selection
5.20.3. Contained-Event Selection and Joins
5.20.4. Sentence and Word Example
5.20.5. More Examples
5.20.6. Contained-Event Limitations
5.21. Updating an Insert Stream: the Update IStream Clause
5.21.1. Immutability and Updates
5.22. Controlling Event Delivery : The For Clause
6. EPL Reference: Patterns
6.1. Event Pattern Overview
6.2. How to use Patterns
6.2.1. Pattern Syntax
6.2.2. Patterns in EPL
6.2.3. Subscribing to Pattern Events
6.2.4. Pulling Data from Patterns
6.2.5. Pattern Error Reporting
6.2.6. Suppressing Same-Event Matches
6.2.7. Discarding Partially Completed Patterns
6.3. Operator Precedence
6.4. Filter Expressions In Patterns
6.4.1. Controlling Event Consumption
6.5. Pattern Operators
6.5.1. Every
6.5.2. Every-Distinct
6.5.3. Repeat
6.5.4. Repeat-Until
6.5.5. And
6.5.6. Or
6.5.7. Not
6.5.8. Followed-by
6.5.9. Pattern Guards
6.6. Pattern Atoms
6.6.1. Filter Atoms
6.6.2. Time-based Observer Atoms
7. EPL Reference: Match Recognize
7.1. Overview
7.2. Comparison of Match Recognize and EPL Patterns
7.3. Syntax
7.3.1. Syntax Example
7.4. Pattern and Pattern Operators
7.4.1. Operator Precedence
7.4.2. Concatenation
7.4.3. Alternation
7.4.4. Quantifiers Overview
7.4.5. Variables Can be Singleton or Group
7.4.6. Eliminating Duplicate Matches
7.4.7. Greedy Or Reluctant
7.4.8. Quantifier - One Or More (+ and +?)
7.4.9. Quantifier - Zero Or More (* and *?)
7.4.10. Quantifier - Zero Or One (? and ??)
7.5. Define Clause
7.5.1. The Prev Operator
7.6. Measure Clause
7.7. Datawindow-Bound
7.8. Interval
7.9. Interval-Or-Terminated
7.10. Use with Different Event Types
7.11. Limitations
8. EPL Reference: Operators
8.1. Arithmetic Operators
8.2. Logical And Comparison Operators
8.2.1. Null-Value Comparison Operators
8.3. Concatenation Operators
8.4. Binary Operators
8.5. Array Definition Operator
8.6. Dot Operator
8.6.1. Duck Typing
8.7. The 'in' Keyword
8.7.1. 'in' for Range Selection
8.8. The 'between' Keyword
8.9. The 'like' Keyword
8.10. The 'regexp' Keyword
8.11. The 'any' and 'some' Keywords
8.12. The 'all' Keyword
8.13. The 'new' Keyword
9. EPL Reference: Functions
9.1. Single-row Function Reference
9.1.1. The Case Control Flow Function
9.1.2. The Cast Function
9.1.3. The Coalesce Function
9.1.4. The Current_Timestamp Function
9.1.5. The Exists Function
9.1.6. The Instance-Of Function
9.1.7. The Istream Function
9.1.8. The Min and Max Functions
9.1.9. The Previous Function
9.1.10. The Previous-Tail Function
9.1.11. The Previous-Window Function
9.1.12. The Previous-Count Function
9.1.13. The Prior Function
9.1.14. The Type-Of Function
9.2. Aggregation Functions
9.2.1. SQL-Standard Functions
9.2.2. Event Aggregation Functions
9.2.3. Additional Aggregation Functions
9.3. User-Defined Functions
9.4. Select-Clause transpose Function
9.4.1. Transpose with Insert-Into
10. EPL Reference: Enumeration Methods
10.1. Overview
10.2. Example Events
10.3. How to Use
10.3.1. Syntax
10.3.2. Introductory Examples
10.3.3. Input, Output and Limitations
10.4. Inputs
10.4.1. Subquery Results
10.4.2. Named Window
10.4.3. Event Property
10.4.4. Event Aggregation Function
10.4.5. prev, prevwindow and prevtail Single-Row Functions as Input
10.4.6. Single-Row Function, User-Defined Function and Enum Types
10.4.7. Declared Expression
10.4.8. Variables
10.4.9. Match-Recognize Group Variable
10.4.10. Pattern Repeat and Repeat-Until Operators
10.5. Example
10.6. Reference
10.6.1. Aggregate
10.6.2. AllOf
10.6.3. AnyOf
10.6.4. Average
10.6.5. CountOf
10.6.6. DistinctOf
10.6.7. Except
10.6.8. FirstOf
10.6.9. GroupBy
10.6.10. Intersect
10.6.11. LastOf
10.6.12. LeastFrequent
10.6.13. Max
10.6.14. MaxBy
10.6.15. Min
10.6.16. MinBy
10.6.17. MostFrequent
10.6.18. OrderBy and OrderByDesc
10.6.19. Reverse
10.6.20. SelectFrom
10.6.21. SequenceEqual
10.6.22. SumOf
10.6.23. Take
10.6.24. TakeLast
10.6.25. TakeWhile
10.6.26. TakeWhileLast
10.6.27. ToMap
10.6.28. Union
10.6.29. Where
11. EPL Reference: Date-Time Methods
11.1. Overview
11.2. How to Use
11.2.1. Syntax
11.3. Calendar and Formatting Reference
11.3.1. Between
11.3.2. Format
11.3.3. Get (By Field)
11.3.4. Get (By Name)
11.3.5. Minus
11.3.6. Plus
11.3.7. RoundCeiling
11.3.8. RoundFloor
11.3.9. RoundHalf
11.3.10. Set (By Field)
11.3.11. WithDate
11.3.12. WithMax
11.3.13. WithMin
11.3.14. WithTime
11.3.15. ToCalendar
11.3.16. ToDate
11.3.17. ToMillisec
11.4. Interval Algebra Reference
11.4.1. Examples
11.4.2. Interval Algebra Parameters
11.4.3. Performance
11.4.4. Limitations
11.4.5. After
11.4.6. Before
11.4.7. Coincides
11.4.8. During
11.4.9. Finishes
11.4.10. Finished By
11.4.11. Includes
11.4.12. Meets
11.4.13. Met By
11.4.14. Overlaps
11.4.15. Overlapped By
11.4.16. Starts
11.4.17. Started By
12. EPL Reference: Views
12.1. A Note on View Parameters
12.2. Data Window Views
12.2.1. Length window (win:length)
12.2.2. Length batch window (win:length_batch)
12.2.3. Time window (win:time)
12.2.4. Externally-timed window (win:ext_timed)
12.2.5. Time batch window (win:time_batch)
12.2.6. Externally-timed batch window (win:ext_timed_batch)
12.2.7. Time-Length combination batch window (win:time_length_batch)
12.2.8. Time-Accumulating window (win:time_accum)
12.2.9. Keep-All window (win:keepall)
12.2.10. First Length (win:firstlength)
12.2.11. First Time (win:firsttime)
12.2.12. Expiry Expression (win:expr)
12.2.13. Expiry Expression Batch (win:expr_batch)
12.3. Standard view set
12.3.1. Unique (std:unique)
12.3.2. Grouped Data Window (std:groupwin)
12.3.3. Size (std:size)
12.3.4. Last Event (std:lastevent)
12.3.5. First Event (std:firstevent)
12.3.6. First Unique (std:firstunique)
12.4. Statistics views
12.4.1. Univariate statistics (stat:uni)
12.4.2. Regression (stat:linest)
12.4.3. Correlation (stat:correl)
12.4.4. Weighted average (stat:weighted_avg)
12.5. Extension View Set
12.5.1. Sorted Window View (ext:sort)
12.5.2. Ranked Window View (ext:rank)
12.5.3. Time-Order View (ext:time_order)
13. EPL Reference: Data Flow
13.1. Introduction
13.2. Usage
13.2.1. Overview
13.2.2. Syntax
13.3. Built-in Operators
13.3.1. BeaconSource
13.3.2. EPStatementSource
13.3.3. EventBusSink
13.3.4. EventBusSource
13.3.5. Filter
13.3.6. LogSink
13.3.7. Select
13.4. API
13.4.1. Declaring a Data Flow
13.4.2. Instantiating a Data Flow
13.4.3. Executing a Data Flow
13.4.4. Instantiation Options
13.4.5. Start Captive
13.4.6. Data Flow Punctuation with Markers
13.4.7. Exception Handling
13.5. Examples
13.6. Operator Implementation
13.6.1. Sample Operator Acting as Source
13.6.2. Sample Tokenizer Operator
13.6.3. Sample Aggregator Operator
14. API Reference
14.1. API Overview
14.2. The Service Provider Interface
14.3. The Administrative Interface
14.3.1. Creating Statements
14.3.2. Receiving Statement Results
14.3.3. Setting a Subscriber Object
14.3.4. Adding Listeners
14.3.5. Using Iterators
14.3.6. Managing Statements
14.3.7. Runtime Configuration
14.4. The Runtime Interface
14.4.1. Event Sender
14.4.2. Receiving Unmatched Events
14.5. On-Demand Fire-And-Forget Query Execution
14.5.1. On-Demand Query Single Execution
14.5.2. On-Demand Query Prepared Unparameterized Execution
14.5.3. On-Demand Query Prepared Parameterized Execution
14.6. Event and Event Type
14.6.1. Event Type Metadata
14.6.2. Event Object
14.6.3. Query Example
14.6.4. Pattern Example
14.7. Engine Threading and Concurrency
14.7.1. Advanced Threading
14.7.2. Processing Order
14.8. Controlling Time-Keeping
14.8.1. Controlling Time Using Time Span Events
14.8.2. Additional Time-Related APIs
14.9. Time Resolution
14.10. Service Isolation
14.10.1. Overview
14.10.2. Example: Suspending a Statement
14.10.3. Example: Catching up a Statement from Historical Data
14.10.4. Isolation for Insert-Into
14.10.5. Isolation for Named Windows
14.10.6. Runtime Considerations
14.11. Exception Handling
14.12. Condition Handling
14.13. Statement Object Model
14.13.1. Building an Object Model
14.13.2. Building Expressions
14.13.3. Building a Pattern Statement
14.13.4. Building a Select Statement
14.13.5. Building a Create-Variable and On-Set Statement
14.13.6. Building Create-Window, On-Delete and On-Select Statements
14.14. Prepared Statement and Substitution Parameters
14.15. Engine and Statement Metrics Reporting
14.15.1. Engine Metrics
14.15.2. Statement Metrics
14.16. Event Rendering to XML and JSON
14.16.1. JSON Event Rendering Conventions and Options
14.16.2. XML Event Rendering Conventions and Options
14.17. Plug-in Loader
14.18. Interrogating EPL Annotations
14.19. Context Partition Selection
14.19.1. Selectors
14.20. Context Partition Administration
14.21. Test and Assertion Support
14.21.1. EPAssertionUtil Summary
14.21.2. SupportUpdateListener Summary
14.21.3. Usage Example
15. Configuration
15.1. Programmatic Configuration
15.2. Configuration via XML File
15.3. XML Configuration File
15.4. Configuration Items
15.4.1. Events represented by Java Classes
15.4.2. Events represented by java.util.Map
15.4.3. Events represented by Object[] (Object-array)
15.4.4. Events represented by org.w3c.dom.Node
15.4.5. Events represented by Plug-in Event Representations
15.4.6. Class and package imports
15.4.7. Cache Settings for From-Clause Method Invocations
15.4.8. Variables
15.4.9. Relational Database Access
15.4.10. Engine Settings related to Concurrency and Threading
15.4.11. Engine Settings related to Event Metadata
15.4.12. Engine Settings related to View Resources
15.4.13. Engine Settings related to Logging
15.4.14. Engine Settings related to Variables
15.4.15. Engine Settings related to Patterns
15.4.16. Engine Settings related to Scripts
15.4.17. Engine Settings related to Stream Selection
15.4.18. Engine Settings related to Time Source
15.4.19. Engine Settings related to JMX Metrics
15.4.20. Engine Settings related to Metrics Reporting
15.4.21. Engine Settings related to Language and Locale
15.4.22. Engine Settings related to Expression Evaluation
15.4.23. Engine Settings related to Execution of Statements
15.4.24. Engine Settings related to Exception Handling
15.4.25. Engine Settings related to Condition Handling
15.4.26. Revision Event Type
15.4.27. Variant Stream
15.5. Type Names
15.6. Runtime Configuration
15.7. Logging Configuration
15.7.1. Log4j Logging Configuration
16. Development Lifecycle
16.1. Authoring
16.2. Testing
16.3. Debugging
16.3.1. @Audit Annotation
16.4. Packaging and Deploying Overview
16.5. EPL Modules
16.6. The Deployment Administrative Interface
16.6.1. Reading Module Content
16.6.2. Ordering Multiple Modules
16.6.3. Deploying and Undeploying
16.6.4. Listing Deployments
16.6.5. State Transitioning a Module
16.6.6. Best Practices
16.7. J2EE Packaging and Deployment
16.7.1. J2EE Deployment Considerations
16.7.2. Servlet Context Listener
16.8. Monitoring and JMX
17. Integration and Extension
17.1. Overview
17.2. Virtual Data Window
17.2.1. How to Use
17.2.2. Implementing the Factory
17.2.3. Implementing the Virtual Data Window
17.2.4. Implementing the Lookup
17.3. Single-Row Function
17.3.1. Implementing a Single-Row Function
17.3.2. Configuring the Single-Row Function Name
17.3.3. Value Cache
17.3.4. Single-Row Functions in Filter Predicate Expressions
17.3.5. Single-Row Functions Taking Events as Parameters
17.3.6. Receiving a Context Object
17.3.7. Exception Handling
17.4. Derived-value and Data Window View
17.4.1. Implementing a View Factory
17.4.2. Implementing a View
17.4.3. View Contract
17.4.4. Configuring View Namespace and Name
17.4.5. Requirement for Data Window Views
17.4.6. Requirement for Derived-Value Views
17.4.7. Requirement for Grouped Views
17.5. Aggregation Function
17.5.1. Aggregation Single-Function Development
17.5.2. Aggregation Multi-Function Development
17.6. Pattern Guard
17.6.1. Implementing a Guard Factory
17.6.2. Implementing a Guard Class
17.6.3. Configuring Guard Namespace and Name
17.7. Pattern Observer
17.7.1. Implementing an Observer Factory
17.7.2. Implementing an Observer Class
17.7.3. Configuring Observer Namespace and Name
17.8. Event Type And Event Object
17.8.1. How It Works
17.8.2. Steps
17.8.3. URI-based Resolution
17.8.4. Example
18. Script Support
18.1. Overview
18.2. Syntax
18.3. Examples
18.4. Built-In EPL Script Attributes
18.5. Performance Notes
18.6. Additional Notes
19. Examples, Tutorials, Case Studies
19.1. Examples Overview
19.2. Running the Examples
19.3. AutoID RFID Reader
19.4. Runtime Configuration
19.5. JMS Server Shell and Client
19.5.1. Overview
19.5.2. JMS Messages as Events
19.5.3. JMX for Remote Dynamic Statement Management
19.6. Market Data Feed Monitor
19.6.1. Input Events
19.6.2. Computing Rates Per Feed
19.6.3. Detecting a Fall-off
19.6.4. Event generator
19.7. OHLC Plug-in View
19.8. Transaction 3-Event Challenge
19.8.1. The Events
19.8.2. Combined event
19.8.3. Real time summary data
19.8.4. Find problems
19.8.5. Event generator
19.9. Self-Service Terminal
19.9.1. Events
19.9.2. Detecting Customer Check-in Issues
19.9.3. Absence of Status Events
19.9.4. Activity Summary Data
19.9.5. Sample Application for J2EE Application Server
19.10. Assets Moving Across Zones - An RFID Example
19.11. StockTicker
19.12. MatchMaker
19.13. Named Window Query
19.14. Sample Virtual Data Window
19.15. Sample Cycle Detection
19.16. Quality of Service
19.17. Trivia Geeks Club
20. Performance
20.1. Performance Results
20.2. Performance Tips
20.2.1. Understand how to tune your Java virtual machine
20.2.2. Input and Output Bottlenecks
20.2.3. Theading
20.2.4. Select the underlying event rather than individual fields
20.2.5. Prefer stream-level filtering over where-clause filtering
20.2.6. Reduce the use of arithmetic in expressions
20.2.7. Remove Unneccessary Constructs
20.2.8. End Pattern Sub-Expressions
20.2.9. Consider using EventPropertyGetter for fast access to event properties
20.2.10. Consider casting the underlying event
20.2.11. Turn off logging and audit
20.2.12. Disable view sharing
20.2.13. Tune or disable delivery order guarantees
20.2.14. Use a Subscriber Object to Receive Events
20.2.15. Consider Data Flows
20.2.16. High-Arrival-Rate Streams and Single Statements
20.2.17. Subqueries versus Joins And Where-clause And Data Windows
20.2.18. Patterns and Pattern Sub-Expression Instances
20.2.19. Pattern Sub-Expression Instance Versus Data Window Use
20.2.20. The Keep-All Data Window
20.2.21. Statement Design for Reduced Memory Consumption - Diagnosing OutOfMemoryError
20.2.22. Performance, JVM, OS and hardware
20.2.23. Consider using Hints
20.2.24. Optimizing Stream Filter Expressions
20.2.25. Statement and Engine Metric Reporting
20.2.26. Expression Evaluation Order and Early Exit
20.2.27. Large Number of Threads
20.2.28. Context Partition Related Information
20.2.29. Prefer Constant Variables over Non-Constant Variables
20.2.30. Prefer Object-array Events
20.2.31. Composite or Compound Keys
20.2.32. Notes on Query Planning
20.2.33. Query Planning Index Hints
20.2.34. Measuring Throughput
20.2.35. Do not create the same EPL Statement X times
20.2.36. Comparing Single-Threaded and Multi-Threaded Performance
20.2.37. Incremental Versus Recomputed Aggregation for Named Window Events
20.2.38. When Does Memory Get Released
20.3. Using the performance kit
20.3.1. How to use the performance kit
20.3.2. How we use the performance kit
21. References
21.1. Reference List
A. Output Reference and Samples
A.1. Introduction and Sample Data
A.2. Output for Un-aggregated and Un-grouped Queries
A.2.1. No Output Rate Limiting
A.2.2. Output Rate Limiting - Default
A.2.3. Output Rate Limiting - Last
A.2.4. Output Rate Limiting - First
A.2.5. Output Rate Limiting - Snapshot
A.3. Output for Fully-aggregated and Un-grouped Queries
A.3.1. No Output Rate Limiting
A.3.2. Output Rate Limiting - Default
A.3.3. Output Rate Limiting - Last
A.3.4. Output Rate Limiting - First
A.3.5. Output Rate Limiting - Snapshot
A.4. Output for Aggregated and Un-grouped Queries
A.4.1. No Output Rate Limiting
A.4.2. Output Rate Limiting - Default
A.4.3. Output Rate Limiting - Last
A.4.4. Output Rate Limiting - First
A.4.5. Output Rate Limiting - Snapshot
A.5. Output for Fully-aggregated and Grouped Queries
A.5.1. No Output Rate Limiting
A.5.2. Output Rate Limiting - Default
A.5.3. Output Rate Limiting - All
A.5.4. Output Rate Limiting - Last
A.5.5. Output Rate Limiting - First
A.5.6. Output Rate Limiting - Snapshot
A.6. Output for Aggregated and Grouped Queries
A.6.1. No Output Rate Limiting
A.6.2. Output Rate Limiting - Default
A.6.3. Output Rate Limiting - All
A.6.4. Output Rate Limiting - Last
A.6.5. Output Rate Limiting - First
A.6.6. Output Rate Limiting - Snapshot
B. Reserved Keywords
Index

Analyzing and reacting to information in real-time oftentimes requires the development of custom applications. Typically these applications must obtain the data to analyze, filter data, derive information and then indicate this information through some form of presentation or communication. Data may arrive with high frequency requiring high throughput processing. And applications may need to be flexible and react to changes in requirements while the data is processed. Esper is an event stream processor that aims to enable a short development cycle from inception to production for these types of applications.

This document is a resource for software developers who develop event driven applications. It also contains information that is useful for business analysts and system architects who are evaluating Esper.

It is assumed that the reader is familiar with the Java programming language.

This document is relevant in all phases of your software development project: from design to deployment and support.

If you are new to Esper, please follow these steps:

  1. Read the tutorials, case studies and solution patterns available on the Esper public web site at http://esper.codehaus.org

  2. Read Section 1.1, “Introduction to CEP and event series analysis” if you are new to CEP and ESP (complex event processing, event stream processing)

  3. Read Chapter 2, Event Representations that explains the different ways of representing events to Esper

  4. Read Chapter 3, Processing Model to gain insight into EPL continuous query results

  5. Read Section 5.1, “EPL Introduction” for an introduction to event stream processing via EPL

  6. Read Section 6.1, “Event Pattern Overview” for an overview over event patterns

  7. Read Section 7.1, “Overview” for an overview over event patterns using the match recognize syntax.

  8. Then glance over the examples Section 19.1, “Examples Overview”

  9. Finally to test drive Esper performance, read Chapter 20, Performance

This section outlines the different means to model and represent events.

Esper uses the term event type to describe the type information available for an event representation.

Your application may configure predefined event types at startup time or dynamically add event types at runtime via API or EPL syntax. See Section 15.4, “Configuration Items” for startup-time configuration and Section 14.3.7, “Runtime Configuration” for the runtime configuration API.

The EPL create schema syntax allows declaring an event type at runtime using EPL, see Section 5.16, “Declaring an Event Type: Create Schema”.

In Section 14.6, “Event and Event Type” we explain how an event type becomes visible in EPL statements and output events delivered by the engine.

An event is an immutable record of a past occurrence of an action or state change. Event properties capture the state information for an event.

In Esper, an event can be represented by any of the following underlying Java objects:


Esper provides multiple choices for representing an event. There is no absolute need for you to create new Java classes to represent an event.

Event representations have the following in common:

  • All event representations support nested, indexed and mapped properties (aka. property expression), as explained in more detail below. There is no limitation to the nesting level.

  • All event representations provide event type metadata. This includes type metadata for nested properties.

  • All event representations allow transposing the event itself and parts of all of its property graph into new events. The term transposing refers to selecting the event itself or event properties that are themselves nestable property graphs, and then querying the event's properties or nested property graphs in further statements. The Apache Axiom event representation is an exception and does not currently allow transposing event properties but does allow transposing the event itself.

  • The Java object, Map and Object-array representations allow supertypes.

The API behavior for all event representations is the same, with minor exceptions noted in this chapter.

The benefits of multiple event representations are:

  • For applications that already have events in one of the supported representations, there is no need to transform events into a Java object before processing.

  • Event representations are exchangeable, reducing or eliminating the need to change statements when the event representation changes.

  • Event representations are interoperable, allowing all event representations to interoperate in same or different statements.

  • The choice makes its possible to consciously trade-off performance, ease-of-use, the ability to evolve and effort needed to import or externalize events and use existing event type metadata.

Event properties capture the state information for an event. Event properties be simple as well as indexed, mapped and nested event properties. The table below outlines the different types of properties and their syntax in an event expression. This syntax allows statements to query deep JavaBean objects graphs, XML structures and Map events.


Combinations are also possible. For example, a valid combination could be person.address('home').street[0].

You may use any expression as a mapped property key or indexed property index by putting the expression within parenthesis after the mapped or index property name. Please find examples below.

The key or index expression must be placed in parenthesis. When using an expression as key for a mapped property, the expression must return a String-typed value. When using an expression as index for an indexed property, the expression must return an int-typed value.

This example below uses Java classes to illustrate;The same principles apply to all event representations.

Assume a class declares these properties (getters not shown for brevity):

public class MyEventType {
  String myMapKey;
  int myIndexValue;
  int myInnerIndexValue;
  Map<String, InnerType> innerTypesMap;	// mapped property
  InnerType[] innerTypesArray; // indexed property
}

public class InnerType {
  String name;
  int[] ids;
}

A sample EPL statement demonstrating expressions as map keys or indexes is:

select innerTypesMap('somekey'),  // returns map value for 'somekey'
  innerTypesMap(myMapKey),        // returns map value for myMapKey value (an expression)
  innerTypesArray[1],             // returns array value at index 1
  innerTypesArray(myIndexValue)   // returns array value at index myIndexValue (an expression)
  from MyEventType

The dot-operator can be used to access methods on the value objects returned by the mapped or indexed properties. By using the dot-operator the syntax follows the chained method invocation described at Section 8.6, “Dot Operator”.

A sample EPL statement demonstrating the dot-operator as well as expressions as map keys or indexes is:

 select innerTypesMap('somekey').ids[1],
  innerTypesMap(myMapKey).getIds(myIndexValue),
  innerTypesArray[1].ids[2],
  innerTypesArray(myIndexValue).getIds(myInnerIndexValue)
  from MyEventType

Please note the following limitations:

  • The square brackets-syntax for indexed properties does now allow expressions and requires a constant index value.

  • When using the dot-operator with mapped or indexed properties that have expressions as map keys or indexes you must follow the chained method invocation syntax.

Dynamic (unchecked) properties are event properties that need not be known at statement compilation time. Such properties are resolved during runtime: they provide duck typing functionality.

The idea behind dynamic properties is that for a given underlying event representation we don't always know all properties in advance. An underlying event may have additional properties that are not known at statement compilation time, that we want to query on. The concept is especially useful for events that represent rich, object-oriented domain models.

The syntax of dynamic properties consists of the property name and a question mark. Indexed, mapped and nested properties can also be dynamic properties:


Dynamic properties always return the java.lang.Object type. Also, dynamic properties return a null value if the dynamic property does not exist on events processed at runtime.

As an example, consider an OrderEvent event that provides an "item" property. The "item" property is of type Object and holds a reference to an instance of either a Service or Product.

Assume that both Service and Product classes provide a property named "price". Via a dynamic property we can specify a query that obtains the price property from either object (Service or Product):

select item.price? from OrderEvent

As a second example, assume that the Service class contains a "serviceName" property that the Product class does not possess. The following query returns the value of the "serviceName" property for Service objects. It returns a null-value for Product objects that do not have the "serviceName" property:

select item.serviceName? from OrderEvent

Consider the case where OrderEvent has multiple implementation classes, some of which have a "timestamp" property. The next query returns the timestamp property of those implementations of the OrderEvent interface that feature the property:

select timestamp? from OrderEvent

The query as above returns a single column named "timestamp?" of type Object.

When dynamic properties are nested, then all properties under the dynamic property are also considered dynamic properties. In the below example the query asks for the "direction" property of the object returned by the "detail" dynamic property:

select detail?.direction from OrderEvent

Above is equivalent to:

select detail?.direction? from OrderEvent

The functions that are often useful in conjunction with dynamic properties are:

  • The cast function casts the value of a dynamic property (or the value of an expression) to a given type.

  • The exists function checks whether a dynamic property exists. It returns true if the event has a property of that name, or false if the property does not exist on that event.

  • The instanceof function checks whether the value of a dynamic property (or the value of an expression) is of any of the given types.

  • The typeof function returns the string type name of a dynamic property.

Dynamic event properties work with all event representations outlined next: Java objects, Map-based, Object-array-based and XML DOM-based events.

Plain-old Java object events are object instances that expose event properties through JavaBeans-style getter methods. Events classes or interfaces do not have to be fully compliant to the JavaBean specification; however for the Esper engine to obtain event properties, the required JavaBean getter methods must be present or an accessor-style and accessor-methods may be defined via configuration.

Esper supports JavaBeans-style event classes that extend a superclass or implement one or more interfaces. Also, Esper event pattern and EPL statements can refer to Java interface classes and abstract classes.

Classes that represent events should be made immutable. As events are recordings of a state change or action that occurred in the past, the relevant event properties should not be changeable. However this is not a hard requirement and the Esper engine accepts events that are mutable as well.

The hashCode and equals methods do not need to be implemented. The implementation of these methods by a Java event class does not affect the behavior of the engine in any way.

Please see Chapter 15, Configuration on options for naming event types represented by Java object event classes. Java classes that do not follow JavaBean conventions, such as legacy Java classes that expose public fields, or methods not following naming conventions, require additional configuration. Via configuration it is also possible to control case sensitivity in property name resolution. The relevant section in the chapter on configuration is Section 15.4.1.3, “Non-JavaBean and Legacy Java Event Classes”.

As outlined earlier, the different property types are supported by the standard JavaBeans specification, and some of which are uniquely supported by Esper:

Assume there is an NewEmployeeEvent event class as shown below. The mapped and indexed properties in this example return Java objects but could also return Java language primitive types (such as int or String). The Address object and Employee can themselves have properties that are nested within them, such as a street name in the Address object or a name of the employee in the Employee object.

public class NewEmployeeEvent {
	public String getFirstName();
	public Address getAddress(String type);
	public Employee getSubordinate(int index);
	public Employee[] getAllSubordinates();
}

Simple event properties require a getter-method that returns the property value. In this example, the getFirstName getter method returns the firstName event property of type String.

Indexed event properties require either one of the following getter-methods. A method that takes an integer-type key value and returns the property value, such as the getSubordinate method, or a method that returns an array-type, or a class that implements Iterable. An example is the getAllSubordinates getter method, which returns an array of Employee but could also return an Iterable. In an EPL or event pattern statement, indexed properties are accessed via the property[index] syntax.

Mapped event properties require a getter-method that takes a String-typed key value and returns the property value, such as the getAddress method. In an EPL or event pattern statement, mapped properties are accessed via the property('key') syntax.

Nested event properties require a getter-method that returns the nesting object. The getAddress and getSubordinate methods are mapped and indexed properties that return a nesting object. In an EPL or event pattern statement, nested properties are accessed via the property.nestedProperty syntax.

All event pattern and EPL statements allow the use of indexed, mapped and nested properties (or a combination of these) anywhere where one or more event property names are expected. The below example shows different combinations of indexed, mapped and nested properties in filters of event pattern expressions (each line is a separate EPL statement):

every NewEmployeeEvent(firstName='myName')
every NewEmployeeEvent(address('home').streetName='Park Avenue')
every NewEmployeeEvent(subordinate[0].name='anotherName')
every NewEmployeeEvent(allSubordinates[1].name='thatName')
every NewEmployeeEvent(subordinate[0].address('home').streetName='Water Street')

Similarly, the syntax can be used in EPL statements in all places where an event property name is expected, such as in select lists, where-clauses or join criteria.

select firstName, address('work'), subordinate[0].name, subordinate[1].name
from NewEmployeeEvent
where address('work').streetName = 'Park Ave'

Events can also be represented by objects that implement the java.util.Map interface. Event properties of Map events are the values in the map accessible through the get method exposed by the java.util.Map interface.

Similar to the Object-array event type, the Map event type takes part in the comprehensive type system that can eliminate the need to use Java classes as event types, thereby making it easier to change types at runtime or generate type information from another source.

A given Map event type can have one or more supertypes that must also be Map event types. All properties available on any of the Map supertypes are available on the type itself. In addition, anywhere within EPL that an event type name of a Map supertype is used, any of its Map subtypes and their subtypes match that expression.

Your application can add properties to an existing Map event type during runtime using the configuration operation updateMapEventType. Properties may not be updated or deleted - properties can only be added, and nested properties can be added as well. The runtime configuration also allows removing Map event types and adding them back with new type information.

After your application configures a Map event type by providing a type name, the type name can be used when defining further Map or Object-array event types by specifying the type name as a property type or an array property type.

One-to-Many relationships in Map event types are represented via arrays. A property in a Map event type may be an array of primitive, an array of Java object, an array of Map or an an array of Object-array.

The engine can process java.util.Map events via the sendEvent(Map map, String eventTypeName) method on the EPRuntime interface. Entries in the Map represent event properties. Keys must be of type java.util.String for the engine to be able to look up event property names specified by pattern or EPL statements.

The engine does not validate Map event property names or values. Your application should ensure that objects passed in as event properties match the create schema property names and types, or the configured event type information when using runtime or static configuration.

Map event properties can be of any type. Map event properties that are Java application objects or that are of type java.util.Map (or arrays thereof) or that are of type Object[] (object-array) (or arrays thereof) offer additional power:

In order to use Map events, the event type name and property names and types must be made known to the engine via Configuration or create schema EPL syntax. Please see examples in Section 5.16, “Declaring an Event Type: Create Schema” and Section 15.4.2, “Events represented by java.util.Map”.

The code snippet below defines a Map event type, creates a Map event and sends the event into the engine. The sample defines the CarLocUpdateEvent event type via runtime configuration interface (create schema or static configuration could have been used instead).

// Define CarLocUpdateEvent event type (example for runtime-configuration interface)
Map<String, Object> def = new HashMap<String, Object>;
def.put("carId", String.class);
def.put("direction", int.class);

epService.getEPAdministrator().getConfiguration().
  addEventType("CarLocUpdateEvent", def);

The CarLocUpdateEvent can now be used in a statement:

select carId from CarLocUpdateEvent.win:time(1 min) where direction = 1
// Create a CarLocUpdateEvent event and send it into the engine for processing
Map<String, Object> event = new HashMap<String, Object>();
event.put("carId", carId);
event.put("direction", direction);

epRuntime.sendEvent(event, "CarLocUpdateEvent");

The engine can also query Java objects as values in a Map event via the nested property syntax. Thus Map events can be used to aggregate multiple data structures into a single event and query the composite information in a convenient way. The example below demonstrates a Map event with a transaction and an account object.

Map event = new HashMap();
event.put("txn", txn);
event.put("account", account);
epRuntime.sendEvent(event, "TxnEvent");

An example statement could look as follows.

select account.id, account.rate * txn.amount 
from TxnEvent.win:time(60 sec) 
group by account.id

Your Map event type may declare one or more supertypes when configuring the type at engine initialization time or at runtime through the administrative interface.

Supertypes of a Map event type must also be Map event types. All property names and types of a supertype are also available on a subtype and override such same-name properties of the subtype. In addition, anywhere within EPL that an event type name of a Map supertype is used, any of its Map subtypes also matches that expression (similar to the concept of interface in Java).

This example assumes that the BaseUpdate event type has been declared and acts as a supertype to the AccountUpdate event type (both Map event types):

epService.getEPAdministrator().getConfiguration().
    addEventType("AccountUpdate", accountUpdateDef, 
    new String[] {"BaseUpdate"});

Your application EPL statements may select BaseUpdate events and receive both BaseUpdate and AccountUpdate events, as well as any other subtypes of BaseUpdate and their subtypes.

// Receive BaseUpdate and any subtypes including subtypes of subtypes
select * from BaseUpdate

Your application Map event type may have multiple supertypes. The multiple inheritance hierarchy between Maps can be arbitrarily deep, however cyclic dependencies are not allowed. If using runtime configuration, supertypes must exist before a subtype to a supertype can be added.

See Section 15.4.2, “Events represented by java.util.Map” for more information on configuring Map event types.

Strongly-typed nested Map-within-Map events can be used to build rich, type-safe event types on the fly. Use the addEventType method on Configuration or ConfigurationOperations for initialization-time and runtime-time type definition, or the create schema EPL syntax.

Noteworthy points are:

For demonstration, in this example our top-level event type is an AccountUpdate event, which has an UpdatedFieldType structure as a property. Inside the UpdatedFieldType structure the example defines various fields, as well as a property by name 'history' that holds a JavaBean class UpdateHistory to represent the update history for the account. The code snippet to define the event type is thus:

Map<String, Object> updatedFieldDef = new HashMap<String, Object>();
updatedFieldDef.put("name", String.class);
updatedFieldDef.put("addressLine1", String.class);
updatedFieldDef.put("history", UpdateHistory.class);
epService.getEPAdministrator().getConfiguration().
    addEventType("UpdatedFieldType", updatedFieldDef);

Map<String, Object> accountUpdateDef = new HashMap<String, Object>();
accountUpdateDef.put("accountId", long.class);
accountUpdateDef.put("fields", "UpdatedFieldType");	
// the latter can also be:  accountUpdateDef.put("fields", updatedFieldDef);

epService.getEPAdministrator().getConfiguration().
    addEventType("AccountUpdate", accountUpdateDef);

The next code snippet populates a sample event and sends the event into the engine:

Map<String, Object> updatedField = new HashMap<String, Object>();
updatedField.put("name", "Joe Doe");
updatedField.put("addressLine1", "40 Popular Street");
updatedField.put("history", new UpdateHistory());

Map<String, Object> accountUpdate = new HashMap<String, Object>();
accountUpdate.put("accountId", 10009901);
accountUpdate.put("fields", updatedField);

epService.getEPRuntime().sendEvent(accountUpdate, "AccountUpdate");

Last, a sample query to interrogate AccountUpdate events is as follows:

select accountId, fields.name, fields.addressLine1, fields.history.lastUpdate
from AccountUpdate

An event can also be represented by an array of objects. Event properties of Object[] events are the array element values.

Similar to the Map event type, the object-array event type takes part in the comprehensive type system that can eliminate the need to use Java classes as event types, thereby making it easier to change types at runtime or generate type information from another source.

A given Object-array event type can have only a single supertype that must also be an Object-array event type. All properties available on the Object-array supertype is also available on the type itself. In addition, anywhere within EPL that an event type name of an Object-array supertype is used, any of its Object-array subtypes and their subtypes match that expression.

Your application can add properties to an existing Object-array event type during runtime using the configuration operation updateObjectArrayEventType. Properties may not be updated or deleted - properties can only be added, and nested properties can be added as well. The runtime configuration also allows removing Object-array event types and adding them back with new type information.

After your application configures an Object-array event type by providing a type name, the type name can be used when defining further Object-array or Map event types by specifying the type name as a property type or an array property type.

One-to-Many relationships in Object-array event types are represented via arrays. A property in an Object-array event type may be an array of primitive, an array of Java object, an array of Map or an array of Object-array.

The engine can process Object[] events via the sendEvent(Object[] array, String eventTypeName) method on the EPRuntime interface. Entries in the Object array represent event properties.

The engine does not validate Object array length or value types. Your application must ensure that Object array values match the declaration of the event type: The type and position of property values must match property names and types in the same exact order and object array length must match the number of properties declared via create schema or the static or runtime configuration.

Object-array event properties can be of any type. Object-array event properties that are Java application objects or that are of type java.util.Map (or arrays thereof) or that are of type Object-array (or arrays thereof) offer additional power:

In order to use Object[] (object-array) events, the event type name and property names and types, in a well-defined order that must match object-array event properties, must be made known to the engine via configuration or create schema EPL syntax. Please see examples in Section 5.16, “Declaring an Event Type: Create Schema” and Section 15.4.3, “Events represented by Object[] (Object-array)”.

The code snippet below defines an Object-array event type, creates an Object-array event and sends the event into the engine. The sample defines the CarLocUpdateEvent event type via the runtime configuration interface (create schema or static configuration could have been used instead).

// Define CarLocUpdateEvent event type (example for runtime-configuration interface)
String[] propertyNames = {"carId", "direction"};   // order is important
Object[] propertyTypes = {String.class, int.class};  // type order matches name order

epService.getEPAdministrator().getConfiguration().
  addEventType("CarLocUpdateEvent", propertyNames, propertyTypes);

The CarLocUpdateEvent can now be used in a statement:

select carId from CarLocUpdateEvent.win:time(1 min) where direction = 1
// Send an event
Object[] event = {carId, direction};
epRuntime.sendEvent(event, "CarLocUpdateEvent");

The engine can also query Java objects as values in an Object[] event via the nested property syntax. Thus Object[] events can be used to aggregate multiple data structures into a single event and query the composite information in a convenient way. The example below demonstrates a Object[] event with a transaction and an account object.

epRuntime.sendEvent(new Object[] {txn, account}, "TxnEvent");

An example statement could look as follows:

select account.id, account.rate * txn.amount 
from TxnEvent.win:time(60 sec) 
group by account.id

Strongly-typed nested Object[]-within-Object[] events can be used to build rich, type-safe event types on the fly. Use the addEventType method on Configuration or ConfigurationOperations for initialization-time and runtime-time type definition, or the create schema EPL syntax.

Noteworthy points are:

For demonstration, in this example our top-level event type is an AccountUpdate event, which has an UpdatedFieldType structure as a property. Inside the UpdatedFieldType structure the example defines various fields, as well as a property by name 'history' that holds a JavaBean class UpdateHistory to represent the update history for the account. The code snippet to define the event type is thus:

String[] propertyNamesUpdField = {"name", "addressLine1", "history"};
Object[] propertyTypesUpdField = {String.class, String.class, UpdateHistory.class};
epService.getEPAdministrator().getConfiguration().
    addEventType("UpdatedFieldType", propertyNamesUpdField, propertyTypesUpdField);

String[] propertyNamesAccountUpdate = {"accountId", "fields"};
Object[] propertyTypesAccountUpdate = {long.class, "UpdatedFieldType"};
epService.getEPAdministrator().getConfiguration().
    addEventType("AccountUpdate", propertyNamesAccountUpdate, propertyTypesAccountUpdate);

The next code snippet populates a sample event and sends the event into the engine:

Object[] updatedField = {"Joe Doe", "40 Popular Street", new UpdateHistory()};
Object[] accountUpdate = {10009901, updatedField};

epService.getEPRuntime().sendEvent(accountUpdate, "AccountUpdate");

Last, a sample query to interrogate AccountUpdate events is as follows:

select accountId, fields.name, fields.addressLine1, fields.history.lastUpdate
from AccountUpdate

Events can be represented as org.w3c.dom.Node instances and send into the engine via the sendEvent method on EPRuntime or via EventSender. Please note that configuration is required so the event type name and root element name is known. See Chapter 15, Configuration.

If a XML schema document (XSD file) can be made available as part of the configuration, then Esper can read the schema and appropriately present event type metadata and validate statements that use the event type and its properties. See Section 2.8.1, “Schema-Provided XML Events”.

When no XML schema document is provided, XML events can still be queried, however the return type and return values of property expressions are string-only and no event type metadata is available other then for explicitly configured properties. See Section 2.8.2, “No-Schema-Provided XML Events”.

In all cases Esper allows you to configure explicit XPath expressions as event properties. You can specify arbitrary XPath functions or expressions and provide a property name and type by which result values will be available for use in EPL statements. See Section 2.8.3, “Explicitly-Configured Properties”.

Nested, mapped and indexed event properties are also supported in expressions against org.w3c.dom.Node events. Thus XML trees can conveniently be interrogated via the property expression syntax.

Only one event type per root element name may be configured. The engine recognizes each event by its root element name or you may use EventSender to send events.

This section uses the following XML document as an example:

<?xml version="1.0" encoding="UTF-8"?>
<Sensor xmlns="SensorSchema">
  <ID>urn:epc:1:4.16.36</ID>
  <Observation Command="READ_PALLET_TAGS_ONLY">
    <ID>00000001</ID>
    <Tag>
      <ID>urn:epc:1:2.24.400</ID>
    </Tag>
    <Tag>
      <ID>urn:epc:1:2.24.401</ID>
    </Tag>
  </Observation>
</Sensor>

The schema for the example is:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">

  <xs:element name="Sensor">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="ID" type="xs:string"/>
        <xs:element ref="Observation" />
      </xs:sequence>
    </xs:complexType>
  </xs:element>

  <xs:element name="Observation">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="ID" type="xs:string"/>
        <xs:element ref="Tag" maxOccurs="unbounded" />
      </xs:sequence>
      <xs:attribute name="Command" type="xs:string" use="required" />
    </xs:complexType>
  </xs:element>

  <xs:element name="Tag">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="ID" type="xs:string"/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>
</xs:schema>

If you have a XSD schema document available for your XML events, Esper can interrogate the schema. The benefits are:

The engine reads a XSD schema file from an URL you provide. Make sure files imported by the XSD schema file can also be resolved.

The configuration accepts a schema URL. This is a sample code snippet to determine a schema URL from a file in classpath:

URL schemaURL = this.getClass().getClassLoader().getResource("sensor.xsd");

Here is a sample use of the runtime configuration API, please see Chapter 15, Configuration for further examples.

epService = EPServiceProviderManager.getDefaultProvider();
ConfigurationEventTypeXMLDOM sensorcfg = new ConfigurationEventTypeXMLDOM();
sensorcfg.setRootElementName("Sensor");
sensorcfg.setSchemaResource(schemaURL.toString());
epService.getEPAdministrator().getConfiguration()
    .addEventType("SensorEvent", sensorcfg);

You must provide a root element name. This name is used to look up the event type for the sendEvent(org.w3c.Node node) method. An EventSender is a useful alternative method for sending events if the type lookup based on the root or document element name is not desired.

After adding the event type, you may create statements and send events. Next is a sample statement:

select ID, Observation.Command, Observation.ID, 
  Observation.Tag[0].ID, Observation.Tag[1].ID
from SensorEvent

As you can see from the example above, property expressions can query property values held in the XML document's elements and attributes.

There are multiple ways to obtain a XML DOM document instance from a XML string. The next code snippet shows how to obtain a XML DOM org.w3c.Document instance:

InputSource source = new InputSource(new StringReader(xml));
DocumentBuilderFactory builderFactory = DocumentBuilderFactory.newInstance();
builderFactory.setNamespaceAware(true);
Document doc = builderFactory.newDocumentBuilder().parse(source);

Send the org.w3c.Node or Document object into the engine for processing:

epService.getEPRuntime().sendEvent(doc);

By setting the xpath-property-expr option the engine rewrites each property expression as an XPath expression, effectively handing the evaluation over to the underlying XPath implementation available from classpath. Most JVM have a built-in XPath implementation and there are also optimized, fast implementations such as Jaxen that can be used as well.

Set the xpath-property-expr option if you need namespace-aware document traversal, such as when your schema mixes several namespaces and element names are overlapping.

The below table samples several property expressions and the XPath expression generated for each, without namespace prefixes to keep the example simple:


For mapped properties that are specified via the syntax name('key'), the algorithm looks for an attribute by name id and generates a XPath expression as mapped[@id='key'].

Finally, here is an example that includes all different types of properties and their XPath expression equivalent in one property expression:

select nested.mapped('key').indexed[1].attribute from MyEvent

The equivalent XPath expression follows, this time including n0 as a sample namespace prefix:

/n0:rootelement/n0:nested/n0:mapped[@id='key']/n0:indexed[position() = 2]/@attribute

When providing a XSD document, the default configuration allows to transpose property values that are themselves complex elements, as defined in the XSD schema, into a new stream. This behavior can be controlled via the flag auto-fragment.

For example, consider the next query:

insert into ObservationStream
select ID, Observation from SensorEvent

The Observation as a property of the SensorEvent gets itself inserted into a new stream by name ObservationStream. The ObservationStream thus consists of a string-typed ID property and a complex-typed property named Observation, as described in the schema.

A further statement can use this stream to query:

select Observation.Command, Observation.Tag[0].ID from ObservationStream

Before continuing the discussion, here is an alternative syntax using the wildcard-select, that is also useful:

insert into TagListStream
select ID as sensorId, Observation.* from SensorEvent

The new TagListStream has a string-typed ID and Command property as well as an array of Tag properties that are complex types themselves as defined in the schema.

Next is a sample statement to query the new stream:

select sensorId, Command, Tag[0].ID from TagListStream

Please note the following limitations:

Esper automatically registers a new event type for transposed properties. It generates the type name of the new XML event type from the XML event type name and the property names used in the expression. The synposis is type_name.property_name[.property_name...]. The type name can be looked up, for example for use with EventSender or can be created in advance.

Regardless of whether or not you provide a XSD schema for the XML event type, you can always fall back to configuring explicit properties that are backed by XPath expressions.

For further documentation on XPath, please consult the XPath standard or other online material. Consider using Jaxen or Apache Axiom, for example, to provide faster XPath evaluation then your Java VM built-in XPath provider may offer.

Part of the extension and plug-in features of Esper is an event representation API. This set of classes allow an application to create new event types and event instances based on information available elsewhere, statically or dynamically at runtime when EPL statements are created. Please see Section 17.8, “Event Type And Event Object” for details.

Creating a plug-in event representation can be useful when your application has existing Java classes that carry event metadata and event property values and your application does not want to (or cannot) extract or transform such event metadata and event data into one of the built-in event representations (POJO Java objects, Map, Object-array or XML DOM).

Further use of a plug-in event representation is to provide a faster or short-cut access path to event data. For example, access to event data stored in a XML format through the Streaming API for XML (StAX) is known to be very efficient. A plug-in event representation can also provide network lookup and dynamic resolution of event type and dynamic sourcing of event instances.

Currently, EsperIO provides the following additional event representations:

  • Apache Axiom: Streaming API for XML (StAX) implementation

Please see the EsperIO documentation for details on the above.

The chapter on Section 17.8, “Event Type And Event Object” explains how to create your own custom event representation.

To summarize, an event is an immutable record of a past occurrence of an action or state change, and event properties contain useful information about an event.

The length of time an event is of interest to the event processing engine (retention time) depends on your EPL statements, and especially the data window, pattern and output rate limiting clauses of your statements.

During the retention time of an event more information about the event may become available, such as additional properties or changes to existing properties. Esper provides three concepts for handling updates to events.

The first means to handle updating events is the update istream clause as further described in Section 5.21, “Updating an Insert Stream: the Update IStream Clause”. It is useful when you need to update events as they enter a stream, before events are evaluated by any particular consuming statement to that stream.

The second means to update events is the on-merge and on-update clauses, for use with named windows only, as further described in Section 5.15.12, “Triggered Upsert using the On-Merge Clause” and Section 5.15.8, “Updating Named Windows: the On Update clause”. On-merge is similar to the SQL merge clause and provides what is known as an "Upsert" operation: Update existing events or if no existing event(s) are found then insert a new event, all in one atomic operation provided by a single EPL statement. On-update can be used to update individual properties of events held in a named window.

The third means to handle updating events is the revision event types, for use with named windows only, as further described in Section 5.15.14, “Versioning and Revision Event Type Use with Named Windows”. With revision event types one can declare, via configuration only, multiple different event types and then have the engine present a merged event type that contains a superset of properties of all merged types, and have the engine merge events as they arrive without additional EPL statements.

Note that patterns do not reflect changes to past events. For the temporal nature of patterns, any changes to events that were observed in the past do not reflect upon current pattern state.

The insert into clause can populate instantiate new instances of Java object events, java.util.Map events and Object[] (object array) events directly from the results of select clause expressions and populate such instances. Simply use the event type name as the stream name in the insert into clause as described in Section 5.10, “Merging Streams and Continuous Insertion: the Insert Into Clause”.

If instead you have an existing instance of a Java object returned by an expression, such as a single-row function or static method invocation for example, you can transpose that expression result object to a stream. This is described further in Section 5.10.7, “Transposing an Expression Result” and Section 9.4, “Select-Clause transpose Function”.

The column names specified in the select and insert into clause must match available writable properties in the event object to be populated (the target event type). The expression result types of any expressions in the select clause must also be compatible with the property types of the target event type.

If populating a POJO-based event type and the class provides a matching constructor, the expression result types of expressions in the select clause must be compatible with the constructor parameters in the order listed by the constructor. The insert into clause column names are not relevant in this case.

Consider the following example statement:

insert into com.mycompany.NewEmployeeEvent 
select fname as firstName, lname as lastName from HRSystemEvent

The above example specifies the fully-qualified class name of NewEmployeeEvent. The engine instantianes NewEmployeeEvent for each result row and populates the firstName and lastName properties of each instance from the result of select clause expressions. The HRSystemEvent in the example is assumed to have lname and fname properties, and either setter-methods and a default constructor, or a matching constructor.

Note how the example uses the as-keyword to assign column names that match the property names of the NewEmployeeEvent target event. If the property names of the source and target events are the same, the as-keyword is not required.

The next example is an alternate form and specifies property names within the insert into clause instead. The example also assumes that NewEmployeeEvent has been defined or imported via configuration since it does not specify the event class package name:

insert into NewEmployeeEvent(firstName, lastName) 
select fname, lname from HRSystemEvent

Finally, this example populates HRSystemEvent events. The example populates the value of a type property where the event has the value 'NEW' and populates a new event object with the value 'HIRED', copying the fname and lname property values to the new event object:

insert into HRSystemEvent 
select fname, lname, 'HIRED' as type from HRSystemEvent(type='NEW')

The matching of the select or insert into-clause column names to target event type's property names is case-sensitive. It is allowed to only populate a subset of all available columns in the target event type. Wildcard (*) is also allowed and copies all fields of the events or multiple events in a join.

For Java object events, your event class must provide setter-methods according to JavaBean conventions or, alternatively, a matching constructor. If the event class provides setter methods the class should also provide a default constructor taking no parameters. If the event class provides a matching constructor there is no need for setter-methods. If your event class does not have a default constructor and setter methods, or a matching constructor, your application may configure a factory method via ConfigurationEventTypeLegacy.

The engine follows Java standards in terms of widening, performing widening automatically in cases where widening type conversion is allowed without loss of precision, for both boxed and primitive types and including BigInteger and BigDecimal.

When inserting array-typed properties into a Java, Map-type or Object-array underlying event the event definition should declare the target property as an array.

Please note the following limitations:

  • Event types that utilize XML org.w3c.dom.Node underlying event objects cannot be target of an insert into clause.

The Esper processing model is continuous: Update listeners and/or subscribers to statements receive updated data as soon as the engine processes events for that statement, according to the statement's choice of event streams, views, filters and output rates.

As outlined in Chapter 14, API Reference the interface for listeners is com.espertech.esper.client.UpdateListener. Implementations must provide a single update method that the engine invokes when results become available:

A second, strongly-typed and native, highly-performant method of result delivery is provided: A subscriber object is a direct binding of query results to a Java object. The object, a POJO, receives statement results via method invocation. The subscriber class need not implement an interface or extend a superclass. Please see Section 14.3.3, “Setting a Subscriber Object”.

The engine provides statement results to update listeners by placing results in com.espertech.esper.client.EventBean instances. A typical listener implementation queries the EventBean instances via getter methods to obtain the statement-generated results.

The get method on the EventBean interface can be used to retrieve result columns by name. The property name supplied to the get method can also be used to query nested, indexed or array properties of object graphs as discussed in more detail in Chapter 2, Event Representations and Section 14.6, “Event and Event Type”

The getUnderlying method on the EventBean interface allows update listeners to obtain the underlying event object. For wildcard selects, the underlying event is the event object that was sent into the engine via the sendEvent method. For joins and select clauses with expressions, the underlying object implements java.util.Map.

A length window instructs the engine to only keep the last N events for a stream. The next statement applies a length window onto the Withdrawal event stream. The statement serves to illustrate the concept of data window and events entering and leaving a data window:

select * from Withdrawal.win:length(5)

The size of this statement's length window is five events. The engine enters all arriving Withdrawal events into the length window. When the length window is full, the oldest Withdrawal event is pushed out the window. The engine indicates to listeners all events entering the window as new events, and all events leaving the window as old events.

While the term insert stream denotes new events arriving, the term remove stream denotes events leaving a data window, or changing aggregation values. In this example, the remove stream is the stream of Withdrawal events that leave the length window, and such events are posted to listeners as old events.

The next diagram illustrates how the length window contents change as events arrive and shows the events posted to an update listener.


As before, all arriving events are posted as new events to listeners. In addition, when event W1 leaves the length window on arrival of event W6, it is posted as an old event to listeners.

Similar to a length window, a time window also keeps the most recent events up to a given time period. A time window of 5 seconds, for example, keeps the last 5 seconds of events. As seconds pass, the time window actively pushes the oldest events out of the window resulting in one or more old events posted to update listeners.

Filters to event streams allow filtering events out of a given stream before events enter a data window. The statement below shows a filter that selects Withdrawal events with an amount value of 200 or more.

select * from Withdrawal(amount>=200).win:length(5)

With the filter, any Withdrawal events that have an amount of less then 200 do not enter the length window and are therefore not passed to update listeners. Filters are discussed in more detail in Section 5.4.1, “Filter-based Event Streams” and Section 6.4, “Filter Expressions In Patterns”.


The where-clause and having-clause in statements eliminate potential result rows at a later stage in processing, after events have been processed into a statement's data window or other views.

The next statement applies a where-clause to Withdrawal events. Where-clauses are discussed in more detail in Section 5.5, “Specifying Search Conditions: the Where Clause”.

select * from Withdrawal.win:length(5) where amount >= 200

The where-clause applies to both new events and old events. As the diagram below shows, arriving events enter the window however only events that pass the where-clause are handed to update listeners. Also, as events leave the data window, only those events that pass the conditions in the where-clause are posted to listeners as old events.


The where-clause can contain complex conditions while event stream filters are more restrictive in the type of filters that can be specified. The next statement's where-clause applies the ceil function of the java.lang.Math Java library class in the where clause. The insert-into clause makes the results of the first statement available to the second statement:

insert into WithdrawalFiltered select * from Withdrawal where Math.ceil(amount) >= 200
select * from WithdrawalFiltered

In this section we explain the output model of statements employing a time window view and a time batch view.

A time window is a moving window extending to the specified time interval into the past based on the system time. Time windows enable us to limit the number of events considered by a query, as do length windows.

As a practical example, consider the need to determine all accounts where the average withdrawal amount per account for the last 4 seconds of withdrawals is greater then 1000. The statement to solve this problem is shown below.

select account, avg(amount) 
from Withdrawal.win:time(4 sec) 
group by account
having amount > 1000

The next diagram serves to illustrate the functioning of a time window. For the diagram, we assume a query that simply selects the event itself and does not group or filter events.

select * from Withdrawal.win:time(4 sec)

The diagram starts at a given time t and displays the contents of the time window at t + 4 and t + 5 seconds and so on.


The activity as illustrated by the diagram:

  1. At time t + 4 seconds an event W1 arrives and enters the time window. The engine reports the new event to update listeners.

  2. At time t + 5 seconds an event W2 arrives and enters the time window. The engine reports the new event to update listeners.

  3. At time t + 6.5 seconds an event W3 arrives and enters the time window. The engine reports the new event to update listeners.

  4. At time t + 8 seconds event W1 leaves the time window. The engine reports the event as an old event to update listeners.

The built-in data windows that act on batches of events are the win:time_batch and the win:length_batch views, among others. The win:time_batch data window collects events arriving during a given time interval and posts collected events as a batch to listeners at the end of the time interval. The win:length_batch data window collects a given number of events and posts collected events as a batch to listeners when the given number of events has collected.

Related to batch data windows is output rate limiting. While batch data windows retain events the output clause offered by output rate limiting can control or stabilize the rate at which events are output, see Section 5.7, “Stabilizing and Controlling Output: the Output Clause”.

Let's look at how a time batch window may be used:

select account, amount from Withdrawal.win:time_batch(1 sec)

The above statement collects events arriving during a one-second interval, at the end of which the engine posts the collected events as new events (insert stream) to each listener. The engine posts the events collected during the prior batch as old events (remove stream). The engine starts posting events to listeners one second after it receives the first event and thereon.

For statements containing aggregation functions and/or a group by clause, the engine posts consolidated aggregation results for an event batch. For example, consider the following statement:

select sum(amount) as mysum from Withdrawal.win:time_batch(1 sec)

Note that output rate limiting also generates batches of events following the output model as discussed here.

Following SQL (Standard Query Language) standards for queries against relational databases, the presence or absence of aggregation functions and the presence or absence of the group by clause dictates the number of rows posted by the engine to listeners. The next sections outline the output model for batched events under aggregation and grouping. The examples also apply to data windows that don't batch events and post results continously as events arrive or leave data windows. The examples also apply to patterns providing events when a complete pattern matches.

In summary, as in SQL, if your query only selects aggregation values, the engine provides one row of aggregated values. It provides that row every time the aggregation is updated (insert stream), which is when events arrive or a batch of events gets processed, and when the events leave a data window or a new batch of events arrives. The remove stream then consists of prior aggregation values.

Also as in SQL, if your query selects non-aggregated values along with aggregation values in the select clause, the engine provides a row per event. The insert stream then consists of the aggregation values at the time the event arrives, while the remove stream is the aggregation value at the time the event leaves a data window, if any is defined in your query.

The documentation provides output examples for query types in Appendix A, Output Reference and Samples, and the next sections outlines each query type.

An event sent by your application or generated by statements is visible to all other statements in the same engine instance. Similarly, current time (the time horizon) moves forward for all statements in the same engine instance. Please see the Chapter 14, API Reference chapter for how to send events and how time moves forward through system time or via simulated time, and the possible threading models.

Within an Esper engine instance you can additionally control event visibility and current time on a statement level, under the term isolated service as described in Section 14.10, “Service Isolation”.

An isolated service provides a dedicated execution environment for one or more statements. Events sent to an isolated service are visible only within that isolated service. In the isolated service you can move time forward at the pace and resolution desired without impacting other statements that reside in the engine runtime or other isolated services. You can move statements between the engine and an isolated service.

This section discusses the notion of context and its role in the Esper event processing language (EPL).

When you look up the word context in a dictionary, you may find: Context is the set of circumstances or facts that surround a particular event, situation, etc..

Context-dependent event processing occurs frequently: For example, consider a requirement that monitors banking transactions. For different customers your analysis considers customer-specific aggregations, patterns or data windows. In this example the context of detection is the customer. For a given customer you may want to analyze the banking transactions of that customer by using aggregations, data windows, patterns including other EPL constructs.

In a second example, consider traffic monitoring to detect speed violations. Assume the speed limit must be enforced only between 9 am and 5 pm. The context of detection is of temporal nature.

A context takes a cloud of events and classifies them into one or more sets. These sets are called context partitions. An event processing operation that is associated with a context operates on each of these context partitions independently. (Credit: Taken from the book "Event Processing in Action" by Opher Etzion and Peter Niblett.)

A context is a declaration of dimension and may thus result in one or more context partitions. In the banking transaction example there the context dimension is the customer and a context partition exists per customer. In the traffic monitoring example there is a single context partition that exists only between 9 am and 5 pm and does not exist outside of that daily time period.

In an event processing glossary you may find the term event processing agent. An EPL statement is an event processing agent. An alternative term for context partition is event processing agent instance.

Esper EPL allows you to declare contexts explicitly, offering the following benefits:

Esper EPL allows you to declare a context explicitly via the create context syntax introduced below.

After you have declared a context, one or more EPL statements can refer to that context by specifying context name. When an EPL statement refers to a context, all EPL-statement related state such as aggregations, patterns or data windows etc. exists once per context partition.

If an EPL statement does not declare a context, it implicitly has a single context partition. The single context partition lives as long as the EPL statement is started and ends when the EPL statement is stopped.

Variables are global state and are visible across context partitions. The same is true for event types and external data.

For more information on locking and threading please see Section 14.7, “Engine Threading and Concurrency”. For performance related information please refer to Chapter 20, Performance.

The create context statement declares a context by specifying a context name and context dimension information.

A context declaration by itself does not consume any resources or perform any logic until your application starts at least one statement that refers to that context. Until then the context is inactive and not in use.

When your application creates or starts the first statement that refers to the context, the engine activates the context.

As soon as your application stops or destroys all statements that refer to the context, the context becomes inactive again.

When your application stops or destroys a statement that refers to a context, the context partitions associated to that statement also end (context partitions associated to other started statements live on).

When your application stops or destroys the statement that declared the context and does not also stop or destroy any statements that refer to the context, the context partitions associated to each such statement do not end.

When your application destroys the statement that declared the context and destroys all statements that refer to that context then the engine removes the context declaration entirely.

The create context statement posts no output events to listeners or subscribers and does not return any rows when iterated.

This context assigns events to context partitions based on the values of one or more event properties, using the value of these property(s) as a key that picks a unique context partition directly. Each event thus belongs to exactly one context partition or zero context partitions if the event does not match the optional filter predicate expression(s). Each context partition handles exactly one set of key values.

The syntax for creating a keyed segmented context is as follows:

create context context_name partition [by]
  event_property [and event_property [and ...]] from stream_def
  [, event_property [...] from stream_def]
  [, ...]

The context_name you assign to the context can be any identifier.

Following the context name is one or more lists of event properties and a stream definition for each entry, separated by comma (,).

The event_property is the name(s) of the event properties that provide the value(s) to pick a unique partition. Multiple event property names are separated by the and keyword.

The stream_def is a stream definition which consists of an event type name optionally followed by parenthesis that contains filter expressions. If providing filter expressions, only events matching the provided filter expressions for that event type are considered by context partitions. The name of a named window is not allowed.

You may list multiple event properties for each stream definition. You may list multiple stream definitions. Please refer to usage guidelines below when specifying multiple event properties and/or multiple stream definitions.

The next statement creates a context SegmentedByCustomer that considers the value of the custId property of the BankTxn event type to pick the context partition to assign events to:

create context SegmentedByCustomer partition by custId from BankTxn

The following statement refers to the context created as above to compute a total withdrawal amount per account for each customer:

context SegmentedByCustomer
select custId, account, sum(amount) from BankTxn group by account

The following statement refers to the context created as above and detects a withdrawal of more then 400 followed by a second withdrawal of more then 400 that occur within 10 minutes of the first withdrawal, all for the same customer:

context SegmentedByCustomer
select * from pattern [
  every a=BankTxn(amount > 400) -> b=BankTxn(amount > 400) where timer:within(10 minutes)
]

The EPL statement that refers to a keyed segmented context must have at least one filter expression, at any place within the EPL statement that looks for events of any of the event types listed in the context declaration.

For example, the following is not valid:

// Neither LoginEvent nor LogoutEvent are listed in the context declaration
context SegmentedByCustomer
select * from pattern [every a=LoginEvent -> b=LogoutEvent where timer:within(10 minutes)]

If the context declaration lists multiple streams, each event type must be unrelated: You may not list the same event type twice and you may not list a sub- or super-type of any event type already listed.

The following is not a valid declaration since the BankTxn event type is listed twice:

// Not valid
create context SegmentedByCustomer partition by custId from BankTxn, account from BankTxn

If the context declaration lists multiple streams, the number of event properties provided for each event type must also be the same. The value type returned by event properties of each event type must match within the respective position it is listed in, i.e. the first property listed for each event type must have the same type, the second property listed for each event type must have the same type, and so on.

The following is not a valid declaration since the customer id of BankTxn and login time of LoginEvent is not the same type:

// Invalid: Type mismatch between properties
create context SegmentedByCustomer partition by custId from BankTxn, loginTime from LoginEvent

The next statement creates a context SegmentedByCustomer that also considers LoginEvent and LogoutEvent:

create context SegmentedByCustomer partition by 
  custId from BankTxn, loginId from LoginEvent, loginId from LogoutEvent

As you may have noticed, the above example refers to loginId as the event property name for LoginEvent and LogoutEvent events. The assumption is that the loginId event property of the login and logout events has the same type and carries the same exact value as the custId of bank transaction events, thereby allowing all events of the three event types to apply to the same customer-specific context partition.

This context assigns events to context partitions based on result of a hash function and modulo operation. Each event thus belongs to exactly one context partition or zero context partitions if the event does not match the optional filter predicate expression(s). Each context partition handles exactly one result of hash value modulo granularity.

The syntax for creating a hashed segmented context is as follows:

create context context_name coalesce [by] 
  hash_func_name(hash_func_param) from stream_def
  [, hash_func_name(hash_func_param) from stream_def ]
  [, ...]
  granularity granularity_value
  [preallocate]

The context_name you assign to the context can be any identifier.

Following the context name is one or more lists of hash function name and parameters pairs and a stream definition for each entry, separated by comma (,).

The hash_func_name can either be consistent_hash_crc32 or hash_code or a plug-in single-row function. The hash_func_param is a list of parameter expressions.

The stream_def is a stream definition which consists of an event type name optionally followed by parenthesis that contains filter expressions. If providing filter expressions, only events matching the provided filter expressions for that event type are considered by context partitions. The name of a named window is not allowed.

You may list multiple stream definitions. Please refer to usage guidelines below when specifying multiple stream definitions.

The granularity is required and is an integer number that defines the maximum number of context partitions. The engine computes hash code modulo granularity hash(params) mod granularity to determine the context partition. When you specify the hash_code function the engine uses the object hash code and the computation is params.hashCode() %granularity.

Since the engine locks on the level of context partition to protect state, the granularity defines the maximum degree of parallelism. For example, a granularity of 1024 means that 1024 context partitions handle events and thus a maximum 1024 threads can process each assigned statement concurrently.

The optional preallocate keyword instructs the engine to allocate all context partitions at once at the time a statement refers to the context. This is beneficial for performance as the engine does not need to determine whether a context partition exists and dynamically allocate, but may require more memory.

The next statement creates a context SegmentedByCustomerHash that considers the CRC-32 hash code of the custId property of the BankTxn event type to pick the context partition to assign events to, with up to 16 different context partitions that are preallocated:

create context SegmentedByCustomerHash
  coalesce by consistent_hash_crc32(custId) from BankTxn granularity 16 preallocate

The following statement refers to the context created as above to compute a total withdrawal amount per account for each customer:

context SegmentedByCustomerHash
select custId, account, sum(amount) from BankTxn group by custId, account

Note that the statement above groups by custId: Since the events for different customer ids can be assigned to the same context partition, it is necessary that the EPL statement also groups by customer id.

The context declaration shown next assumes that the application provides a computeHash single-row function that accepts BankTxn as a parameter, wherein the result of this function must be an integer value that returns the context partition id for each event:

create context MyHashContext
  coalesce by computeHash(*) from BankTxn granularity 16 preallocate

The EPL statement that refers to a hash segmented context must have at least one filter expression, at any place within the EPL statement that looks for events of any of the event types listed in the context declaration.

For example, the following is not valid:

// Neither LoginEvent nor LogoutEvent are listed in the context declaration
context SegmentedByCustomerHash
select * from pattern [every a=LoginEvent -> b=LogoutEvent where timer:within(10 minutes)]

This context assigns events to context partitions based on the values of one or more event properties, using a predicate expression(s) to define context partition membership. Each event can thus belong to zero, one or many context partitions depending on the outcome of the predicate expression(s).

The syntax for creating a category segmented context is as follows:

create context context_name
  group [by] group_expression as category_label
  [, group [by] group_expression as category_label]
  [, ...]
  from stream_def

The context_name you assign to the context can be any identifier.

Following the context name is a list of groups separated by the group keyword. The list of group is followed by the from keyword and a stream definition.

The group_expression is an expression that categorizes events. Each group expression must be followed by the as keyword and a category label which can be any identifier.

Group expressions are predicate expression and must return a Boolean true or false when applied to an event. For a given event, any number of the group expressions may return true thus categories can be overlapping.

The stream_def is a stream definition which consists of an event type name optionally followed by parenthesis that contains filter expressions. If providing filter expressions, only events matching the provided filter expressions for that event type are considered by context partitions.

The next statement creates a context CategoryByTemp that consider the value of the temperature property of the SensorEvent event type to pick context partitions to assign events to:

create context CategoryByTemp
  group temp < 65 as cold,
  group temp between 65 and 85 as normal,
  group temp > 85 as large
  from SensorEvent

The following statement simply counts, for each category, the number of events and outputs the category label and count:

context CategoryByTemp select context.label, count(*) from SensorEvent

You may declare a non-overlapping context that exists once or that repeats in a regular fashion as controlled by start and end conditions. The number of context partitions is always either one or zero: Context partitions do not overlap.

The syntax for creating a non-overlapping context is as follows:

create context context_name
  start (@now | start_condition) 
  end end_condition

The context_name you assign to the context can be any identifier.

Following the context name is the start keyword, either @now or a start_condition, the end keyword and an end_condition.

Both the start (if specified) and end condition can be an event filter, a pattern, a crontab or a time period. The syntax of start and end conditions is described in Section 4.2.7, “Context Conditions”.

Once the start condition occurs, the engine no longer observes the start condition and begins observing the end condition. Once the end condition occurs, the engine observes the start condition again. If you specified @now instead of a start condition, the engine begins observing the end condition instead.

If you specified an event filter as the start condition, then the event also counts towards the statement(s) that refer to that context. If you specified a pattern as the start condition, then the events that may constitute the pattern match can also count towards the statement(s) that refer to the context provided that @inclusive and event tags are both specified (see below).

At the time of context activation when your application creates a statement that utilizes the context, the engine checks whether the start and end condition are crontab expressions. The engine evaluates the start and end crontab expressions and determines whether the current time is a time between start and end. If the current time is between start and end times, the engine allocates the context partition and waits for observing the end time. Otherwise the engine waits to observe the start time and does not allocate a context partition.

The built-in context properties that are available are the same as described in Section 4.2.6.2, “Built-In Context Properties”.

The next statement creates a context NineToFive that declares a daily time period that starts at 9 am and ends at 5 pm:

create context NineToFive start (0, 9, *, *, *) end (0, 17, *, *, *)

The following statement outputs speed violations between 9 am and 5 pm, considering a speed of 100 or greater as a violation:

context NineToFive select * from TrafficEvent(speed >= 100)

The example that follows demonstrates the use of an event filter as the start condition and a pattern as the end condition.

The next statement creates a context PowerOutage that starts when the first PowerOutageEvent event arrives and that ends 5 seconds after a subsequent PowerOnEvent arrives:

create context PowerOutage start PowerOutageEvent end pattern [PowerOnEvent -> timer:interval(5)]

The following statement outputs the temperature during a power outage and for 5 seconds after the power comes on:

context PowerOutage select * from TemperatureEvent

To output only the last value when a context partition ends (terminates, expires), please read on to the description of output rate limiting.

The next statement creates a context Every15Minutes that starts immediately and lasts for 15 minutes, repeatedly allocating a new context partition at the end of 15 minute intervals:

create context Every15Minutes start @now end after 15 minutes

Tip

A non-overlapping context with @now is always-on: A context partition is always allocated at any given point in time.

Note

If you specified an event filter or pattern as the end condition for a context partition, and statements that refer to the context specify an event filter or pattern that matches the same conditions, use @Priority to instruct the engine whether the context management or the statement evaluation takes priority (see below for configuring prioritized execution).

For example, if your context declaration looks like this:

create context MyCtx start MyStartEvent end MyEndEvent

And a statement managed by the context is this:

context MyCtx select count(*) as cnt from MyEndEvent output when terminated

By using @Priority(1) for create-context and @Priority(0) for the counting statement the counting statement does not count the last MyEndEvent since context partition management takes priority.

By using @Priority(0) for create-context and @Priority(1) for the counting statement the counting statement will count the last MyEndEvent since the statement evaluation takes priority.

This context initiates a new context partition when an initiating condition occurs, and terminates one or more context partitions when the terminating condition occurs. The engine maintains as many context partitions as the initiating condition fired, and discards context partitions that terminate when the termination condition fires.

The syntax for creating an overlapping context is as follows:

create context context_name
  initiated [by] [distinct (distinct_value_expr [,...])] [@now and] initiating_condition
  terminated [by] terminating_condition

The context_name you assign to the context can be any identifier.

Following the context name is the initiated keyword. After the initiated keyword you can optionally specify the distinct keyword and, within parenthesis, list one or more distinct value expressions. After the initiated keyword you can also specify @now and as explained below.

After the initiated keyword you must specify the initiating condition. It follows the terminated keyword followed by the terminating condition.

Both the initiating and terminating condition can be an event filter, a pattern, a crontab or a time period. The syntax of initiating and terminating conditions is described in Section 4.2.7, “Context Conditions”.

If you specified @now and before the initiating condition then the engine initiates a new context partition immediately. The @now is only allowed in conjunction with initiation conditions that specify a pattern, crontab or time period and not with event filters.

If you specified an event filter for the initiating condition, then the event that initiates a new context partition also counts towards the statement(s) that refer to that context. If you specified a pattern to initiate a new context partition, then the events that may constitute the pattern match can also count towards the statement(s) that refer to the context provided that @inclusive and event tags are both specified (see below).

The next statement creates a context CtxTrainEnter that allocates a new context partition when a train enters a station, and that terminates each context partition 5 minutes after the time the context partition was allocated:

create context CtxTrainEnter
  initiated by TrainEnterEvent as te
  terminated after 5 minutes

The context declared above assigns the stream name te. Thereby the initiating event's properties can be accessed, for example, by specifying context.te.trainId.

The following statement detects when a train enters a station as indicated by a TrainEnterEvent, but does not leave the station within 5 minutes as would be indicated by a matching TrainLeaveEvent:

context CtxTrainEnter
select t1 from pattern [
  t1=TrainEnterEvent -> timer:interval(5 min) and not TrainLeaveEvent(trainId = context.te.trainId)
  ]

Since the TrainEnterEvent that initiates a new context partition also counts towards the statement, the first part of the pattern (the t1=TrainEnterEvent) is satisfied by that initiating event.

The next statement creates a context CtxEachMinute that allocates a new context partition immediately and every 1 minute, and that terminates each context partition 1 minute after the time the context partition was allocated:

create context CtxEachMinute
  initiated @now and pattern [every timer:interval(1 minute)]
  terminated after 1 minutes

The statement above specifies @now to instruct the engine to allocate a new context partition immediately as well as when the pattern fires. Without the @now the engine would only allocate a new context partition when the pattern fires after 1 minute and every minute thereafter.

The following statement averages the temperature, starting anew every 1 minute and outputs the aggregate value continuously:

context CtxEachMinute select avg(temp) from SensorEvent

To output only the last value when a context partition ends (terminates, expires), please read on to the description of output rate limiting.

Note

If you specified an event filter or pattern as the termination condition for a context partition, and statements that refer to the context specify an event filter or pattern that matches the same conditions, use @Priority to instruct the engine whether the context management or the statement evaluation takes priority (see below for configuring prioritized execution). See the note above for more information.

Context start/initiating and end/terminating conditions are for use with overlapping and non-overlapping contexts. Any combination of conditions may be specified.

Use the syntax described here to define the stream that starts/initiates a context partition or that ends/terminates a context partition.

The syntax is:

event_stream_name [(filter_criteria)] [as stream_name]

The event_stream_name is either the name of an event type or name of an event stream populated by an insert into statement. The filter_criteria is optional and consists of a list of expressions filtering the events of the event stream, within parenthesis after the event stream name.

Two examples are:

// A non-overlapping context that starts when MyStartEvent arrives and ends when MyEndEvent arrives
create context MyContext start MyStartEvent end MyEndEvent
// An overlapping context where each MyEvent with level greater zero 
// initiates a new context partition that terminates after 10 seconds
create context MyContext initiated MyEvent(level > 0) terminated after 10 seconds

You may correlate the start/initiating and end/terminating streams by providing a stream name following the as keyword, and by referring to that stream name in the filter criteria of the end condition.

Two examples that correlate the start/initiating and end/terminating condition are:

// A non-overlapping context that starts when MyEvent arrives
// and ends when a matching MyEvent arrives (same id)
create context MyContext 
start MyEvent as myevent
end MyEvent(id=myevent.id)
// An overlapping context where each MyInitEvent initiates a new context partition 
// that terminates when a matching MyTermEvent arrives 
create context MyContext 
initiated by MyInitEvent as e1 
terminated by MyTermEvent(id=e1.id, level <> e1.level)

You can define a pattern that starts/initiates a context partition or that ends/terminates a context partition.

The syntax is:

pattern [pattern_expression] [@inclusive]

The pattern_expression is a pattern at Chapter 6, EPL Reference: Patterns.

Specify @inclusive after the pattern to have those same events that constitute the pattern match also count towards any statements that are associated to the context. You must also provide a tag for each event in a pattern that should be included.

Examples are:

// A non-overlapping context that starts when either StartEventOne or StartEventTwo arrive
// and that ends after 5 seconds.
// Here neither StartEventOne or StartEventTwo count towards any statements
// that are referring to the context.
create context MyContext 
  start pattern [StartEventOne or StartEventTwo] 
  end after 5 seconds
// Same as above. 
// Here both StartEventOne or StartEventTwo do count towards any statements
// that are referring to the context.
create context MyContext 
  start pattern [a=StartEventOne or b=StartEventTwo] @inclusive 
  end after 5 seconds
// An overlapping context where each distinct MyInitEvent initiates a new context 
// and each context partition terminates after 20 seconds
// We use @inclusive to say that the same MyInitEvent that fires the pattern
// also applies to statements that are associated to the context.
create context MyContext
  initiated by pattern [every-distinct(a.id, 20 sec) a=MyInitEvent]@inclusive
  terminated after 20 sec
// An overlapping context where each pattern match initiates a new context 
// and all context partitions terminate when MyTermEvent arrives.
// The MyInitEvent and MyOtherEvent that trigger the pattern are themselves not included 
// in any statements that are associated to the context.
create context MyContext
  initiated by pattern [every MyInitEvent -> MyOtherEvent where timer:within(5)]
  terminated by MyTermEvent

You may correlate the start and end streams by providing tags as part of the pattern, and by referring to the tag name(s) in the filter criteria of the end condition.

An example that correlates the start and end condition is:

// A non-overlapping context that starts when either StartEventOne or StartEventTwo arrive
// and that ends when either a matching EndEventOne or EndEventTwo arrive
create context MyContext 
  start pattern [a=StartEventOne or b=StartEventTwo]@inclusive
  end pattern [EndEventOne(id=a.id) or EndEventTwo(id=b.id)]

Crontab expression are described in Section 6.6.2.2, “timer:at”.

Examples are:

// A non-overlapping context started daily between 9 am to 5 pm
// and not started outside of these hours:
create context NineToFive start (0, 9, *, *, *) end (0, 17, *, *, *)
// An overlapping context where crontab initiates a new context every 1 minute
// and each context partition terminates after 10 seconds:
create context MyContext initiated (*, *, *, *, *) terminated after 10 seconds

You may specify a time period that the engine observes before the condition fires. Time period expressions are described in Section 5.2.1, “Specifying Time Periods”.

The syntax is:

after time_period_expression

Examples are:

// A non-overlapping context started after 10 seconds 
// that ends 1 minute after it starts and that again starts 10 seconds thereafter.
create context NonOverlap10SecFor1Min start after 10 seconds end after 1 minute
// An overlapping context that starts a new context partition every 5 seconds
// and each context partition lasts 1 minute
create context Overlap5SecFor1Min initiated after 5 seconds terminated after 1 minute

A nested context is a context that is composed from two or more contexts.

The syntax for creating a nested context is as follows:

create context context_name
  context nested_context_name [as] nested_context_definition ,
  context nested_context_name [as] nested_context_definition [, ...]

The context_name you assign to the context can be any identifier.

Following the context name is a comma-separated list of nested contexts. For each nested context specify the context keyword followed a nested context name and the nested context declaration. Any of the context declarations as outlined in Section 4.2, “Context Declaration” are allowed for nested contexts. The order of nested context declarations matters as outlined below.

The next statement creates a nested context NineToFiveSegmented that, between 9 am and 5 pm, allocates a new context partition for each customer id:

create context NineToFiveSegmented
  context NineToFive start (0, 9, *, *, *) end (0, 17, *, *, *),
  context SegmentedByCustomer partition by custId from BankTxn

The following statement refers to the nested context to compute a total withdrawal amount per account for each customer but only between 9 am and 5 pm:

context NineToFiveSegmented
select custId, account, sum(amount) from BankTxn group by account

Esper implements nested contexts as a context tree: The context declared first controls the lifecycle of the context(s) declared thereafter. Thereby, in the above example, outside of the 9am-to-5pm time the engine has no memory and consumes no resources in relationship to bank transactions or customer ids.

When combining segmented contexts, the set of context partitions for the nested context effectively is the Cartesian product of the partition sets of the nested segmented contexts.

When combining temporal contexts with other contexts, since temporal contexts may overlap and may terminate, it is important to understand that temporal contexts control the lifecycle of sub-contexts (contexts declared thereafter). The order of declaration of contexts in a nested context can thereby change resource usage and output result.

The next statement creates a context that allocates context partition only when a train enters a station and then for each hash of the tag id of a passenger as indicated by PassengerScanEvent events, and terminates all context partitions after 5 minutes:

create context CtxNestedTrainEnter
  context InitCtx initiated by TrainEnterEvent as te terminated after 5 minutes,
  context HashCtx coalesce by consistent_hash_crc32(tagId) from PassengerScanEvent
    granularity 16 preallocate

In the example above the engine does not start tracking PassengerScanEvent events or hash codes or allocate context partitions until a TrainEnterEvent arrives.

You may use output rate limiting to trigger output when a context partition ends, as further described in Section 5.7, “Stabilizing and Controlling Output: the Output Clause”.

Consider the fixed temporal context: A new context partition gets allocated at the designated start time and the current context partition ends at the designated end time. To trigger output when the context partition ends and before it gets removed, read on.

The same is true for the initiated temporal context: That context starts a new context partition when trigger events arrive or when a pattern matches. Each context partition expires (ends, terminates) after the specified time period passed. To trigger output at the time the context partition expires, read on.

You may use the when terminated syntax with output rate limiting to trigger output when a context partition ends. The following example demonstrates the idea by declaring an initiated temporal context.

The next statement creates a context CtxEachMinute that initiates a new context partition every 1 minute, and that expires each context partition after 5 minutes:

create context CtxEachMinute
initiated by pattern [every timer:interval(1 min)]
terminated after 5 minutes

The following statement computes an ongoing average temperature however only outputs the last value of the average temperature after 5 minutes when a context partition ends:

context CtxEachMinute
select context.id, avg(temp) from SensorEvent output snapshot when terminated

The when terminated syntax can be combined with other output rates.

The next example outputs every 1 minute and also when the context partition ends:

context CtxEachMinute
select context.id, avg(temp) from SensorEvent output snapshot every 1 minute and when terminated

In the case that the end/terminating condition of the context partition is an event or pattern, the context properties contain the information of the tagged events in the pattern or the single event that ended/terminated the context partition.

For example, consider the following context wherein the engine initializes a new context partition for each arriving MyStartEvent event and that terminates a context partition when a matching MyEndEvent arrives:

create context CtxSample
initiated by MyStartEvent as startevent
terminated by MyEndEvent(id = startevent.id) as endevent

The following statement outputs the id property of the initiating and terminating event and only outputs when a context partition ends:

context CtxSample 
select context.startevent.id, context.endevent.id, count(*) from MyEvent
output snapshot when terminated

You may in addition specify a termination expression that the engine evaluates when a context partition terminates. Only when the terminaton expression evaluates to true does output occur. The expression may refer to built-in properties as described in Section 5.7.1.1, “Controlling Output Using an Expression”. The syntax is as follows:

...output when terminated and termination_expression

The next example statement outputs when a context partition ends but only if at least two events are available for output:

context CtxEachMinute
select * from SensorEvent output when terminated and count_insert >= 2

The final example EPL outputs when a context partition ends and sets the variable myvar to a new value:

context CtxEachMinute
select * from SensorEvent output when terminated then set myvar=3

Named windows are globally-visible data window that may be referred to by multiple statements. You may refer to named windows in statements that declare a context without any special considerations.

You may also create a named window and declare a context for the named window. In this case the engine in effect manages separate named windows, one for each context partition. Limitations apply in this case that we discuss herein.

For example, consider the 9 am to 5 pm fixed temoral context as shown earlier:

create context NineToFive start (0, 9, *, *, *) end (0, 17, *, *, *)

You may create a named window that only exists between 9 am and 5 pm:

context NineToFive create window SpeedingEvents1Hour.win:time(30 min) as TrafficEvent

You can insert into the named window:

insert into SpeedingEvents1Hour select * from TrafficEvent(speed > 100)

Any on-merge, on-select, on-update and on-delete statements must however declare the same context.

The following is not a valid statement as it does not declare the same context that was used to declare the named window:

// You must declare the same context for on-trigger statements
on TruncateEvent delete from SpeedingEvents1Hour

The following is valid:

context NineToFive on TruncateEvent delete from SpeedingEvents1Hour

For context declarations that require specifying event types, such as the hash segmented context and keyed segmented context, please provide the named window underlying event type.

The following sample EPL statements define a type for the named window, declare a context and associate the named window to the context:

create schema ScoreCycle (userId string, keyword string, productId string, score long)
create context HashByUserCtx as 
  coalesce by consistent_hash_crc32(userId) from ScoreCycle granularity 64
context HashByUserCtx create window ScoreCycleWindow.std:unique(productId, keyword) as ScoreCycle

Selecting specific context partitions and interrogating context partitions is useful for:

Esper provides APIs to identify, filter and select context partitions for statement iteration and on-demand queries. The APIs are described in detail at Section 14.19, “Context Partition Selection”.

For statement iteration, your application can provide context selector objects to the iterate and safeIterate methods on EPStatement. If your code does not provide context selectors the iteration considers all context partitions. At the time of iteration, the engine obtains the current set of context partitions and iterates each independently. If your statement has an order-by clause, the order-by clause orders within the context partition and does not order across context partitions.

For on-demand queries, your application can provide context selector objects to the executeQuery method on EPRuntime and to the execute method on EPOnDemandPreparedQuery. If your code does not provide context selectors the on-demand query considers all context partitions. At the time of on-demand query execution, the engine obtains the current set of context partitions and queries each independently. If the on-demand query has an order-by clause, the order-by clause orders within the context partition and does not order across context partitions.

5.1. EPL Introduction
5.2. EPL Syntax
5.2.1. Specifying Time Periods
5.2.2. Using Comments
5.2.3. Reserved Keywords
5.2.4. Escaping Strings
5.2.5. Data Types
5.2.6. Using Constants and Enum Types
5.2.7. Annotation
5.2.8. Expression Declaration
5.2.9. Script Declaration
5.2.10. Referring to a Context
5.3. Choosing Event Properties And Events: the Select Clause
5.3.1. Choosing all event properties: select *
5.3.2. Choosing specific event properties
5.3.3. Expressions
5.3.4. Renaming event properties
5.3.5. Choosing event properties and events in a join
5.3.6. Choosing event properties and events from a pattern
5.3.7. Selecting insert and remove stream events
5.3.8. Qualifying property names and stream names
5.3.9. Select Distinct
5.3.10. Transposing an Expression Result to a Stream
5.3.11. Selecting EventBean instead of Underlying Event
5.4. Specifying Event Streams: the From Clause
5.4.1. Filter-based Event Streams
5.4.2. Pattern-based Event Streams
5.4.3. Specifying Views
5.4.4. Multiple Data Window Views
5.4.5. Using the Stream Name
5.5. Specifying Search Conditions: the Where Clause
5.6. Aggregates and grouping: the Group-by Clause and the Having Clause
5.6.1. Using aggregate functions
5.6.2. Organizing statement results into groups: the Group-by clause
5.6.3. Selecting groups of events: the Having clause
5.6.4. How the stream filter, Where, Group By and Having clauses interact
5.6.5. Comparing Keyed Segmented Context, the Group By clause and the std:groupwin view
5.7. Stabilizing and Controlling Output: the Output Clause
5.7.1. Output Clause Options
5.7.2. Aggregation, Group By, Having and Output clause interaction
5.7.3. Runtime Considerations
5.8. Sorting Output: the Order By Clause
5.9. Limiting Row Count: the Limit Clause
5.10. Merging Streams and Continuous Insertion: the Insert Into Clause
5.10.1. Transposing a Property To a Stream
5.10.2. Merging Streams By Event Type
5.10.3. Merging Disparate Types of Events: Variant Streams
5.10.4. Decorated Events
5.10.5. Event as a Property
5.10.6. Instantiating and Populating an Underlying Event Object
5.10.7. Transposing an Expression Result
5.10.8. Select-Clause Expression And Inserted-Into Column Event Type
5.11. Subqueries
5.11.1. The 'exists' Keyword
5.11.2. The 'in' and 'not in' Keywords
5.11.3. The 'any' and 'some' Keywords
5.11.4. The 'all' Keyword
5.11.5. Subquery With Group By Clause
5.11.6. Multi-Column Selection
5.11.7. Multi-Row Selection
5.11.8. Hints Related to Subqueries
5.12. Joining Event Streams
5.12.1. Introducing Joins
5.12.2. Inner (Default) Joins
5.12.3. Outer, Left and Right Joins
5.12.4. Unidirectional Joins
5.12.5. Hints Related to Joins
5.13. Accessing Relational Data via SQL
5.13.1. Joining SQL Query Results
5.13.2. SQL Query and the EPL Where Clause
5.13.3. Outer Joins With SQL Queries
5.13.4. Using Patterns to Request (Poll) Data
5.13.5. Polling SQL Queries via Iterator
5.13.6. JDBC Implementation Overview
5.13.7. Oracle Drivers and No-Metadata Workaround
5.13.8. SQL Input Parameter and Column Output Conversion
5.13.9. SQL Row POJO Conversion
5.14. Accessing Non-Relational Data via Method Invocation
5.14.1. Joining Method Invocation Results
5.14.2. Polling Method Invocation Results via Iterator
5.14.3. Providing the Method
5.14.4. Using a Map Return Type
5.15. Creating and Using Named Windows
5.15.1. Creating Named Windows: the Create Window clause
5.15.2. Inserting Into Named Windows
5.15.3. Inserting Into Named Windows Using Fire-And-Forget Queries
5.15.4. Selecting From Named Windows
5.15.5. Triggered Select on Named Windows: the On Select clause
5.15.6. Triggered Select+Delete on Named Windows: the On Select Delete clause
5.15.7. Populating a Named Window from an Existing Named Window
5.15.8. Updating Named Windows: the On Update clause
5.15.9. Updating Named Windows Using Fire-And-Forget Queries
5.15.10. Deleting From Named Windows: the On Delete clause
5.15.11. Deleting From Named Windows Using Fire-And-Forget Queries
5.15.12. Triggered Upsert using the On-Merge Clause
5.15.13. Explicitly Indexing Named Windows
5.15.14. Versioning and Revision Event Type Use with Named Windows
5.16. Declaring an Event Type: Create Schema
5.16.1. Declare an Event Type by Providing Names and Types
5.16.2. Declare an Event Type by Providing a Class Name
5.16.3. Declare a Variant Stream
5.17. Splitting and Duplicating Streams
5.18. Variables and Constants
5.18.1. Creating Variables: the Create Variable clause
5.18.2. Setting Variable Values: the On Set clause
5.18.3. Using Variables
5.18.4. Object-Type Variables
5.18.5. Class and Event-Type Variables
5.19. Declaring Global Expressions And Scripts: Create Expression
5.19.1. Declaring a Global Expression
5.19.2. Declaring a Global Script
5.20. Contained-Event Selection
5.20.1. Select-Clause in a Contained-Event Selection
5.20.2. Where Clause in a Contained-Event Selection
5.20.3. Contained-Event Selection and Joins
5.20.4. Sentence and Word Example
5.20.5. More Examples
5.20.6. Contained-Event Limitations
5.21. Updating an Insert Stream: the Update IStream Clause
5.21.1. Immutability and Updates
5.22. Controlling Event Delivery : The For Clause

The Event Processing Language (EPL) is a SQL-like language with SELECT, FROM, WHERE, GROUP BY, HAVING and ORDER BY clauses. Streams replace tables as the source of data with events replacing rows as the basic unit of data. Since events are composed of data, the SQL concepts of correlation through joins, filtering and aggregation through grouping can be effectively leveraged.

The INSERT INTO clause is recast as a means of forwarding events to other streams for further downstream processing. External data accessible through JDBC may be queried and joined with the stream data. Additional clauses such as the PATTERN and OUTPUT clauses are also available to provide the missing SQL language constructs specific to event processing.

The purpose of the UPDATE clause is to update event properties. Update takes place before an event applies to any selecting statements or pattern statements.

EPL statements are used to derive and aggregate information from one or more streams of events, and to join or merge event streams. This section outlines EPL syntax. It also outlines the built-in views, which are the building blocks for deriving and aggregating information from event streams.

EPL statements contain definitions of one or more views. Similar to tables in a SQL statement, views define the data available for querying and filtering. Some views represent windows over a stream of events. Other views derive statistics from event properties, group events or handle unique event property values. Views can be staggered onto each other to build a chain of views. The Esper engine makes sure that views are reused among EPL statements for efficiency.

The built-in set of views is:

EPL provides the concept of named window. Named windows are data windows that can be inserted-into and deleted-from by one or more statements, and that can queried by one or more statements. Named windows have a global character, being visible and shared across an engine instance beyond a single statement. Use the CREATE WINDOW clause to create named windows. Use the ON MERGE clause to atomically merge events into named window state, the INSERT INTO clause to insert data into a named window, the ON DELETE clause to remove events from a named window, the ON UPDATE clause to update events held by a named window and the ON SELECT clause to perform a query triggered by a pattern or arriving event on a named window. Finally, the name of the named window can occur in a statement's FROM clause to query a named window or include the named window in a join or subquery.

EPL allows execution of on-demand (fire-and-forget, non-continuous, triggered by API) queries against named windows through the runtime API. The query engine automatically indexes named window data for fast access by ON SELECT/UPDATE/INSERT/DELETE without the need to create an index explicitly. For fast on-demand query execution via runtime API use the CREATE INDEX syntax to create an explicit index.

Use CREATE SCHEMA to declare an event type.

Variables can come in handy to parameterize statements and change parameters on-the-fly and in response to events. Variables can be used in an expression anywhere in a statement as well as in the output clause for dynamic control of output rates.

Esper can be extended by plugging-in custom developed views and aggregation functions.

EPL queries are created and stored in the engine, and publish results to listeners as events are received by the engine or timer events occur that match the criteria specified in the query. Events can also be obtained from running EPL queries via the safeIterator and iterator methods that provide a pull-data API.

The select clause in an EPL query specifies the event properties or events to retrieve. The from clause in an EPL query specifies the event stream definitions and stream names to use. The where clause in an EPL query specifies search conditions that specify which event or event combination to search for. For example, the following statement returns the average price for IBM stock ticks in the last 30 seconds.

select avg(price) from StockTick.win:time(30 sec) where symbol='IBM'

EPL queries follow the below syntax. EPL queries can be simple queries or more complex queries. A simple select contains only a select clause and a single stream definition. Complex EPL queries can be build that feature a more elaborate select list utilizing expressions, may join multiple streams, may contain a where clause with search conditions and so on.

[annotations]
[expression_declarations]
[context context_name]
[insert into insert_into_def]
select select_list
from stream_def [as name] [, stream_def [as name]] [,...]
[where search_conditions]
[group by grouping_expression_list]
[having grouping_search_conditions]
[output output_specification]
[order by order_by_expression_list]
[limit num_rows]

Certain words such as select, delete or set are reserved and may not be used as identifiers. Please consult Appendix B, Reserved Keywords for the list of reserved keywords and permitted keywords.

Names of built-in functions and certain auxiliary keywords are permitted as event property names and in the rename syntax of the select clause. For example, count is acceptable.

Consider the example below, which assumes that 'last' is an event property of MyEvent:

// valid
select last, count(*) as count from MyEvent

This example shows an incorrect use of a reserved keyword:

// invalid
select insert from MyEvent

EPL offers an escape syntax for reserved keywords: Event properties as well as event or stream names may be escaped via the backwards apostrophe ` (ASCII 96) character.

The next example queries an event type by name Order (a reserved keyword) that provides a property by name insert (a reserved keyword):

// valid
select `insert` from `Order`

You may surround string values by either double-quotes (") or single-quotes ('). When your string constant in an EPL statement itself contains double quotes or single quotes, you must escape the quotes.

Double and single quotes may be escaped by the backslash (\) character or by unicode notation. Unicode 0027 is a single quote (') and 0022 is a double quote (").

Escaping event property names is described in Section 2.2.1, “Escape Characters”.

The sample EPL below escapes the single quote in the string constant John's, and filters out order events where the name value matches:

select * from OrderEvent(name='John\'s')
// ...equivalent to...
select * from OrderEvent(name='John\u0027s')

The next EPL escapes the string constant Quote "Hello":

select * from OrderEvent(description like "Quote \"Hello\"")
// is equivalent to
select * from OrderEvent(description like "Quote \u0022Hello\u0022")

When building an escape string via the API, escape the backslash, as shown in below code snippet:

epService.getEPAdministrator().createEPL("select * from OrderEvent(name='John\\'s')");
// ... and for double quotes...
epService.getEPAdministrator().createEPL("select * from OrderEvent(
  description like \"Quote \\\"Hello\\\"\")");

EPL honors all Java built-in primitive and boxed types, including java.math.BigInteger and java.math.BigDecimal.

EPL also follows Java standards in terms of widening, performing widening automatically in cases where widening type conversion is allowed without loss of precision, for both boxed and primitive types and including BigInteger and BigDecimal:

  1. byte to short, int, long, float, double, BigInteger or BigDecimal

  2. short to int, long, float, or double, BigInteger or BigDecimal

  3. char to int, long, float, or double, BigInteger or BigDecimal

  4. int to long, float, or double, BigInteger or BigDecimal

  5. long to float or double, BigInteger or BigDecimal

  6. float to double or BigDecimal

  7. double to BigDecimal

In cases where loss of precision is possible because of narrowing requirements, EPL compilation outputs a compilation error.

EPL supports casting via the cast function.

EPL returns double-type values for division regardless of operand type. EPL can also be configured to follow Java rules for integer arithmetic instead as described in Section 15.4.22, “Engine Settings related to Expression Evaluation”.

Division by zero returns positive or negative infinity. Division by zero can be configured to return null instead.

This chapter is about Java language constants and enum types and their use in EPL expressions.

Java language constants are public static final fields in Java that may participate in expressions of all kinds, as this example shows:

select * from MyEvent where property = MyConstantClass.FIELD_VALUE

Event properties that are enumeration values can be compared by their enum type value:

select * from MyEvent where enumProp = EnumClass.ENUM_VALUE_1

Event properties can also be passed to enum type functions or compared to an enum type method result:

select * from MyEvent where somevalue = EnumClass.ENUM_VALUE_1.getSomeValue()
  or EnumClass.ENUM_VALUE_2.analyze(someothervalue)

Enum types have a valueOf method that returns the enum type value:

select * from MyEvent where enumProp = EnumClass.valueOf('ENUM_VALUE_1')

If your application does not import, through configuration, the package that contains the enumeration class, then it must also specify the package name of the class. Enum types that are inner classes must be qualified with $ following Java conventions.

For example, the Color enum type as an inner class to MyEvent in package org.myorg can be referenced as shown:

select * from MyEvent(enumProp=org.myorg.MyEvent$Color.GREEN).std:firstevent()

Instance methods may also be invoked on event instances by specifying a stream name, as shown below:

select myevent.computeSomething() as result from MyEvent as myevent

Chaining instance methods is supported as this example shows:

select myevent.getComputerFor('books', 'movies').calculate() as result 
from MyEvent as myevent

An annotation is an addition made to information in a statement. Esper provides certain built-in annotations for defining statement name, adding a statement description or for tagging statements such as for managing statements or directing statement output. Other then the built-in annotations, applications can provide their own annotation classes that the EPL compiler can populate.

An annotation is part of the statement text and precedes the EPL select or pattern statement. Annotations are therefore part of the EPL grammar. The syntax for annotations follows the host language (Java, .NET) annotation syntax:

@annotation_name [(annotation_parameters)]

An annotation consists of the annotation name and optional annotation parameters. The annotation_name is the simple class name or fully-qualified class name of the annotation class. The optional annotation_parameters are a list of key-value pairs following the syntax:

@annotation_name (attribute_name = attribute_value, [name=value, ...])

The attribute_name is an identifier that must match the attributes defined by the annotation class. An attribute_value is a constant of any of the primitive types or string, an array, an enum type value or another (nested) annotation. Null values are not allowed as annotation attribute values. Enumeration values are supported in EPL statements and not support in statements created via the createPattern method.

Use the getAnnotations method of EPStatement to obtain annotations provided via statement text.

The name of built-in annotations is not case-sensitive, allowing both @NAME or @name, for example.

The list of built-in EPL annotations is:

Table 5.2. Built-In EPL Annotations

NamePurpose and AttributesExample
Name

Provides a statement name. Attributes are:

value : Statement name.

@Name("MyStatementName")
Description

Provides a statement textual description. Attributes are:

value : Statement description.

@Description("Place statement
description here.")
Tag

For tagging a statement with additional information. Attributes are:

name : Tag name.

value : Tag value.

@Tag(name="MyTagName", 
 value="MyTagValue")
Priority

Applicable when an event (or schedule) matches filter criteria for multiple statements: Defines the order of statement processing (requires an engine-level setting).

Attributes are:

value : priority value.

@Priority(10)
Drop

Applicable when an event (or schedule) matches filter criteria for multiple statements, drops the event after processing the statement (requires an engine-level setting).

No attributes.

@Drop
Hint

For providing one or more hints towards how the engine should execute a statement. Attributes are:

value : A comma-separated list of one or more case-insensitive keywords.

@Hint('ITERATE_ONLY')
Hook

Use this annotation to register one or more statement-specific hooks providing a hook type for each individual hook, such as for SQL parameter, column or row conversion.

Attributes are the hook type and the hook itself (usually a import or class name):

@Hook(type=HookType.SQLCOL,
  hook='MyDBTypeConvertor')
Audit

Causes the engine to output detailed processing information for a statement.

optional value : A comma-separated list of one or more case-insensitive keywords.

@Audit
EventRepresentation

Causes the engine to use object-array event representation, if possible, for output and internal event types.

@EventRepresentation(array=true)

The following example statement text specifies some of the built-in annotations in combination:

@Name("RevenuePerCustomer")
@Description("Outputs revenue per customer considering all events encountered so far.")
@Tag(name="grouping", value="customer")

select customerId, sum(revenue) from CustomerRevenueEvent

This annotation only takes effect if the engine-level setting for prioritized execution is set via configuration, as described in Section 15.4.23, “Engine Settings related to Execution of Statements”.

Use the @Priority EPL annotation to tag statements with a priority value. The default priority value is zero (0) for all statements. When an event (or single timer execution) requires processing the event for multiple statements, processing begins with the highest priority statement and ends with the lowest-priority statement.

Example:

@Priority(10) select * from SecurityFilter(ip="127.0.0.1")

This annotation only takes effect if the engine-level setting for prioritized execution is set via configuration, as described in Section 15.4.23, “Engine Settings related to Execution of Statements”.

Use the @Drop EPL annotation to tag statements that preempt all other same or lower-priority statements. When an event (or single timer execution) requires processing the event for multiple statements, processing begins with the highest priority statement and ends with the first statement marked with @Drop, which becomes the last statement to process that event.

Unless a different priority is specified, the statement with the @Drop EPL annotation executes at priority 1. Thereby @Drop alone is an effective means to remove events from a stream.

Example:

@Drop select * from SecurityFilter(ip="127.0.0.1")

An EPL statement can contain expression declarations. Expressions that are common to multiple places in the same EPL statement can be moved to a named expression declaration and reused within the same statement without duplicating the expression itself.

For declaring expressions that are visible across multiple EPL statements i.e. globally visible expressions please consult Section 5.19.1, “Declaring a Global Expression” that explains the create expression clause.

An expression declaration follows the lambda-style expression syntax. This syntax was chosen as it typically allows for a shorter and more concise expression body that can be easier to read then most procedural code.

The syntax for an expression declaration is:

expression expression_name { expression_body }

An expression declaration consists of the expression name and an expression body. The expression_name is any identifier. The expression_body contains optional parameters and the expression. The parameter types and the return type of the expression is determined by the engine and do not need to be specified.

Parameters to a declared expression can be a stream name, pattern tag name or wildcard (*). Use wildcard to pass the event itself to the expression. In a join or subquery, or more generally in an expression where multiple streams or pattern tags are available, the EPL must specify the stream name or pattern tag name and cannot use wildcard.

In the expression body the => lambda operator reads as "goes to". The left side of the lambda operator specifies the input parameters (if any) and the right side holds the expression. The lambda expression x => x * x is read "x goes to x times x".

In the expression body, if your expression takes no parameters, you may simply specify the expression and do not need the => lambda operator.

If your expression takes one parameters, specify the input parameter name followed by the => lambda operator and followed by the expression. The synopsis for use with a single input parameter is:

expression_body:   input_param_name => expression 

If your expression takes two or more parameters, specify the input parameter names in parenthesis followed by the => lambda operator followed by the expression. The synopsis for use with a multiple input parameter is:

expression_body:   (input_param [,input_param [,...]]) => expression 

The following example declares an expression that returns two times PI (ratio of the circumference of a circle to its diameter) and demonstrates its use in a select-clause:

expression twoPI { Math.PI * 2} select twoPI() from SampleEvent

The next example declares an expression that accepts one parameter: a MarketData event. The expression computes a new "mid" price based on the buy and sell price:

expression midPrice { x => (x.buy + x.sell) / 2 } 
select midPrice(md) from MarketDataEvent as md

The variable name can be left off if event property names resolve without ambiguity.

This example EPL removes the variable name x:

expression midPrice { x => (buy + sell) / 2 } 
select midPrice(md) from MarketDataEvent as md

The next example EPL specifies wildcard instead:

expression midPrice { x => (buy + sell) / 2 } 
select midPrice(*) from MarketDataEvent

A further example that demonstrates two parameters is listed next. The example joins two streams and uses the price value from MarketDataEvent and the sentiment value of NewsEvent to compute a weighted sentiment:

expression weightedSentiment { (x, y) => x.price * y.sentiment } 
select weightedSentiment(md, news) 
from MarketDataEvent.std:lastevent() as md, NewsEvent.std:lastevent() news

Any expression can be used in the expression body including aggregations, variables, subqueries or further declared expressions. Sub-queries, when used without in or exists, must be placed within parenthesis.

An example subquery within a declared expression is shown next:

expression newsSubq(md) { 
    (select sentiment from NewsEvent.std:unique(symbol) where symbol = md.symbol) 
} 
select newsSubq(mdstream)
from MarketDataEvent mdstream

When using declared expressions please note these limitations:

  1. Parameters to a declared expression can only be a stream name, pattern tag name or wildcard (*).

The following scope rules apply for declared expressions:

  1. The scope of the expression body of a declared expression only includes the parameters explicitly listed.

You may refer to a context in the EPL text by specifying the context keyword followed by a context name. Context are described in more detail at Chapter 4, Context and Context Partitions

The effect of referring to a context is that your statement operates according to the context dimensional information as declared for the context.

The synopsis is:

... context context_name ...

You may refer to a context in all statements except for the following types of statements:

  1. create schema for declaring event types.

  2. create variable for declaring a variable.

  3. create index for creating an index on a named window.

  4. update istream for updating insert stream events.

The select clause is required in all EPL statements. The select clause can be used to select all properties via the wildcard *, or to specify a list of event properties and expressions. The select clause defines the event type (event property names and types) of the resulting events published by the statement, or pulled from the statement via the iterator methods.

The select clause also offers optional istream, irstream and rstream keywords to control whether input stream, remove stream or input and remove stream events are posted to UpdateListener instances and observers to a statement. By default, the engine provides only the insert stream to listener and observers. See Section 15.4.17, “Engine Settings related to Stream Selection” on how to change the default.

The syntax for the select clause is summarized below.

select [istream | irstream | rstream] [distinct] * | expression_list ... 

The istream keyword is the default, and indicates that the engine only delivers insert stream events to listeners and observers. The irstream keyword indicates that the engine delivers both insert and remove stream. Finally, the rstream keyword tells the engine to deliver only the remove stream.

The distinct keyword outputs only unique rows depending on the column list you have specified after it. It must occur after the select and after the optional stream keywords, as described in more detail below.

The syntax for selecting all event properties in a stream is:

select * from stream_def

The following statement selects StockTick events for the last 30 seconds of IBM stock ticks.

select * from StockTick(symbol='IBM').win:time(30 sec)

You may well be asking: Why does the statement specify a time window here? First, the statement is meant to demonstrate the use of * wildcard. When the engine pushes statement results to your listener and as the statement does not select remove stream events via rstream keyword, the listener receives only new events and the time window could be left off. By adding the time window the pull API (iterator API or JDBC driver) returns the last 30 seconds of events.

The * wildcard and expressions can also be combined in a select clause. The combination selects all event properties and in addition the computed values as specified by any additional expressions that are part of the select clause. Here is an example that selects all properties of stock tick events plus a computed product of price and volume that the statement names 'pricevolume':

select *, price * volume as pricevolume from StockTick

When using wildcard (*), Esper does not actually read or copy your event properties out of your event or events, neither does it copy the event object. It simply wraps your native type in an EventBean interface. Your application has access to the underlying event object through the getUnderlying method and has access to the property values through the get method.

In a join statement, using the select * syntax selects one event property per stream to hold the event for that stream. The property name is the stream name in the from clause.

If your statement is joining multiple streams, your may specify property names that are unique among the joined streams, or use wildcard (*) as explained earlier.

In case the property name in your select or other clauses is not unique considering all joined streams, you will need to use the name of the stream as a prefix to the property.

This example is a join between the two streams StockTick and News, respectively named as 'tick' and 'news'. The example selects from the StockTick event the symbol value using the 'tick' stream name as a prefix:

select tick.symbol from StockTick.win:time(10) as tick, News.win:time(10) as news
where news.symbol = tick.symbol

Use the wildcard (*) selector in a join to generate a property for each stream, with the property value being the event itself. The output events of the statement below have two properties: the 'tick' property holds the StockTick event and the 'news' property holds the News event:

select * from StockTick.win:time(10) as tick, News.win:time(10) as news

The following syntax can also be used to specify what stream's properties to select:

select stream_name.* [as name] from ...

The selection of tick.* selects the StockTick stream events only:

select tick.* from StockTick.win:time(10) as tick, News.win:time(10) as news
where tick.symbol = news.symbol

The next example uses the as keyword to name each stream's joined events. This instructs the engine to create a property for each named event:

select tick.* as stocktick, news.* as news 
from StockTick.win:time(10) as tick, News.win:time(10) as news
where stock.symbol = news.symbol

The output events of the above example have two properties 'stocktick' and 'news' that are the StockTick and News events.

The stream name itself, as further described in Section 5.4.5, “Using the Stream Name”, may be used within expressions or alone.

This example passes events to a user-defined function named compute and also shows insert-into to populate an event stream of combined events:

insert into TickNewStream select tick, news, MyLib.compute(news, tick) as result
from StockTick.win:time(10) as tick, News.win:time(10) as news
where tick.symbol = news.symbol
// second statement that uses the TickNewStream stream
select tick.price, news.text, result from TickNewStream

In summary, the stream_name.* streamname wildcard syntax can be used to select a stream as the underlying event or as a property, but cannot appear within an expression. While the stream_name syntax (without wildcard) always selects a property (and not as an underlying event), and can occur anywhere within an expression.

If your statement employs pattern expressions, then your pattern expression tags events with a tag name. Each tag name becomes available for use as a property in the select clause and all other clauses.

For example, here is a very simple pattern that matches on every StockTick event received within 30 seconds after start of the statement. The sample selects the symbol and price properties of the matching events:

select tick.symbol as symbol, tick.price as price
from pattern[every tick=StockTick where timer:within(10 sec)]

The use of the wildcard selector, as shown in the next statement, creates a property for each tagged event in the output. The next statement outputs events that hold a single 'tick' property whose value is the event itself:

select * from pattern[every tick=StockTick where timer:within(10 sec)]

You may also select the matching event itself using the tick.* syntax. The engine outputs the StockTick event itself to listeners:

select tick.* from pattern[every tick=StockTick where timer:within(10 sec)]

A tag name as specified in a pattern is a valid expression itself. This example uses the insert into clause to make available the events matched by a pattern to further statements:

// make a new stream of ticks and news available
insert into StockTickAndNews 
select tick, news from pattern [every tick=StockTick -> news=News(symbol=tick.symbol)]
// second statement to select from the stream of ticks and news
select tick.symbol, tick.price, news.text from StockTickAndNews

The optional istream, irstream and rstream keywords in the select clause control the event streams posted to listeners and observers to a statement.

If neither keyword is specified, and in the default engine configuration, the engine posts only insert stream events via the newEvents parameter to the update method of UpdateListener instances listening to the statement. The engine does not post remove stream events, by default.

The insert stream consists of the events entering the respective window(s) or stream(s) or aggregations, while the remove stream consists of the events leaving the respective window(s) or the changed aggregation result. See Chapter 3, Processing Model for more information on insert and remove streams.

The engine posts remove stream events to the oldEvents parameter of the update method only if the irstream keyword occurs in the select clause. This behavior can be changed via engine-wide configuration as described in Section 15.4.17, “Engine Settings related to Stream Selection”.

By specifying the istream keyword you can instruct the engine to only post insert stream events via the newEvents parameter to the update method on listeners. The engine will then not post any remove stream events, and the oldEvents parameter is always a null value.

By specifying the irstream keyword you can instruct the engine to post both insert stream and remove stream events.

By specifying the rstream keyword you can instruct the engine to only post remove stream events via the newEvents parameter to the update method on listeners. The engine will then not post any insert stream events, and the oldEvents parameter is also always a null value.

The following statement selects only the events that are leaving the 30 second time window.

select rstream * from StockTick.win:time(30 sec)

The istream and rstream keywords in the select clause are matched by same-name keywords available in the insert into clause. While the keywords in the select clause control the event stream posted to listeners to the statement, the same keywords in the insert into clause specify the event stream that the engine makes available to other statements.

The optional distinct keyword removes duplicate output events from output. The keyword must occur after the select keyword and after the optional irstream keyword.

The distinct keyword in your select instructs the engine to consolidate, at time of output, the output event(s) and remove output events with identical property values. Duplicate removal only takes place when two or more events are output together at any one time, therefore distinct is typically used with a batch data window, output rate limiting, on-demand queries, on-select or iterator pull API.

If two or more output event objects have same property values for all properties of the event, the distinct removes all but one duplicated event before outputting events to listeners. Indexed, nested and mapped properties are considered in the comparison, if present in the output event.

The next example outputs sensor ids of temperature sensor events, but only every 10 seconds and only unique sensor id values during the 10 seconds:

select distinct sensorId from TemperatureSensorEvent output every 10 seconds

Use distinct with wildcard (*) to remove duplicate output events considering all properties of an event.

This example statement outputs all distinct events either when 100 events arrive or when 10 seconds passed, whichever occurs first:

select distinct * from TemperatureSensorEvent.win:time_length_batch(10, 100)

When selecting nested, indexed, mapped or dynamic properties in a select clause with distinct, it is relevant to know that the comparison uses hash code and the Java equals semantics.

By default, for certain select-clause expressions that output events or a collection of events, the engine outputs the underlying event objects. With outputs we refer to the data passed to listeners, subscribers and inserted-into into another stream via insert-into.

The select-clause expressions for which underlying event objects are output by default are:

To have the engine output EventBean instance(s) instead, add @eventbean to the relevant expressions of the select-clause.

The sample EPL shown below outputs current data window contents as EventBean instances into the stream OutStream, thereby statements consuming the stream may operate on such instances:

insert into OutStream 
select prevwindow(s0) @eventbean as win 
from MyEvent.win:length(2) as s0

The next EPL consumes the stream and selects the last event:

select win.lastOf() from OutStream

It is not necessary to use @eventbean if an event type by the same name (OutStream in the example) is already declared and a property exist on the type by the same name (win in this example) and the type of the property is the event type (MyEvent in the example) returned by the expression. This is further described in Section 5.10.8, “Select-Clause Expression And Inserted-Into Column Event Type”.

The from clause is required in all EPL statements. It specifies one or more event streams or named windows. Each event stream or named window can optionally be given a name by means of the as keyword.

from stream_def [as name] [unidirectional] [retain-union | retain-intersection] 
    [, stream_def [as stream_name]] [, ...]

The event stream definition stream_def as shown in the syntax above can consists of either a filter-based event stream definition or a pattern-based event stream definition.

For joins and outer joins, specify two or more event streams. Joins between pattern-based and filter-based event streams are also supported. Joins and the unidirectional keyword are described in more detail in Section 5.12, “Joining Event Streams”.

Esper supports joins against relational databases for access to historical or reference data as explained in Section 5.13, “Accessing Relational Data via SQL”. Esper can also join results returned by an arbitrary method invocation, as discussed in Section 5.14, “Accessing Non-Relational Data via Method Invocation”.

The stream_name is an optional identifier assigned to the stream. The stream name can itself occur in any expression and provides access to the event itself from the named stream. Also, a stream name may be combined with a method name to invoke instance methods on events of that stream.

For all streams with the exception of historical sources your query may employ data window views as outlined below. The retain-intersection (the default) and retain-union keywords build a union or intersection of two or more data windows as described in Section 5.4.4, “Multiple Data Window Views”.

The stream_def syntax for a filter-based event stream is as below:

event_stream_name [(filter_criteria)] [contained_selection] [.view_spec] [.view_spec] [...]

The event_stream_name is either the name of an event type or name of an event stream populated by an insert into statement or the name of a named window.

The filter_criteria is optional and consists of a list of expressions filtering the events of the event stream, within parenthesis after the event stream name.

The contained_selection is optional and is for use with coarse-grained events that have properties that are themselves one or more events, see Section 5.20, “Contained-Event Selection” for the synopsis and examples.

The view_spec are optional view specifications, which are combinable definitions for retaining events and for deriving information from events.

The following EPL statement shows event type, filter criteria and views combined in one statement. It selects all event properties for the last 100 events of IBM stock ticks for volume. In the example, the event type is the fully qualified Java class name org.esper.example.StockTick. The expression filters for events where the property symbol has a value of "IBM". The optional view specifications for deriving data from the StockTick events are a length window and a view for computing statistics on volume. The name for the event stream is "volumeStats".

select * from 
  org.esper.example.StockTick(symbol='IBM').win:length(100).stat:uni(volume) as volumeStats

Esper filters out events in an event stream as defined by filter criteria before it sends events to subsequent views. Thus, compared to search conditions in a where clause, filter criteria remove unneeded events early. In the above example, events with a symbol other then IBM do not enter the time window.

The filtering criteria to filter for events with certain event property values are placed within parenthesis after the event type name:

select * from RfidEvent(category="Perishable")

All expressions can be used in filters, including static methods that return a boolean value:

select * from com.mycompany.RfidEvent(MyRFIDLib.isInRange(x, y) or (x < 0 and y < 0))

Filter expressions can be separated via a single comma ','. The comma represents a logical AND between filter expressions:

select * from RfidEvent(zone=1, category=10)
...is equivalent to...
select * from RfidEvent(zone=1 and category=10)

The following operators are highly optimized through indexing and are the preferred means of filtering in high-volume event streams and especially in the presence of a larger number of filters or statements:

At compile time as well as at run time, the engine scans new filter expressions for sub-expressions that can be indexed. Indexing filter values to match event properties of incoming events enables the engine to match incoming events faster, especially if your application creates a large number of statements or requires many similar filters. The above list of operators represents the set of operators that the engine can best convert into indexes. The use of comma or logical and in filter expressions does not impact optimizations by the engine.

Event pattern expressions can also be used to specify one or more event streams in an EPL statement. For pattern-based event streams, the event stream definition stream_def consists of the keyword pattern and a pattern expression in brackets []. The syntax for an event stream definition using a pattern expression is below. As in filter-based event streams, an optional list of views that derive data from the stream can be supplied.

pattern [pattern_expression] [.view_spec] [.view_spec] [...]

The next statement specifies an event stream that consists of both stock tick events and trade events. The example tags stock tick events with the name "tick" and trade events with the name "trade".

select * from pattern [every tick=StockTickEvent or every trade=TradeEvent]

This statement generates an event every time the engine receives either one of the event types. The generated events resemble a map with "tick" and "trade" keys. For stock tick events, the "tick" key value is the underlying stock tick event, and the "trade" key value is a null value. For trade events, the "trade" key value is the underlying trade event, and the "tick" key value is a null value.

Lets further refine this statement adding a view the gives us the last 30 seconds of either stock tick or trade events. Lets also select prices and a price total.

select tick.price as tickPrice, trade.price as tradePrice, 
       sum(tick.price) + sum(trade.price) as total
  from pattern [every tick=StockTickEvent or every trade=TradeEvent].win:time(30 sec)

Note that in the statement above tickPrice and tradePrice can each be null values depending on the event processed. Therefore, an aggregation function such as sum(tick.price + trade.price)) would always return null values as either of the two price properties are always a null value for any event matching the pattern. Use the coalesce function to handle null values, for example: sum(coalesce(tick.price, 0) + coalesce(trade.price, 0)).

Views are used to specify an expiry policy for events (data window views) and also to derive data. Views can be staggered onto each other. See the section Chapter 12, EPL Reference: Views on the views available that also outlines the different types of views: Data Window views and Derived-Value views.

Views can optionally take one or more parameters. These parameters are expressions themselves that may consist of any combination of variables, arithmetic, user-defined function or substitution parameters for prepared statements, for example.

The example statement below outputs a count per expressway for car location events (contains information about the location of a car on a highway) of the last 60 seconds:

select expressway, count(*) from CarLocEvent.win:time(60) 
group by expressway

The next example serves to show staggering of views. It uses the std:groupwin view to create a separate length window per car id:

select cardId, expressway, direction, segment, count(*) 
from CarLocEvent.std:groupwin(carId).win:length(4) 
group by carId, expressway, direction, segment

The first view std:groupwin(carId) groups car location events by car id. The second view win:length(4) keeps a length window of the 4 last events, with one separate length window for each car id. The example reports the number of events per car id and per expressway, direction and segment considering the last 4 events for each car id only.

Note that the group by syntax is generally preferable over std:groupwin for grouping information as it is SQL-compliant, easier to read and does not create a separate data window per group. The std:groupwin in above example creates a separate data window (length window in the example) per group, demonstrating staggering views.

When views are staggered onto each other as a chain of views, then the insert and remove stream received by each view is the insert and remove stream made available by the view (or stream) earlier in the chain.

The special keep-all view keeps all events: It does not provide a remove stream, i.e. events are not removed from the keep-all view unless by means of the on-delete syntax or by revision events.

Data window views provide an expiry policy that indicates when to remove events from the data window, with the exception of the keep-all data window which has no expiry policy and the std:groupwin grouped-window view for allocating a new data window per group.

EPL allows the freedom to use multiple data window views onto a stream and thus combine expiry policies. Combining data windows into an intersection (the default) or a union can achieve a useful strategy for retaining events and expiring events that are no longer of interest. Named windows and the on-delete syntax provide an additional degree of freedom.

In order to combine two or more data window views there is no keyword required. The retain-intersection keyword is the default and the retain-union keyword may instead be provided for a stream.

The concept of union and intersection come from Set mathematics. In the language of Set mathematics, two sets A and B can be "added" together: The intersection of A and B is the set of all things which are members of both A and B, i.e. the members two sets have "in common". The union of A and B is the set of all things which are members of either A or B.

Use the retain-intersection (the default) keyword to retain an intersection of all events as defined by two or more data windows. All events removed from any of the intersected data windows are entered into the remove stream. This is the default behavior if neither retain keyword is specified.

Use the retain-union keyword to retain a union of all events as defined by two or more data windows. Only events removed from all data windows are entered into the remove stream.

The next example statement totals the price of OrderEvent events in a union of the last 30 seconds and unique by product name:

select sum(price) from OrderEvent.win:time(30 sec).std:unique(productName) retain-union

In the above statement, all OrderEvent events that are either less then 30 seconds old or that are the last event for the product name are considered.

Here is an example statement totals the price of OrderEvent events in an intersection of the last 30 seconds and unique by product name:

select sum(price) from OrderEvent.win:time(30 sec).std:unique(productName) retain-intersection

In the above statement, only those OrderEvent events that are both less then 30 seconds old and are the last event for the product name are considered. The number of events that the engine retains is the number of unique events per product name in the last 30 seconds (and not the number of events in the last 30 seconds).

For an intersection the engine retains the minimal number of events representing that intersection. Thus when combining a time window of 30 seconds and a last-event window, for example, the number of events retained at any time is zero or one event (and not 30 seconds of events).

When combining a batch window into an intersection with another data window the combined data window gains batching semantics: Only when the batch criteria is fulfilled does the engine provide the batch of intersecting insert stream events. Multiple batch data windows may not be combined into an intersection.

In below table we provide additional examples for data window intersections:


For advanced users and for backward compatibility, it is possible to configure Esper to allow multiple data window views without either of the retain keywords, as described in Section 15.4.12.2, “Configuring Multi-Expiry Policy Defaults”.

Your from clause may assign a name to each stream. This assigned stream name can serve any of the following purposes.

First, the stream name can be used to disambiguate property names. The stream_name.property_name syntax uniquely identifies which property to select if property names overlap between streams. Here is an example:

select prod.productId, ord.productId from ProductEvent as prod, OrderEvent as ord

Second, the stream name can be used with a wildcard (*) character to select events in a join, or assign new names to the streams in a join:

// Select ProductEvent only
select prod.* from ProductEvent as prod, OrderEvent
// Assign column names 'product' and 'order' to each event
select prod.* as product, ord.* as order from ProductEvent as prod, OrderEvent as ord

Further, the stream name by itself can occur in any expression: The engine passes the event itself to that expression. For example, the engine passes the ProductEvent and the OrderEvent to the user-defined function 'checkOrder':

select prod.productId, MyFunc.checkOrder(prod, ord) 
from ProductEvent as prod, OrderEvent as ord

Last, you may invoke an instance method on each event of a stream, and pass parameters to the instance method as well. Instance method calls are allowed anywhere in an expression.

The next statement demonstrates this capability by invoking a method 'computeTotal' on OrderEvent events and a method 'getMultiplier' on ProductEvent events:

select ord.computeTotal(prod.getMultiplier()) from ProductEvent as prod, OrderEvent as ord

Instance methods may also be chained: Your EPL may invoke a method on the result returned by a method invocation.

Assume that your product event exposes a method getZone which returns a zone object. Assume that the Zone class declares a method checkZone. This example statement invokes a method chain:

select prod.getZone().checkZone("zone 1") from ProductEvent as prod

The aggregate functions are further documented in Section 9.2, “Aggregation Functions”. You can use aggregate functions to calculate and summarize data from event properties. For example, to find out the total price for all stock tick events in the last 30 seconds, type:

select sum(price) from StockTickEvent.win:time(30 sec)

Here is the syntax for aggregate functions:

aggregate_function( [all | distinct] expression)

You can apply aggregate functions to all events in an event stream window or other view, or to one or more groups of events. From each set of events to which an aggregate function is applied, Esper generates a single value.

Expression is usually an event property name. However it can also be a constant, function, or any combination of event property names, constants, and functions connected by arithmetic operators.

For example, to find out the average price for all stock tick events in the last 30 seconds if the price was doubled:

select avg(price * 2) from StockTickEvent.win:time(30 seconds)

You can use the optional keyword distinct with all aggregate functions to eliminate duplicate values before the aggregate function is applied. The optional keyword all which performs the operation on all events is the default.

You can use aggregation functions in a select clause and in a having clause. You cannot use aggregate functions in a where clause, but you can use the where clause to restrict the events to which the aggregate is applied. The next query computes the average and sum of the price of stock tick events for the symbol IBM only, for the last 10 stock tick events regardless of their symbol.

select 'IBM stats' as title, avg(price) as avgPrice, sum(price) as sumPrice
from StockTickEvent.win:length(10)
where symbol='IBM'

In the above example the length window of 10 elements is not affected by the where clause, i.e. all events enter and leave the length window regardless of their symbol. If we only care about the last 10 IBM events, we need to add filter criteria as below.

select 'IBM stats' as title, avg(price) as avgPrice, sum(price) as sumPrice
from StockTickEvent(symbol='IBM').win:length(10)
where symbol='IBM'

You can use aggregate functions with any type of event property or expression, with the following exceptions:

  1. You can use sum, avg, median, stddev, avedev with numeric event properties only

Esper ignores any null values returned by the event property or expression on which the aggregate function is operating, except for the count(*) function, which counts null values as well. All aggregate functions return null if the data set contains no events, or if all events in the data set contain only null values for the aggregated expression.

The group by clause is optional in all EPL statements. The group by clause divides the output of an EPL statement into groups. You can group by one or more event property names, or by the result of computed expressions. When used with aggregate functions, group by retrieves the calculations in each subgroup. You can use group by without aggregate functions, but generally that can produce confusing results.

For example, the below statement returns the total price per symbol for all stock tick events in the last 30 seconds:

select symbol, sum(price) from StockTickEvent.win:time(30 sec) group by symbol

The syntax of the group by clause is:

group by aggregate_free_expression [, aggregate_free_expression] [, ...]

Esper places the following restrictions on expressions in the group by clause:

  1. Expressions in the group by cannot contain aggregate functions.

  2. When grouping an unbound stream, i.e. no data window is specified onto the stream providing groups, or when using output rate limiting with the ALL keyword, you should ensure your group-by expression does not return an unlimited number of values. If, for example, your group-by expression is a fine-grained timestamp, group state that accumulates for an unlimited number of groups potentially reduces available memory significantly. Use a @Hint as described below to instruct the engine when to discard group state.

You can list more then one expression in the group by clause to nest groups. Once the sets are established with group by the aggregation functions are applied. This statement posts the median volume for all stock tick events in the last 30 seconds per symbol and tick data feed. Esper posts one event for each group to statement listeners:

select symbol, tickDataFeed, median(volume) 
from StockTickEvent.win:time(30 sec) 
group by symbol, tickDataFeed

In the statement above the event properties in the select list (symbol, tickDataFeed) are also listed in the group by clause. The statement thus follows the SQL standard which prescribes that non-aggregated event properties in the select list must match the group by columns.

Esper also supports statements in which one or more event properties in the select list are not listed in the group by clause. The statement below demonstrates this case. It calculates the standard deviation for the last 30 seconds of stock ticks aggregating by symbol and posting for each event the symbol, tickDataFeed and the standard deviation on price.

select symbol, tickDataFeed, stddev(price) from StockTickEvent.win:time(30 sec) group by symbol

The above example still aggregates the price event property based on the symbol, but produces one event per incoming event, not one event per group.

Additionally, Esper supports statements in which one or more event properties in the group by clause are not listed in the select list. This is an example that calculates the mean deviation per symbol and tickDataFeed and posts one event per group with symbol and mean deviation of price in the generated events. Since tickDataFeed is not in the posted results, this can potentially be confusing.

select symbol, avedev(price) 
from StockTickEvent.win:time(30 sec) 
group by symbol, tickDataFeed

Expressions are also allowed in the group by list:

select symbol * price, count(*) from StockTickEvent.win:time(30 sec) group by symbol * price

If the group by expression resulted in a null value, the null value becomes its own group. All null values are aggregated into the same group. If you are using the count(expression) aggregate function which does not count null values, the count returns zero if only null values are encountered.

You can use a where clause in a statement with group by. Events that do not satisfy the conditions in the where clause are eliminated before any grouping is done. For example, the statement below posts the number of stock ticks in the last 30 seconds with a volume larger then 100, posting one event per group (symbol).

select symbol, count(*) from StockTickEvent.win:time(30 sec) where volume > 100 group by symbol

The Esper engine reclaims aggregation state agressively when it determines that a group has no data points, based on the data in the data windows. When your application data creates a large number of groups with a small or zero number of data points then performance may suffer as state is reclaimed and created anew. Esper provides the @Hint('disable_reclaim_group') hint that you can specify as part of an EPL statement text to avoid group reclaim.

When aggregating values over an unbound stream (i.e. no data window is specified onto the stream) and when your group-by expression returns an unlimited number of values, for example when a timestamp expression is used, then please note the next hint.

A sample statement that aggregates stock tick events by timestamp, assuming the event type offers a property by name timestamp that, reflects time in high resolution, for example arrival or system time:

// Note the below statement could lead to an out-of-memory problem:
select symbol, sum(price) from StockTickEvent group by timestamp

As the engine has no means of detecting when aggregation state (sums per symbol) can be discarded, you may use the following hints to control aggregation state lifetime.

The @Hint("reclaim_group_aged=age_in_seconds") hint instructs the engine to discard aggregation state that has not been updated for age_in_seconds seconds.

The optional @Hint("reclaim_group_freq=sweep_frequency_in_seconds") can be used in addition to control the frequency at which the engine sweeps aggregation state to determine aggregation state age and remove state that is older then age_in_seconds seconds. If the hint is not specified, the frequency defaults to the same value as age_in_seconds.

The updated sample statement with both hints:

// Instruct engine to remove state older then 10 seconds and sweep every 5 seconds
@Hint('reclaim_group_aged=10,reclaim_group_freq=5')
select symbol, sum(price) from StockTickEvent group by timestamp

Variables may also be used to provide values for age_in_seconds and sweep_frequency_in_seconds.

This example statement uses a variable named varAge to control how long aggregation state remains in memory, and the engine defaults the sweep frequency to the same value as the variable provides:

@Hint('reclaim_group_aged=varAge')
select symbol, sum(price) from StockTickEvent group by timestamp

Use the having clause to pass or reject events defined by the group-by clause. The having clause sets conditions for the group by clause in the same way where sets conditions for the select clause, except where cannot include aggregate functions, while having often does.

This statement is an example of a having clause with an aggregate function. It posts the total price per symbol for the last 30 seconds of stock tick events for only those symbols in which the total price exceeds 1000. The having clause eliminates all symbols where the total price is equal or less then 1000.

select symbol, sum(price) 
from StockTickEvent.win:time(30 sec) 
group by symbol 
having sum(price) > 1000

To include more then one condition in the having clause combine the conditions with and, or or not. This is shown in the statement below which selects only groups with a total price greater then 1000 and an average volume less then 500.

select symbol, sum(price), avg(volume)
from StockTickEvent.win:time(30 sec) 
group by symbol 
having sum(price) > 1000 and avg(volume) < 500

A statement with the having clause should also have a group by clause. If you omit group-by, all the events not excluded by the where clause return as a single group. In that case having acts like a where except that having can have aggregate functions.

The having clause can also be used without group by clause as the below example shows. The example below posts events where the price is less then the current running average price of all stock tick events in the last 30 seconds.

select symbol, price, avg(price) 
from StockTickEvent.win:time(30 sec) 
having price < avg(price)

When you include filters, the where condition, the group by clause and the having condition in an EPL statement the sequence in which each clause affects events determines the final result:

The following query illustrates the use of filter, where, group by and having clauses in one statement with a select clause containing an aggregate function.

select tickDataFeed, stddev(price)
from StockTickEvent(symbol='IBM').win:length(10) 
where volume > 1000
group by tickDataFeed 
having stddev(price) > 0.8

Esper filters events using the filter criteria for the event stream StockTickEvent. In the example above only events with symbol IBM enter the length window over the last 10 events, all other events are simply discarded. The where clause removes any events posted by the length window (events entering the window and event leaving the window) that do not match the condition of volume greater then 1000. Remaining events are applied to the stddev standard deviation aggregate function for each tick data feed as specified in the group by clause. Each tickDataFeed value generates one event. Esper applies the having clause and only lets events pass for tickDataFeed groups with a standard deviation of price greater then 0.8.

The keyed segmented context create context ... partition by and the group by clause as well as the built-in std:groupwin view are similar in their ability to group events but very different in their semantics. This section explains the key differences in their behavior and use.

The keyed segmented context as declared with create context ... partition by and context .... select ... creates a new context partition per key value(s). The engine maintains separate data window views as well as separate aggregations per context partition; thereby the keyed segmented context applies to both. See Section 4.2.2, “Keyed Segmented Context” for additional examples.

The group by clause works together with aggregation functions in your statement to produce an aggregation result per group. In greater detail, this means that when a new event arrives, the engine applies the expressions in the group by clause to determine a grouping key. If the engine has not encountered that grouping key before (a new group), the engine creates a set of new aggregation results for that grouping key and performs the aggregation changing that new set of aggregation results. If the grouping key points to an existing set of prior aggregation results (an existing group), the engine performs the aggregation changing the prior set of aggregation results for that group.

The std:groupwin view is a built-in view that groups events into data windows. The view is described in greater detail in Section 12.3.2, “Grouped Data Window (std:groupwin)”. Its primary use is to create a separate data window per group, or more generally to create separate instances of all its sub-views for each grouping key encountered.

The table below summarizes the point:


Please review the performance section for advice related to performance or memory-use.

The next example shows queries that produce equivalent results. The query using the group by clause is generally preferable as is easier to read. The second form introduces the stat:uni view which computes univariate statistics for a given property:

select symbol, avg(price) from StockTickEvent group by symbol
// ... is equivalent to ...
select symbol, average from StockTickEvent.std:groupwin(symbol).stat:uni(price)

The next example shows two queries that are NOT equivalent as the length window is ungrouped in the first query, and grouped in the second query:

select symbol, sum(price) from StockTickEvent.win:length(10) group by symbol
// ... NOT equivalent to ...
select symbol, sum(price) from StockTickEvent.std:groupwin(symbol).win:length(10)

The key difference between the two statements is that in the first statement the length window is ungrouped and applies to all events regardless of group. While in the second query each group gets its own instance of a length window. For example, in the second query events arriving for symbol "ABC" get a length window of 10 events, and events arriving for symbol "DEF" get their own length window of 10 events.

The output clause is optional in Esper and is used to control or stabilize the rate at which events are output and to suppress output events. The EPL language provides for several different ways to control output rate.

Here is the syntax for the output clause that specifies a rate in time interval or number of events:

output [after suppression_def] 
  [[all | first | last | snapshot] every output_rate [seconds | events]]
[and when terminated]

An alternate syntax specifies the time period between output as outlined in Section 5.2.1, “Specifying Time Periods” :

output [after suppression_def] 
  [[all | first | last | snapshot] every time_period]
[and when terminated]

A crontab-like schedule can also be specified. The schedule parameters follow the pattern observer parameters and are further described in Section 6.6.2.2, “timer:at” :

output [after suppression_def] 
  [[all | first | last | snapshot] at 
   (minutes, hours, days of month, months, days of week [, seconds])]
[and when terminated]

For use with contexts, in order to trigger output only when a context partition terminates, specify when terminated as further described in Section 4.5, “Output When Context Partition Ends”:

output [after suppression_def] 
  [[all | first | last | snapshot] when terminated 
  [and termination_expression]
  [then set variable_name = assign_expression [, variable_name = assign_expression [,...]]]
  ]

Last, output can be controlled by an expression that may contain variables, user-defined functions and information about the number of collected events. Output that is controlled by an expression is discussed in detail below.

The after keyword and suppression_def can appear alone or together with further output conditions and suppresses output events.

For example, the following statement outputs, every 60 seconds, the total price for all orders in the 30-minute time window:

select sum(price) from OrderEvent.win:time(30 min) output snapshot every 60 seconds

The all keyword is the default and specifies that all events in a batch should be output, each incoming row in the batch producing an output row. Note that for statements that group via the group by clause, the all keyword provides special behavior as below.

The first keyword specifies that only the first event in an output batch is to be output. Using the first keyword instructs the engine to output the first matching event as soon as it arrives, and then ignores matching events for the time interval or number of events specified. After the time interval elapsed, or the number of matching events has been reached, the next first matching event is output again and the following interval the engine again ignores matching events. For statements that group via the group by clause, the first keywords provides special behavior as below.

The last keyword specifies to only output the last event at the end of the given time interval or after the given number of matching events have been accumulated. Again, for statements that group via the group by clause the last keyword provides special behavior as below.

The snapshot keyword indicates that the engine output current computation results considering all events as per views specified and/or current aggregation results. While the other keywords control how a batch of events between output intervals is being considered, the snapshot keyword outputs all current state of a statement independent of the last batch. Its output is equivalent to the iterator method provided by a statement. The snapshot keyword requires a data window declaration if not used within a join and outputs only the last event if used without a data window.

The output_rate is the frequency at which the engine outputs events. It can be specified in terms of time or number of events. The value can be a number to denote a fixed output rate, or the name of a variable whose value is the output rate. By means of a variable the output rate can be controlled externally and changed dynamically at runtime.

Please consult the Appendix A, Output Reference and Samples for detailed information on insert and remove stream output for the various output clause keywords.

For use with contexts you may append the keywords and when terminated to trigger output at the rate defined and in addition trigger output when the context partition terminates. Please see Section 4.5, “Output When Context Partition Ends” for details.

The time interval can also be specified in terms of minutes; the following statement is identical to the first one.

select * from StockTickEvent output every 1.5 minutes

A second way that output can be stabilized is by batching events until a certain number of events have been collected:

select * from StockTickEvent output every 5 events

Additionally, event output can be further modified by the optional last keyword, which causes output of only the last event to arrive into an output batch.

select * from StockTickEvent output last every 5 events

Using the first keyword you can be notified at the start of the interval. The allows to watch for situations such as a rate falling below a threshold and only be informed every now and again after the specified output interval, but be informed the moment it first happens.

select * from TickRate where rate<100 output first every 60 seconds

A sample statement using the Unix "crontab"-command schedule is shown next. See Section 6.6.2.2, “timer:at” for details on schedule syntax. Here, output occurs every 15 minutes from 8am to 5:45pm (hours 8 to 17 at 0, 15, 30 and 45 minutes past the hour):

select symbol, sum(price) from StockTickEvent group by symbol output at (*/15, 8:17, *, *, *)

Output can also be controlled by an expression that may check variable values, use user-defined functions and query built-in properties that provide additional information. The synopsis is as follows:

output [after suppression_def] 
  [[all | first | last | snapshot] when trigger_expression 
    [then set variable_name = assign_expression [, variable_name = assign_expression [,...]]]
  [and when terminated 
    [and termination_expression]
    [then set variable_name = assign_expression [, variable_name = assign_expression [,...]]]
  ]

The when keyword must be followed by a trigger expression returning a boolean value of true or false, indicating whether to output. Use the optional then keyword to change variable values after the trigger expression evaluates to true. An assignment expression assigns a new value to variable(s).

For use with contexts you may append the keywords and when terminated to also trigger output when the context partition terminates. Please see Section 4.5, “Output When Context Partition Ends” for details. You may optionally specify a termination expression. If that expression is provided the engine evaluates the expression when the context partition terminates: The evaluation result of true means output occurs when the context partition terminates, false means no output occurs when the context partition terminates. You may specify then set followed by a list of assignments to assign variables. Assignments are executed on context partition termination regardless of the termination expression, if present.

Lets consider an example. The next statement assumes that your application has defined a variable by name OutputTriggerVar of boolean type. The statement outputs rows only when the OutputTriggerVar variable has a boolean value of true:

select sum(price) from StockTickEvent output when OutputTriggerVar = true

The engine evaluates the trigger expression when streams and data views post one or more insert or remove stream events after considering the where clause, if present. It also evaluates the trigger expression when any of the variables used in the trigger expression, if any, changes value. Thus output occurs as follows:

  1. When there are insert or remove stream events and the when trigger expression evaluates to true, the engine outputs the resulting rows.

  2. When any of the variables in the when trigger expression changes value, the engine evaluates the expression and outputs results. Result output occurs within the minimum time interval of timer resolution (100 milliseconds).

By adding a then part to the EPL, we can reset any variables after the trigger expression evaluated to true:

select sum(price) from StockTickEvent 
  output when OutputTriggerVar = true  
  then set OutputTriggerVar = false

Expressions in the when and then may, for example, use variables, user defined functions or any of the built-in named properties that are described in the below list.

The following built-in properties are available for use:


The values provided by count_insert and count_remove are non-continues: The number returned for these properties may 'jump' up rather then count up by 1. The counts reset to zero upon output.

The following restrictions apply to expressions used in the output rate clause:

  • Event property names cannot be used in the output clause.

  • Aggregation functions cannot be used in the output clause.

  • The prev previous event function and the prior prior event function cannot be used in the output clause.

Remove stream events can also be useful in conjunction with aggregation and the output clause: When the engine posts remove stream events for fully-aggregated queries, it presents the aggregation state before the expiring event leaves the data window. Your application can thus easily obtain a delta between the new aggregation value and the prior aggregation value.

The engine evaluates the having-clause at the granularity of the data posted by views. That is, if you utilize a time window and output every 10 events, the having clause applies to each individual event or events entering and leaving the time window (and not once per batch of 10 events).

The output clause interacts in two ways with the group by and having clauses. First, in the output every n events case, the number n refers to the number of events arriving into the group by clause. That is, if the group by clause outputs only 1 event per group, or if the arriving events don't satisfy the having clause, then the actual number of events output by the statement could be fewer than n.

Second, the last, all and first keywords have special meanings when used in a statement with aggregate functions and the group by clause:

Please consult the Appendix A, Output Reference and Samples for detailed information on insert and remove stream output for aggregation and group-by.

By adding an output rate limiting clause to a statement that contains a group by clause we can control output of groups to obtain one row for each group, generating an event per group at the given output frequency.

The next statement outputs total price per symbol cumulatively (no data window was used here). As it specifies the all keyword, the statement outputs the current value for all groups seen so far, regardless of whether the group was updated in the last interval. Output occurs after an interval of 5 seconds passed and at the end of each subsequent interval:

select symbol, sum(price) from StockTickEvent group by symbol output all every 5 seconds

The below statement outputs total price per symbol considering events in the last 3 minutes. When events leave the 3-minute data window output also occurs as new aggregation values are computed. The last keyword instructs the engine to output only those groups that had changes. Output occurs after an interval of 10 seconds passed and at the end of each subsequent interval:

select symbol, sum(price) from StockTickEvent.win:time(3 min)
group by symbol output last every 10 seconds

This statement also outputs total price per symbol considering events in the last 3 minutes. The first keyword instructs the engine to output as soon as there is a new value for a group. After output for a given group the engine suppresses output for the same group for 10 seconds and does not suppress output for other groups. Output occurs again for that group after the interval when the group has new value(s):

select symbol, sum(price) from StockTickEvent.win:time(3 min)
group by symbol output first every 10 seconds

The order by clause is optional. It is used for ordering output events by their properties, or by expressions involving those properties. .

For example, the following statement outputs batches of 5 or more stock tick events that are sorted first by price ascending and then by volume ascending:

select symbol from StockTickEvent.win:time(60 sec) 
output every 5 events 
order by price, volume

Here is the syntax for the order by clause:

order by expression [asc | desc] [, expression [asc | desc]] [, ...]

If the order by clause is absent then the engine still makes certain guarantees about the ordering of output:

  • If the statement is not a join, does not group via group by clause and does not declare grouped data windows via std:groupwin view, the order in which events are delivered to listeners and through the iterator pull API is the order of event arrival.

  • If the statement is a join or outer join, or groups, then the order in which events are delivered to listeners and through the iterator pull API is not well-defined. Use the order by clause if your application requires events to be delivered in a well-defined order.

Esper places the following restrictions on the expressions in the order by clause:

  1. All aggregate functions that appear in the order by clause must also appear in the select expression.

Otherwise, any kind of expression that can appear in the select clause, as well as any name defined in the select clause, is also valid in the order by clause.

By default all sort operations on string values are performed via the compare method and are thus not locale dependent. To account for differences in language or locale, see Section 15.4.21, “Engine Settings related to Language and Locale” to change this setting.

The limit clause is typically used together with the order by and output clause to limit your query results to those that fall within a specified range. You can use it to receive the first given number of result rows, or to receive a range of result rows.

There are two syntaxes for the limit clause, each can be parameterized by integer constants or by variable names. The first syntax is shown below:

limit row_count [offset offset_count]

The required row_count parameter specifies the number of rows to output. The row_count can be an integer constant and can also be the name of the integer-type variable to evaluate at runtime.

The optional offset_count parameter specifies the number of rows that should be skipped (offset) at the beginning of the result set. A variable can also be used for this parameter.

The next sample EPL query outputs the top 10 counts per property 'uri' every 1 minute.

select uri, count(*) from WebEvent 
group by uri 
output snapshot every 1 minute
order by count(*) desc 
limit 10

The next statement demonstrates the use of the offset keyword. It outputs ranks 3 to 10 per property 'uri' every 1 minute:

select uri, count(*) from WebEvent 
group by uri 
output snapshot every 1 minute
order by count(*) desc 
limit 8 offset 2

The second syntax for the limit clause is for SQL standard compatibility and specifies the offset first, followed by the row count:

limit offset_count[, row_count]

The following are equivalent:

limit 8 offset 2
// ...equivalent to
limit 2, 8

A negative value for row_count returns an unlimited number or rows, and a zero value returns no rows. If variables are used, then the current variable value at the time of output dictates the row count and offset. A variable returning a null value for row_count also returns an unlimited number or rows.

A negative value for offset is not allowed. If your variable returns a negative or null value for offset then the value is assumed to be zero (i.e. no offset).

The iterator pull API also honors the limit clause, if present.

The insert into clause is optional in Esper. The clause can be specified to make the results of a statement available as an event stream for use in further statements, or to insert events into a named window. The clause can also be used to merge multiple event streams to form a single stream of events.

The syntax for the insert into clause is as follows:

insert [istream | irstream | rstream] into event_stream_name  [ (property_name [, property_name] ) ]

The istream (default) and rstream keywords are optional. If no keyword or the istream keyword is specified, the engine supplies the insert stream events generated by the statement. The insert stream consists of the events entering the respective window(s) or stream(s). If the rstream keyword is specified, the engine supplies the remove stream events generated by the statement. The remove stream consists of the events leaving the respective window(s).

If your application specifies irstream, the engine inserts into the new stream both the insert and remove stream. This is often useful in connection with the istream built-in function that returns an inserted/removed boolean indicator for each event, see Section 9.1.7, “The Istream Function”.

The event_stream_name is an identifier that names the event stream (and also implicitly names the types of events in the stream) generated by the engine. The identifier can be used in further statements to filter and process events of that event stream. The insert into clause can consist of just an event stream name, or an event stream name and one or more property names.

The engine also allows listeners to be attached to a statement that contain an insert into clause. Listeners receive all events posted to the event stream.

To merge event streams, simply use the same event_stream_name identifier in all EPL statements that merge their result event streams. Make sure to use the same number and names of event properties and event property types match up.

Esper places the following restrictions on the insert into clause:

  1. The number of elements in the select clause must match the number of elements in the insert into clause if the clause specifies a list of event property names

  2. If the event stream name has already been defined by a prior statement or configuration, and the event property names and/or event types do not match, an exception is thrown at statement creation time.

The following sample inserts into an event stream by name CombinedEvent:

insert into CombinedEvent
select A.customerId as custId, A.timestamp - B.timestamp as latency
  from EventA.win:time(30 min) A, EventB.win:time(30 min) B
 where A.txnId = B.txnId

Each event in the CombinedEvent event stream has two event properties named "custId" and "latency". The events generated by the above statement can be used in further statements, such as shown in the next statement:

select custId, sum(latency)
  from CombinedEvent.win:time(30 min)
 group by custId

The example statement below shows the alternative form of the insert into clause that explicitly defines the property names to use.

insert into CombinedEvent (custId, latency)
select A.customerId, A.timestamp - B.timestamp 
...

The rstream keyword can be useful to indicate to the engine to generate only remove stream events. This can be useful if we want to trigger actions when events leave a window rather then when events enter a window. The statement below generates CombinedEvent events when EventA and EventB leave the window after 30 minutes.

insert rstream into CombinedEvent
select A.customerId as custId, A.timestamp - B.timestamp as latency
  from EventA.win:time(30 min) A, EventB.win:time(30 min) B
 where A.txnId = B.txnId

The insert into clause can be used in connection with patterns to provide pattern results to further statements for analysis:

insert into ReUpEvent
select linkUp.ip as ip 
from pattern [every linkDown=LinkDownEvent -> linkUp=LinkUpEvent(ip=linkDown.ip)]

The insert into clause allows to merge multiple event streams into a event single stream. The clause names an event stream to insert into by specifing an event_stream_name. The first statement that inserts into the named stream defines the stream's event types. Further statements that insert into the same event stream must match the type of events inserted into the stream as declared by the first statement.

One approach to merging event streams specifies individual colum names either in the select clause or in the insert into clause of the statement. This approach has been shown in earlier examples.

Another approach to merging event streams specifies the wildcard (*) in the select clause (or the stream wildcard) to select the underlying event. The events in the event stream must then have the same event type as generated by the from clause.

Assume a statement creates an event stream named MergedStream by selecting OrderEvent events:

insert into MergedStream select * from OrderEvent

A statement can use the stream wildcard selector to select only OrderEvent events in a join:

insert into MergedStream select ord.* from ItemScanEvent, OrderEvent as ord

And a statement may also use an application-supplied user-defined function to convert events to OrderEvent instances:

insert into MergedStream select MyLib.convert(item) from ItemScanEvent as item

Esper specifically recognizes a conversion function as follows: A conversion function must be the only selected column, and it must return either a Java object or java.util.Map or Object[] (object array). Your EPL should not use the as keyword to assign a column name.

A variant stream is a predefined stream into which events of multiple disparate event types can be inserted.

A variant stream name may appear anywhere in a pattern or from clause. In a pattern, a filter against a variant stream matches any events of any of the event types inserted into the variant stream. In a from clause including for named windows, views declared onto a variant stream may hold events of any of the event types inserted into the variant stream.

A variant stream is thus useful in problems that require different types of event to be treated the same.

Variant streams can be declared by means of create variant schema or can be predefined via runtime or initialization-time configuration as described in Section 15.4.27, “Variant Stream”. Your application may declare or predefine variant streams to carry events of a limited set of event types, or you may choose the variant stream to carry any and all types of events. This choice affects what event properties are available for consuming statements or patterns of the variant stream.

Assume that an application predefined a variant stream named OrderStream to carry only ServiceOrder and ProductOrder events. An insert into clause inserts events into the variant stream:

insert into OrderStream select * from ServiceOrder
insert into OrderStream select * from ProductOrder

Here is a sample statement that consumes the variant stream and outputs a total price per customer id for the last 30 seconds of ServiceOrder and ProductOrder events:

select customerId, sum(price) from OrderStream.win:time(30 sec) group by customerId

If your application predefines the variant stream to hold specific type of events, as the sample above did, then all event properties that are common to all specified types are visible on the variant stream, including nested, indexed and mapped properties. For access to properties that are only available on one of the types, the dynamic property syntax must be used. In the example above, the customerId and price were properties common to both ServiceOrder and ProductOrder events.

For example, here is a consuming statement that selects a service duraction property that only ServiceOrder events have, and that must therefore be casted to double and null values removed in order to aggregate:

select customerId, sum(coalesce(cast(serviceDuraction?, double), 0)) 
from OrderStream.win:time(30 sec) group by customerId

If your application predefines a variant stream to hold any type of events (the any type variance), then all event properties of the variant stream are effectively dynamic properties.

For example, an application may define an OutgoingEvents variant stream to hold any type of event. The next statement is a sample consumer of the OutgoingEvents variant stream that looks for the destination property and fires for each event in which the property exists with a value of 'email':

select * from OutgoingEvents(destination = 'email')

When you declare the inserted-into event type in advance to the statement that inserts, the engine compares the inserted-into event type information to the return type of expressions in the select-clause. The comparison uses the column alias assigned to each select-clause expression using the as keyword.

When the inserted-into column type is an event type and when using a subquery or the new operator, the engine compares column names assigned to subquery columns or new operator columns.

For example, assume a PurchaseOrder event type that has a property called items that consists of Item rows:

create schema Item(name string, price double)
create schema PurchaseOrder(orderId string, items Item[])

Declare a statement that inserts into the PurchaseOrder stream:

insert into PurchaseOrder 
select '001' as orderId, new {name='i1', price=10} as items
from TriggerEvent

The alias assigned to the first and second expression in the select-clause, namely orderId and items, both match the event property names of the Purchase Order event type. The column names provided to the new operator also both match the event property names of the Item event type.

When the event type declares the column as a single value (and not an array) and when the select-clause expression produces a multiple rows, the engine only populate the first row.

Consider a PurchaseOrder event type that has a property called item that consists of a single Item event:

create schema PurchaseOrder(orderId string, items Item)

The sample subquery below populates only the very first event, discarding remaining subquery result events, since the items property above is declared as holding a single Item-typed event only (versus Item[] to hold multiple Item-typed events).

insert into PurchaseOrder select 
(select 'i1' as name, 10 as price from HistoryEvent.win:length(2)) as items 
from TriggerEvent

Consider using a subquery with filter, or one of the enumeration methods to select a specific subquery result row.

A subquery is a select within another statement. Esper supports subqueries in the select clause, where clause, having clause and in stream and pattern filter expressions. Subqueries provide an alternative way to perform operations that would otherwise require complex joins. Subqueries can also make statements more readable then complex joins.

Esper supports both simple subqueries as well as correlated subqueries. In a simple subquery, the inner query is not correlated to the outer query. Here is an example simple subquery within a select clause:

select assetId, (select zone from ZoneClosed.std:lastevent()) as lastClosed from RFIDEvent

If the inner query is dependent on the outer query, we will have a correlated subquery. An example of a correlated subquery is shown below. Notice the where clause in the inner query, where the condition involves a stream from the outer query:

select * from RfidEvent as RFID where 'Dock 1' = 
  (select name from Zones.std:unique(zoneId) where zoneId = RFID.zoneId)

The example above shows a subquery in the where clause. The statement selects RFID events in which the zone name matches a string constant based on zone id. The statement uses the view std:unique to guarantee that only the last event per zone id is held from processing by the subquery.

The next example is a correlated subquery within a select clause. In this statement the select clause retrieves the zone name by means of a subquery against the Zones set of events correlated by zone id:

select zoneId, (select name from Zones.std:unique(zoneId) 
  where zoneId = RFID.zoneId) as name from RFIDEvent

Note that when a simple or correlated subquery returns multiple rows, the engine returns a null value as the subquery result. To limit the number of events returned by a subquery consider using one of the views std:lastevent, std:unique and std:groupwin or aggregation functions or the multi-row and multi-column selects as described below.

The select clause of a subquery also allows wildcard selects, which return as an event property the underlying event object of the event type as defined in the from clause. An example:

select (select * from MarketData.std:lastevent()) as md 
  from pattern [every timer:interval(10 sec)]

The output events to the statement above contain the underlying MarketData event in a property named "md". The statement populates the last MarketData event into a property named "md" every 10 seconds following the pattern definition, or populates a null value if no MarketData event has been encountered so far.

Aggregation functions may be used in the select clause of the subselect as this example outlines:

select * from MarketData
where price > (select max(price) from MarketData(symbol='GOOG').std:lastevent())

As the sub-select expression is evaluated first (by default), the query above actually never fires for the GOOG symbol, only for other symbols that have a price higher then the current maximum for GOOG. As a sidenote, the insert into clause can also be handy to compute aggregation results for use in multiple subqueries.

When using aggregation functions in a correlated subselect the engine computes the aggregation based on data window or named window contents matching the where-clause.

The following example compares the quantity value provided by the current order event against the total quantity of all order events in the last 1 hour for the same client.

select * from OrderEvent oe
where qty > 
  (select sum(qty) from OrderEvent.win:time(1 hour) pd 
  where pd.client = oe.client)

Filter expressions in a pattern or stream may also employ subqueries. Subqueries can be uncorrelated or can be correlated to properties of the stream or to properties of tagged events in a pattern. Subqueries may reference named windows as well.

The following example filters BarData events that have a close price less then the last moving average (field movAgv) as provided by stream SMA20Stream (an uncorrelated subquery):

select * from BarData(ticker='MSFT', closePrice < 
    (select movAgv from SMA20Stream(ticker='MSFT').std:lastevent()))

A few generic examples follow to demonstrate the point. The examples use short event and property names so they are easy to read. Assume A and B are streams and DNamedWindow is a named window, and properties a_id, b_id, d_id, a_val, b_val, d_val respectively:

// Sample correlated subquery as part of stream filter criteria
select * from A(a_val in 
  (select b_val from B.std:unique(b_val) as b where a.a_id = b.b_id)) as a
// Sample correlated subquery against a named window
select * from A(a_val in 
  (select b_val from DNamedWindow as d where a.a_id = d.d_id)) as a
// Sample correlated subquery in the filter criteria as part of a pattern, querying a named window
select * from pattern [
  a=A -> b=B(bvalue = 
    (select d_val from DNamedWindow as d where d.d_id = b.b_id and d.d_id = a.a_id))
]

Subquery state starts to accumulate as soon as a statement starts (and not only when a pattern-subexpression activates).

The following restrictions apply to subqueries:

  1. The subquery stream definition must define a data window or other view to limit subquery results, reducing the number of events held for subquery execution

  2. Subqueries can only consist of a select clause, a from clause, a where clause and a group by clause. The having clause, as well as joins, outer-joins and output rate limiting are not permitted within subqueries.

  3. If using aggregation functions in a subquery, note these limitations:

    1. None of the properties of the correlated stream(s) can be used within aggregation functions.

    2. The properties of the subselect stream must all be within aggregation functions.

The order of evaluation of subqueries relative to the containing statement is guaranteed: If the containing statement and its subqueries are reacting to the same type of event, the subquery will receive the event first before the containing statement's clauses are evaluated. This behavior can be changed via configuration. The order of evaluation of subqueries is not guaranteed between subqueries.

Performance of your statement containing one or more subqueries principally depends on two parameters. First, if your subquery correlates one or more columns in the subquery stream with the enclosing statement's streams, the engine automatically builds the appropriate indexes for fast row retrieval based on the key values correlated (joined). The second parameter is the number of rows found in the subquery stream and the complexity of the filter criteria (where clause), as each row in the subquery stream must evaluate against the where clause filter.

The any subquery condition is true if the expression returns true for one or more of the values returned by the subquery.

The synopsis for the any keyword is as follows:

expression operator any (subquery)
expression operator some (subquery)

The left-hand expression is evaluated and compared to each row of the subquery result using the given operator, which must yield a Boolean result. The result of any is "true" if any true result is obtained. The result is "false" if no true result is found (including the special case where the subquery returns no rows).

The operator can be any of the following values: =, !=, <>, <, <=, >, >=.

The some keyword is a synonym for any. The in construct is equivalent to = any.

The right-hand side subquery must return exactly one column.

The next statement demonstrates the use of the any subquery condition:

select * from ProductOrder as ord
  where quantity < any
    (select minimumQuantity from MinimumQuantity.win:keepall())

The above query compares ProductOrder event's quantity value with all rows from the MinimumQuantity stream of events and returns only those ProductOrder events that have a quantity that is less then any of the minimum quantity values of the MinimumQuantity events.

Note that if there are no successes and at least one right-hand row yields null for the operator's result, the result of the any construct will be null, not false. This is in accordance with SQL's normal rules for Boolean combinations of null values.

Your subquery may select multiple columns in the select clause including multiple aggregated values from a data window or named window.

The following example is a correlated subquery that selects wildcard and in addition selects the bid and offer properties of the last MarketData event for the same symbol as the arriving OrderEvent:

select *,
  (select bid, offer from MarketData.std:unique(symbol) as md 
   where md.symbol = oe.symbol) as bidoffer
from OrderEvent oe

Output events for the above query contain all properties of the original OrderEvent event. In addition each output event contains a bidoffer nested property that itself contains the bid and offer properties. You may retrieve the bid and offer from output events directly via the bidoffer.bid property name syntax for nested properties.

The next example is similar to the above query but instead selects aggregations and selects from a named window by name OrderNamedWindow (creation not shown here). For each arriving OrderEvent it selects the total quantity and count of all order events for the same client, as currently held by the named window:

select *,
  (select sum(qty) as sumPrice, count(*) as countRows 
   from OrderNamedWindow as onw
   where onw.client = oe.client) as pastOrderTotals
from OrderEvent as oe

The next EPL statement computes a prorated quantity considering the maximum and minimum quantity for the last 1 minute of order events:

expression subq {
  (select max(quantity) as maxq, min(quantity) as minq from OrderEvent.win:time(1 min))
}
select (quantity - minq) / (subq().maxq  - subq().minq) as prorated
from OrderEvent

Output events for the above query contain all properties of the original OrderEvent event. In addition each output event contains a pastOrderTotals nested property that itself contains the sumPrice and countRows properties.

While a subquery cannot change the cardinality of the selected stream, a subquery can return multiple values from the selected data window or named window. This section shows examples of the window aggregation function as well as the use of enumeration methods with subselects.

Consider using an inner join, outer join or unidirectional join instead to achieve a 1-to-many cardinality in the number of output events.

The next example is an uncorrelated subquery that selects all current ZoneEvent events considering the last ZoneEvent per zone for each arriving RFIDEvent.

select assetId,
 (select window(z.*) as winzones from ZoneEvent.std:unique(zone) as z) as zones
 from RFIDEvent

Output events for the above query contain two properties: the assetId property and the zones property. The latter property is a nested property that contains the winzones property. You may retrieve the zones from output events directly via the zones.winzones property name syntax for nested properties.

In this example for a correlated subquery against a named window we assume that the OrderNamedWindow has been created and contains order events. The query returns for each MarketData event the list of order ids for orders with the same symbol:

select price,
 (select window(orderId) as winorders 
  from OrderNamedWindow onw 
  where onw.symbol = md.symbol) as orderIds
 from MarketData md

Output events for the above query contain two properties: the price property and the orderIds property. The latter property is a nested property that contains the winorders property of type array.

Another option to reduce selected rows to a single value is through the use of enumeration methods.

select price,
 (select *  from OrderNamedWindow onw
  where onw.symbol = md.symbol).selectFrom(v => v) as ordersSymbol
 from MarketData md

Output events for the above query also contain a Collection of underlying events in the ordersSymbol property.

The following hints are available to tune performance and memory use of subqueries.

Use the @Hint('set_noindex') hint for a statement that utilizes one or more subqueries. It instructs the engine to always perform a full table scan. The engine does not build an implicit index or use an explicitly-created index when this hint is provided. Use of the hint may result in reduced memory use but poor statement performance.

The following hints are available to tune performance and memory use of subqueries that select from named windows.

Named windows are globally-visible data windows. As such an application may create explicit indexes as discussed in Section 5.15.13, “Explicitly Indexing Named Windows”. The engine may also elect to create implicit indexes (no create-index EPL required) for index-based lookup of rows when executing on-select, on-merge, on-update and on-delete statements and for statements that subquery a named window.

By default and without specifying a hint, each statement that subqueries a named window also maintains its own index for looking up events held by the named window. The engine maintains the index by consuming the named window insert and remove stream. When the statement is destroyed it releases that index.

Specify the @Hint('enable_window_subquery_indexshare') hint to enable subquery index sharing for named windows. When using this hint, indexes for subqueries are maintained by the named window itself (and not each statement context partition), are shared between one or more statements and may also utilize explicit indexes. Specify the hint once as part of the create window statement.

This sample EPL statement creates a named window with subquery index sharing enabled:

@Hint('enable_window_subquery_indexshare')
create window AllOrdersNamedWindow.win:keepall() as OrderMapEventType

When subquery index sharing is enabled, performance may increase as named window stream consumption is no longer needed for correlated subqueries. You may also expect reduced memory use especially if a large number of EPL statements perform similar subqueries against a named window. Subquery index sharing may require additional short-lived object creation and may slightly increase lock held time for named windows.

The following statement performs a correlated subquery against the named window above. When a settlement event arrives it select the order detail for the same order id as provided by the settlement event:

select 
  (select * from AllOrdersNamedWindow as onw 
    where onw.orderId = se.orderId) as orderDetail
  from SettlementEvent as se

With subquery index sharing enabled the engine maintains an index of order events by order id for the named window, and shares that index between additional statements until the time all utilizing statements are destroyed.

You may disable subquery index sharing for a specific statement by specifying the @Hint('disable_window_subquery_indexshare') hint, as this example shows, causing the statement to maintain its own index:

@Hint('disable_window_subquery_indexshare')
select 
  (select * from AllOrdersNamedWindow as onw 
    where onw.orderId = se.orderId) as orderDetail
  from SettlementEvent as se

Two or more event streams can be part of the from-clause and thus both (all) streams determine the resulting events. This section summarizes the important concepts. The sections that follow present more detail on each topic.

The default join is an inner join which produces output events only when there is at least one match in all streams.

Consider the sample statement shown next:

select * from TickEvent.std:lastevent(), NewsEvent.std:lastevent()

The above statement outputs the last TickEvent and the last NewsEvent in one output event when either a TickEvent or a NewsEvent arrives. If no TickEvent was received before a NewsEvent arrives, no output occurs. Similarly when no NewsEvent was received before a TickEvent arrives, no output occurs.

The where-clause lists the join conditions that Esper uses to relate events in the two or more streams.

The next example statement retains the last TickEvent and last NewsEvent per symbol, and joins the two streams based on their symbol value:

select * from TickEvent.std:unique(symbol) as t, NewsEvent.std:unique(symbol) as n
where t.symbol = n.symbol

As before, when a TickEvent arrives for a symbol that has no matching NewsEvent then there is no output event.

An outer join does not require each event in either stream to have a matching event. The full outer join is useful when output is desired when no match is found. The different outer join types (full, left, right) are explained in more detail below.

This example statement is an outer-join and also returns the last TickEvent and last NewsEvent per symbol:

select * from TickEvent.std:unique(symbol) as t
full outer join NewsEvent.std:unique(symbol) as n on t.symbol = n.symbol

In the sample statement above, when a TickEvent arrives for a symbol that has no matching NewsEvent, or when a NewsEvent arrives for a symbol that has no matching TickEvent, the statement still produces an output event with a null column value for the missing event.

Note that each of the sample queries above defines a data window. The sample queries above use the last-event data window (std:lastevent) or the unique data window (std:unique). A data window serves to indicate the subset of events to join from each stream and may be required depending on the join.

In above queries, when either a TickEvent arrives or when a NewsEvent arrives then the query evaluates and there is output. The same holds true if additional streams are added to the from-clause: Each of the streams in the from-clause trigger the join to evaluate.

The unidirectional keyword instructs the engine to evaluate the join only when an event arrives from the single stream that was marked with the unidirectional keyword. In this case no data window should be specified for the stream marked as unidirectional since the keyword implies that the current event of that stream triggers the join.

Here is the sample statement above with unidirectional keyword, so that output occurs only when a TickEvent arrives and not when a NewsEvent arrives:

select * from TickEvent as t unidirectional, NewsEvent.std:unique(symbol) as n 
where t.symbol = n.symbol

It is oftentimes the case that an aggregation (count, sum, average) only needs to be calculated in the context of an arriving event or timer. Consider using the unidirectional keyword when aggregating over joined streams.

An EPL pattern is a normal citizen also providing a stream of data consisting of pattern matches. A time pattern, for example, can be useful to evaluate a join and produce output upon each interval.

This sample statement includes a pattern that fires every 5 seconds and thus triggers the join to evaluate and produce output, computing an aggregated total quantity per symbol every 5 seconds:

select symbol, sum(qty) from pattern[every timer:interval(5 sec)] unidirectional, 
  TickEvent.std:unique(symbol) t, NewsEvent.std:unique(symbol) as n 
where t.symbol = n.symbol group by symbol

Named windows as well as reference and historical data such as stored in your relational database, and data returned by a method invocation, can also be included in joins as discussed in Section 5.13, “Accessing Relational Data via SQL” and Section 5.14, “Accessing Non-Relational Data via Method Invocation”.

Related to joins are subqueries: A subquery is a select within another statement, see Section 5.11, “Subqueries”

The engine performs extensive query analysis and planning, building internal indexes and strategies as required to allow fast evaluation of many types of queries.

Each point in time that an event arrives to one of the event streams, the two event streams are joined and output events are produced according to the where clause when matching events are found for all joined streams.

This example joins 2 event streams. The first event stream consists of fraud warning events for which we keep the last 30 minutes. The second stream is withdrawal events for which we consider the last 30 seconds. The streams are joined on account number.

select fraud.accountNumber as accntNum, fraud.warning as warn, withdraw.amount as amount,
       max(fraud.timestamp, withdraw.timestamp) as timestamp, 'withdrawlFraud' as desc
  from com.espertech.esper.example.atm.FraudWarningEvent.win:time(30 min) as fraud,
       com.espertech.esper.example.atm.WithdrawalEvent.win:time(30 sec) as withdraw
 where fraud.accountNumber = withdraw.accountNumber

Joins can also include one or more pattern statements as the next example shows:

select * from FraudWarningEvent.win:time(30 min) as fraud,
    pattern [every w=WithdrawalEvent -> PINChangeEvent(acct=w.acct)].std:lastevent() as withdraw
 where fraud.accountNumber = withdraw.w.accountNumber

The statement above joins the last 30 minutes of fraud warnings with a pattern. The pattern consists of every withdrawal event that is followed by a PIN change event for the same account number. It joins the two event streams on account number. The last-event view instucts the join to only consider the last pattern match.

In a join and outer join, your statement must declare a data window view or other view onto each stream. Streams that are marked as unidirectional and named windows as well as database or methods in a join are an exception and do not require a view to be specified. If you are joining an event to itself via contained-event selection, views also do not need to be specified. The reason that a data window must be declared is that a data window specifies which events are considered for the join (i.e. last event, last 10 events, all events, last 1 second of events etc.).

The next example joins all FraudWarningEvent events that arrived since the statement was started, with the last 20 seconds of PINChangeEvent events:

select * from FraudWarningEvent.win:keepall() as fraud, PINChangeEvent.win:time(20 sec) as pin
 where fraud.accountNumber = pin.accountNumber

The above example employed the special keep-all view that retains all events.

Esper supports left outer joins, right outer joins, full outer joins and inner joins in any combination between an unlimited number of event streams. Outer and inner joins can also join reference and historical data as explained in Section 5.13, “Accessing Relational Data via SQL”, as well as join data returned by a method invocation as outlined in Section 5.14, “Accessing Non-Relational Data via Method Invocation”.

The keywords left, right, full and inner control the type of the join between two streams. The optional on clause specifies one or more properties that join each stream. The synopsis is as follows:

...from stream_def [as name] 
  ((left|right|full outer) | inner) join stream_def 
  [on property = property [and property = property ...] ]
  [ ((left|right|full outer) | inner) join stream_def [on ...]]...

If the outer join is a left outer join, there will be an output event for each event of the stream on the left-hand side of the clause. For example, in the left outer join shown below we will get output for each event in the stream RfidEvent, even if the event does not match any event in the event stream OrderList.

select * from RfidEvent.win:time(30 sec) as rfid
       left outer join
       OrderList.win:length(10000) as orderlist
     on rfid.itemId = orderList.itemId

Similarly, if the join is a Right Outer Join, then there will be an output event for each event of the stream on the right-hand side of the clause. For example, in the right outer join shown below we will get output for each event in the stream OrderList, even if the event does not match any event in the event stream RfidEvent.

select * from RfidEvent.win:time(30 sec) as rfid
       right outer join
       OrderList.win:length(10000) as orderlist
       on rfid.itemId = orderList.itemId

For all types of outer joins, if the join condition is not met, the select list is computed with the event properties of the arrived event while all other event properties are considered to be null.

The next type of outer join is a full outer join. In a full outer join, each point in time that an event arrives to one of the event streams, one or more output events are produced. In the example below, when either an RfidEvent or an OrderList event arrive, one or more output event is produced. The next example shows a full outer join that joins on multiple properties:

select * from RfidEvent.win:time(30 sec) as rfid
       full outer join
       OrderList.win:length(10000) as orderlist
       on rfid.itemId = orderList.itemId and rfid.assetId = orderList.assetId

The last type of join is an inner join. In an inner join, the engine produces an output event for each event of the stream on the left-hand side that matches at least one event on the right hand side considering the join properties. For example, in the inner join shown below we will get output for each event in the RfidEvent stream that matches one or more events in the OrderList data window:

select * from RfidEvent.win:time(30 sec) as rfid
       inner join
       OrderList.win:length(10000) as orderlist
       on rfid.itemId = orderList.itemId and rfid.assetId = orderList.assetId

Patterns as streams in a join follow this rule: If no data window view is declared for the pattern then the pattern stream retains the last match. Thus a pattern must have matched at least once for the last row to become available in a join. Multiple rows from a pattern stream may be retained by declaring a data window view onto a pattern using the pattern [...].view_specification syntax.

This example outer joins multiple streams. Here the RfidEvent stream is outer joined to both ProductName and LocationDescription via left outer join:

select * from RfidEvent.win:time(30 sec) as rfid
      left outer join ProductName.win:keepall() as refprod
        on rfid.productId = refprod.prodId
      left outer join LocationDescription.win:keepall() as refdesc
        on rfid.location = refdesc.locId

If the optional on clause is specified, it may only employ the = equals operator and property names. Any other operators must be placed in the where-clause. The stream names that appear in the on clause may refer to any stream in the from-clause.

Your EPL may also provide no on clause. This is useful when the streams that are joined do not provide any properties to join on, for example when joining with a time-based pattern.

The next example employs a unidirectional left outer join such that the engine, every 10 seconds, outputs a count of the number of RfidEvent events in the 60-second time window.

select count(*) from
  pattern[every timer:interval(1)] unidirectional 
  left outer join
  RfidEvent.win:time(60 sec)

In a join or outer join your statement lists multiple event streams, views and/or patterns in the from clause. As events arrive into the engine, each of the streams (views, patterns) provides insert and remove stream events. The engine evaluates each insert and remove stream event provided by each stream, and joins or outer joins each event against data window contents of each stream, and thus generates insert and remove stream join results.

The direction of the join execution depends on which stream or streams are currently providing an insert or remove stream event for executing the join. A join is thus multidirectional, or bidirectional when only two streams are joined. A join can be made unidirectional if your application does not want new results when events arrive on a given stream or streams.

The unidirectional keyword can be used in the from clause to identify a single stream that provides the events to execute the join. If the keyword is present for a stream, all other streams in the from clause become passive streams. When events arrive or leave a data window of a passive stream then the join does not generate join results.

For example, consider a use case that requires us to join stock tick events (TickEvent) and news events (NewsEvent). The unidirectional keyword allows to generate results only when TickEvent events arrive, and not when NewsEvent arrive or leave the 10-second time window:

select * from TickEvent unidirectional, NewsEvent.win:time(10 sec) 
where tick.symbol = news.symbol

Aggregation functions in a unidirectional join aggregate within the context of each unidirectional event evaluation and are not cumulative. Thereby aggregation functions when used with unidirectional may evaluate faster as they do not need to consider a remove stream (data removed from data windows or named windows).

The count function in the next query returns, for each TickEvent, the number of matching NewEvent events:

select count(*) from TickEvent unidirectional, NewsEvent.win:time(10 sec) 
where tick.symbol = news.symbol

The following restrictions apply to unidirectional joins:

  1. The unidirectional keyword can only be specified for a single stream in the from clause.

  2. Receiving data from a unidirectional join via the pull API (iterator method) is not allowed. This is because the engine holds no state for the single stream that provides the events to execute the join.

  3. The stream that declares the unidirectional keyword cannot declare a data window view or other view for that stream, since remove stream events are not processed for the single stream.

When joining 3 or more streams (including any relational or non-relational sources as below) it can sometimes help to provide the query planner instructions how to best execute the join. The engine compiles a query plan for the EPL statement at statement creation time. You can output the query plan to logging (see configuration).

An outer join that specifies only inner keywords for all streams is equivalent to an default (inner) join. The following two statements are equivalent:

select * from TickEvent.std:lastevent(), 
    NewsEvent.std:lastevent() where tick.symbol = news.symbol

Equivalent to:

select * from TickEvent.std:lastevent() 
	inner join NewsEvent.std:lastevent() on tick.symbol = news.symbol

For all types of joins, the query planner determines a query graph: The term is used here for all the information regarding what properties or expressions are used to join the streams. The query graph thus includes the where-clause expressions as well as outer-join on-clauses if this statement is an outer join. The query planner also computes a dependency graph which includes information about all historical data streams (relational and non-relational as below) and their input needs.

For default (inner) joins the query planner first attempts to find a path of execution as a nested iteration. For each stream the query planner selects the best order of streams available for the nested iteration considering the query graph and dependency graph. If the full depth of the join is achievable via nested iteration for all streams without full table scan then the query planner uses that nested iteration plan. If not, then the query planner re-plans considering a merge join (Cartesian) approach instead.

Specify the @Hint('PREFER_MERGE_JOIN') to instruct the query planner to prefer a merge join plan instead of a nested iteration plan. Specify the @Hint('FORCE_NESTED_ITER') to instruct the query planner to always use a nested iteration plan.

For example, consider the below statement. Depending on the number of matching rows in OrderBookOne and OrderBookTwo (named windows in this example, and assumed to be defined elsewhere) the performance of the join may be better using the merge join plan.

@Hint('PREFER_MERGE_JOIN') 
select * from TickEvent.std:lastevent() t, 
	OrderBookOne ob1, OrderBookOne ob2
where ob1.symbol = t.symbol and ob2.symbol = t.symbol 
and ob1.price between t.buy and t.sell and ob2.price between t.buy and t.sell

For outer joins the query planner considers nested iteration and merge join (Cartesian) equally and above hints don't apply.

This chapter outlines how reference data and historical data that are stored in a relational database can be queried via SQL within EPL statements.

Esper can access via join and outer join as well as via iterator (poll) API all types of event streams to stored data. In order for such data sources to become accessible to Esper, some configuration is required. The Section 15.4.9, “Relational Database Access” explains the required configuration for database access in greater detail, and includes information on configuring a query result cache.

Esper does not parse or otherwise inspect your SQL query. Therefore your SQL can make use of any database-specific SQL language extensions or features that your database provides.

If you have enabled query result caching in your Esper database configuration, Esper retains SQL query results in cache following the configured cache eviction policy.

Also if you have enabled query result caching in your Esper database configuration and provide EPL where clause and/or on clause (outer join) expressions, then Esper builds indexes on the SQL query results to enable fast lookup. This is especially useful if your queries return a large number of rows. For building the proper indexes, Esper inspects the expression found in your EPL query where clause, if present. For outer joins, Esper also inspects your EPL query on clause. Esper analyzes the EPL on clause and where clause expressions, if present, looking for property comparison with or without logical AND-relationships between properties. When a SQL query returns rows for caching, Esper builds and caches the appropriate index and lookup strategies for fast row matching against indexes.

Joins or outer joins in which only SQL statements or method invocations are listed in the from clause and no other event streams are termed passive joins. A passive join does not produce an insert or remove stream and therefore does not invoke statement listeners with results. A passive join can be iterated on (polled) using a statement's safeIterator and iterator methods.

There are no restrictions to the number of SQL statements or types of streams joined. The following restrictions currently apply:

  • Sub-views on an SQL query are not allowed; That is, one cannot create a time or length window on an SQL query. However one can use the insert into syntax to make join results available to a further statement.

  • Your database software must support JDBC prepared statements that provide statement meta data at compilation time. Most major databases provide this function. A workaround is available for databases that do not provide this function.

  • JDBC drivers must support the getMetadata feature. A workaround is available as below for JDBC drivers that don't support getting metadata.

The next sections assume basic knowledge of SQL (Structured Query Language).

To join an event stream against stored data, specify the sql keyword followed by the name of the database and a parameterized SQL query. The syntax to use in the from clause of an EPL statement is:

sql:database_name [" parameterized_sql_query "]

The engine uses the database_name identifier to obtain configuration information in order to establish a database connection, as well as settings that control connection creation and removal. Please see Section 15.4.9, “Relational Database Access” to configure an engine for database access.

Following the database name is the SQL query to execute. The SQL query can contain one or more substitution parameters. The SQL query string is placed in single brackets [ and ]. The SQL query can be placed in either single quotes (') or double quotes ("). The SQL query grammer is passed to your database software unchanged, allowing you to write any SQL query syntax that your database understands, including stored procedure calls.

Substitution parameters in the SQL query string take the form ${expression}. The engine resolves expression at statement execution time to the actual expression result by evaluating the events in the joined event stream or current variable values, if any event property references or variables occur in the expression. An expression may not contain EPL substitution parameters.

The engine determines the type of the SQL query output columns by means of the result set metadata that your database software returns for the statement. The actual query results are obtained via the getObject on java.sql.ResultSet.

The sample EPL statement below joins an event stream consisting of CustomerCallEvent events with the results of an SQL query against the database named MyCustomerDB and table Customer:

select custId, cust_name from CustomerCallEvent,
  sql:MyCustomerDB [' select cust_name from Customer where cust_id = ${custId} ']

The example above assumes that CustomerCallEvent supplies an event property named custId. The SQL query selects the customer name from the Customer table. The where clause in the SQL matches the Customer table column cust_id with the value of custId in each CustomerCallEvent event. The engine executes the SQL query for each new CustomerCallEvent encountered.

If the SQL query returns no rows for a given customer id, the engine generates no output event. Else the engine generates one output event for each row returned by the SQL query. An outer join as described in the next section can be used to control whether the engine should generate output events even when the SQL query returns no rows.

The next example adds a time window of 30 seconds to the event stream CustomerCallEvent. It also renames the selected properties to customerName and customerId to demonstrate how the naming of columns in an SQL query can be used in the select clause in the EPL query. And the example uses explicit stream names via the as keyword.

select customerId, customerName from
  CustomerCallEvent.win:time(30 sec) as cce,
  sql:MyCustomerDB ["select cust_id as customerId, cust_name as customerName from Customer 
                  where cust_id = ${cce.custId}"] as cq

Any window, such as the time window, generates insert stream (istream) events as events enter the window, and remove stream (rstream) events as events leave the window. The engine executes the given SQL query for each CustomerCallEvent in both the insert stream and the remove stream. As a performance optimization, the istream or rstream keywords in the select clause can be used to instruct the engine to only join insert stream or remove stream events, reducing the number of SQL query executions.

Since any expression may be placed within the ${...} syntax, you may use variables or user-defined functions as well.

The next example assumes that a variable by name varLowerLimit is defined and that a user-defined function getLimit exists on the MyLib imported class that takes a LimitEvent as a parameter:

select * from LimitEvent le, 
  sql:MyCustomerDB [' select cust_name from Customer where 
      amount > ${max(varLowerLimit, MyLib.getLimit(le))} ']

The example above takes the higher of the current variable value or the value returned by the user-defined function to return only those customer names where the amount exceeds the computed limit.

Consider using the EPL where clause to join the SQL query result to your event stream. Similar to EPL joins and outer-joins that join event streams or patterns, the EPL where clause provides join criteria between the SQL query results and the event stream (as a side note, an SQL where clause is a filter of rows executed by your database on your database server before returning SQL query results).

Esper analyzes the expression in the EPL where clause and outer-join on clause, if present, and builds the appropriate indexes from that information at runtime, to ensure fast matching of event stream events to SQL query results, even if your SQL query returns a large number of rows. Your applications must ensure to configure a cache for your database using Esper configuration, as such indexes are held with regular data in a cache. If you application does not enable caching of SQL query results, the engine does not build indexes on cached data.

The sample EPL statement below joins an event stream consisting of OrderEvent events with the results of an SQL query against the database named MyRefDB and table SymbolReference:

select symbol, symbolDesc from OrderEvent as orders,
  sql:MyRefDB ['select symbolDesc from SymbolReference'] as reference
  where reference.symbol = orders.symbol

Notice how the EPL where clause joins the OrderEvent stream to the SymbolReference table. In this example, the SQL query itself does not have a SQL where clause and therefore returns all rows from table SymbolReference.

If your application enables caching, the SQL query fires only at the arrival of the first OrderEvent event. When the second OrderEvent arrives, the join execution uses the cached query result. If the caching policy that you specified in the Esper database configuration evicts the SQL query result from cache, then the engine fires the SQL query again to obtain a new result and places the result in cache.

If SQL result caching is enabled and your EPL where clause, as show in the above example, provides the properties to join, then the engine indexes the SQL query results in cache and retains the index together with the query result in cache. Thus your application can benefit from high performance index-based lookups as long as the SQL query results are found in cache.

The SQL result caches operate on the level of all result rows for a given parameter set. For example, if your query returns 10 rows for a certain set of parameter values then the cache treats all 10 rows as a single entry keyed by the parameter values, and the expiry policy applies to all 10 rows and not to each individual row.

It is also possible to join multiple autonomous database systems in a single query, for example:

select symbol, symbolDesc from OrderEvent as orders,
  sql:My_Oracle_DB ['select symbolDesc from SymbolReference'] as reference,
  sql:My_MySQL_DB ['select orderList from orderHistory'] as history
  where reference.symbol = orders.symbol
  and history.symbol = orders.symbol 

Certain JDBC database drivers are known to not return metadata for precompiled prepared SQL statements. This can be a problem as metadata is required by Esper. Esper obtains SQL result set metadata to validate an EPL statement and to provide column types for output events. JDBC drivers that do not provide metadata for precompiled SQL statements require a workaround. Such drivers do generally provide metadata for executed SQL statements, however do not provide the metadata for precompiled SQL statements.

Please consult the Chapter 15, Configuration for the configuration options available in relation to metadata retrieval.

To obtain metadata for an SQL statement, Esper can alternatively fire a SQL statement which returns the same column names and types as the actual SQL statement but without returning any rows. This kind of SQL statement is referred to as a sample statement in below workaround description. The engine can then use the sample SQL statement to retrieve metadata for the column names and types returned by the actual SQL statement.

Applications can provide a sample SQL statement to retrieve metadata via the metadatasql keyword:

sql:database_name ["parameterized_sql_query" metadatasql "sql_meta_query"] 

The sql_meta_query must be an SQL statement that returns the same number of columns, the same type of columns and the same column names as the parameterized_sql_query, and does not return any rows.

Alternatively, applications can choose not to provide an explicit sample SQL statement. If the EPL statement does not use the metadatasql syntax, the engine applies lexical analysis to the SQL statement. From the lexical analysis Esper generates a sample SQL statement adding a restrictive clause "where 1=0" to the SQL statement.

Alternatively, you can add the following tag to the SQL statement: ${$ESPER-SAMPLE-WHERE}. If the tag exists in the SQL statement, the engine does not perform lexical analysis and simply replaces the tag with the SQL where clause "where 1=0". Therefore this workaround is applicable to SQL statements that cannot be correctly lexically analyzed. The SQL text after the placeholder is not part of the sample query. For example:

select mycol from sql:myDB [
  'select mycol from mytesttable ${$ESPER-SAMPLE-WHERE} where ....'], ...

If your parameterized_sql_query SQL query contains vendor-specific SQL syntax, generation of the metadata query may fail to produce a valid SQL statement. If you experience an SQL error while fetching metadata, use any of the above workarounds with the Oracle JDBC driver.

Your application may need to join data that originates from a web service, a distributed cache, an object-oriented database or simply data held in memory by your application. Esper accommodates this need by allowing a method invocation (or procedure call or function) in the from clause of a statement.

The results of such a method invocation in the from clause plays the same role as a relational database table in an inner and outer join in SQL. The role is thus dissimilar to the role of a user-defined function, which may occur in any expression such as in the select clause or the where clause. Both are backed by one or more static methods provided by your class library.

Esper can join and outer join an unlimited number and all types of event streams to the data returned by your method invocation. In addition, Esper can be configured to cache the data returned by your method invocations.

Joins or outer joins in which only SQL statements or method invocations are listed in the from clause and no other event streams are termed passive joins. A passive join does not produce an insert or remove stream and therefore does not invoke statement listeners with results. A passive join can be iterated on (polled) using a statement's safeIterator and iterator methods.

The following restrictions currently apply:

  • Sub-views on a method invocations are not allowed; That is, one cannot create a time or length window on a method invocation. However one can use the insert into syntax to make join results available to a further statement.

The syntax for a method invocation in the from clause of an EPL statement is:

method:class_name.method_name[(parameter_expressions)]

The method keyword denotes a method invocation. It is followed by a class name and a method name separated by a dot (.) character. If you have parameters to your method invocation, these are placed in round brackets after the method name. Any expression is allowed as a parameter, and individual parameter expressions are separated by a comma. Expressions may also use event properties of the joined stream.

In the sample join statement shown next, the method 'lookupAsset' provided by class 'MyLookupLib' returns one or more rows based on the asset id (a property of the AssetMoveEvent) that is passed to the method:

select * from AssetMoveEvent, method:MyLookupLib.lookupAsset(assetId)

The following statement demonstrates the use of the where clause to join events to the rows returned by a method invocation, which in this example does not take parameters:

select assetId, assetDesc from AssetMoveEvent as asset, 
       method:MyLookupLib.getAssetDescriptions() as desc 
where asset.assetid = desc.assetid

Your method invocation may return zero, one or many rows for each method invocation. If you have caching enabled through configuration, then Esper can avoid the method invocation and instead use cached results. Similar to SQL joins, Esper also indexes cached result rows such that join operations based on the where clause or outer-join on clause can be very efficient, especially if your method invocation returns a large number of rows.

If the time taken by method invocations is critical to your application, you may configure local caches as Section 15.4.7, “Cache Settings for From-Clause Method Invocations” describes.

Esper analyzes the expression in the EPL where clause and outer-join on clause, if present, and builds the appropriate indexes from that information at runtime, to ensure fast matching of event stream events to method invocation results, even if your method invocation returns a large number of rows. Your applications must ensure to configure a cache for your method invocation using Esper configuration, as such indexes are held with regular data in a cache. If you application does not enable caching of method invocation results, the engine does not build indexes on cached data.

Your application must provide a Java class that exposes a public static method. The method must accept the same number and type of parameters as listed in the parameter expression list.

If your method invocation returns either no row or only one row, then the return type of the method can be a Java class or a java.util.Map. If your method invocation can return more then one row, then the return type of the method must be an array of Java class or an array of Map.

If you are using a Java class or an array of Java class as the return type, then the class must adhere to JavaBean conventions: it must expose properties through getter methods.

If you are using java.util.Map as the return type or an array of Map, then the map should have String-type keys and object values (Map<String, Object>). When using Map as the return type, your application must provide a second method that returns property metadata, as the next section outlines.

Your application method must return either of the following:

As an example, consider the method 'getAssetDescriptions' provided by class 'MyLookupLib' as discussed earlier:

select assetId, assetDesc from AssetMoveEvent as asset,
       method:com.mypackage.MyLookupLib.getAssetDescriptions() as desc 
  where asset.assetid = desc.assetid

The 'getAssetDescriptions' method may return multiple rows and is therefore declared to return an array of the class 'AssetDesc'. The class AssetDesc is a POJO class (not shown here):

public class MyLookupLib {
  ...
  public static AssetDesc[] getAssetDescriptions() {
    ...
    return new AssetDesc[] {...};
  }

The example above specifies the full Java class name of the class 'MyLookupLib' class in the EPL statement. The package name does not need to be part of the EPL if your application imports the package using the auto-import configuration through the API or XML, as outlined in Section 15.4.6, “Class and package imports”.

Your application may return java.util.Map or an array of Map from method invocations. If doing so, your application must provide metadata about each row: it must declare the property name and property type of each Map entry of a row. This information allows the engine to perform type checking of expressions used within the statement.

You declare the property names and types of each row by providing a method that returns property metadata. The metadata method must follow these conventions:

In the following example, a class 'MyLookupLib' provides a method to return historical data based on asset id and asset code:

select assetId, location, x_coord, y_coord from AssetMoveEvent as asset,
       method:com.mypackage.MyLookupLib.getAssetHistory(assetId, assetCode) as history

A sample implementation of the class 'MyLookupLib' is shown below.

public class MyLookupLib {
  ...
  // For each column in a row, provide the property name and type
  //
  public static Map<String, Class> getAssetHistoryMetadata() {
    Map<String, Class> propertyNames = new HashMap<String, Class>();
    propertyNames.put("location", String.class);
    propertyNames.put("x_coord", Integer.class);
    propertyNames.put("y_coord", Integer.class);
    return propertyNames;
  }
... 
  // Lookup rows based on assetId and assetCode
  // 
  public static Map<String, Object>[] getAssetHistory(String assetId, String assetCode) {
    Map rows = new Map[2];	// this sample returns 2 rows
    for (int i = 0; i < 2; i++) {
      rows[i] = new HashMap();
      rows[i].put("location", "somevalue");
      rows[i].put("x_coord", 100);
      // ... set more values for each row
    }
    return rows;
  }

In the example above, the 'getAssetHistoryMetadata' method provides the property metadata: the names and types of properties in each row. The engine calls this method once per statement to determine event typing information.

The 'getAssetHistory' method returns an array of Map objects that are two rows. The implementation shown above is a simple example. The parameters to the method are the assetId and assetCode properties of the AssetMoveEvent joined to the method. The engine calls this method for each insert and remove stream event in AssetMoveEvent.

To indicate that no rows are found in a join, your application method may return either a null value or an array of size zero.

A named window is a global data window that can take part in many statement queries, and that can be inserted-into and deleted-from by multiple statements. A named window holds events of the same type or supertype, unless used with a variant stream.

The create window clause declares a new named window. The named window starts up empty unless populated from an existing named window at time of creation. Events must be inserted into the named window using the insert into clause. Events can also be deleted from a named window via the on delete clause.

Events enter the named window by means of insert into clause of a select statement. Events leave a named window either because the expiry policy of the declared data window removes events from the named window, or through statements that use the on delete clause to explicitly delete from a named window.

To query a named window, simply use the window name in the from clause of your statement, including statements that contain subqueries, joins and outer-joins.

A named window may also decorate an event to preserve original events as described in Section 5.10.4, “Decorated Events” and Section 5.15.2.1, “Named Windows Holding Decorated Events”. In addition, columns of a named window are allowed to hold events themselves, as further explained in Section 5.10.5, “Event as a Property” and Section 5.15.2.2, “Named Windows Holding Events As Property”.

To tune subquery performance when the subquery selects from a named window, consider the hints discussed in Section 5.11.8, “Hints Related to Subqueries”.

The create window statement creates a named window by specifying a window name and one or more data window views, as well as the type of event to hold in the named window.

There are two syntaxes for creating a named window: The first syntax allows to model a named window after an existing event type or an existing named window. The second syntax is similar to the SQL create-table syntax and provides a list of column names and column types.

A new named window starts up empty. It must be explicitly inserted into by one or more statements, as discussed below. A named window can also be populated at time of creation from an existing named window.

If your application stops or destroys the statement that creates the named window, any consuming statements no longer receive insert or remove stream events. The named window can also not be deleted from after it was stopped or destroyed.

The create window statement posts to listeners any events that are inserted into the named window as new data. The statement posts all deleted events or events that expire out of the data window to listeners as the remove stream (old data). The named window contents can also be iterated on via the pull API to obtain the current contents of a named window.

The benefit of modelling a named window after an existing event type is that event properties can be nested, indexed, mapped or other types that your event objects may provide as properties, including the type of the underlying event itself. Also, using the wildcard (*) operator means your EPL does not need to list each individual property explicitly.

The syntax for creating a named window by modelling the named window after an existing event type, is as follows:

[context context_name] 
create window window_name.view_specifications 
    [as] [select list_of_properties from] event_type_or_windowname
    [insert [where filter_expression]]

The window_name you assign to the named window can be any identifier. The name should not already be in use as an event type or stream name.

The view_specifications are one or more data window views that define the expiry policy for removing events from the data window. Named windows must explicitly declare a data window view. This is required to ensure that the policy for retaining events in the data window is well defined. To keep all events, use the keep-all view: It indicates that the named window should keep all events and only remove events from the named window that are deleted via the on delete clause. The view specification can only list data window views, derived-value views are not allowed since these don't represent an expiry policy. Data window views are listed in Chapter 12, EPL Reference: Views. View parameterization and staggering are described in Section 5.4.3, “Specifying Views”.

The select clause and list_of_properties are optional. If present, they specify the column names and, implicitly by definition of the event type, the column types of events held by the named window. Expressions other than column names are not allowed in the select list of properties. Wildcards (*) and wildcards with additional properties can also be used.

The event_type_or_windowname is required if using the model-after syntax. It provides the name of the event type of events held in the data window, unless column names and types have been explicitly selected via select. The name of an (existing) other named window is also allowed here. Please find more details in Section 5.15.7, “Populating a Named Window from an Existing Named Window”.

Finally, the insert clause and optional filter_expression are used if the new named windows is modelled after an existing named window, and the data of the new named window is to be populated from the existing named window upon creation. The optional filter_expression can be used to exclude events.

You may refer to a context by specifying the context keyword followed by a context name. Context are described in more detail at Chapter 4, Context and Context Partitions. The effect of referring to a context is that your named window operates according to the context dimensional information as declared for the context. For usage and limitations please see the respective chapter.

The next statement creates a named window 'AllOrdersNamedWindow' for which the expiry policy is simply to keep all events. Assume that the event type 'OrderMapEventType' has been configured. The named window is to hold events of type 'OrderMapEventType':

create window AllOrdersNamedWindow.win:keepall() as OrderMapEventType

The below sample statement demonstrates the select syntax. It defines a named window in which each row has the three properties 'symbol', 'volume' and 'price'. This named window actively removes events from the window that are older then 30 seconds.

create window OrdersTimeWindow.win:time(30 sec) as 
  select symbol, volume, price from OrderEvent

In an alternate form, the as keyword can be used to rename columns, and constants may occur in the select-clause as well:

create window OrdersTimeWindow.win:time(30 sec) as 
  select symbol as sym, volume as vol, price, 1 as alertId from OrderEvent

The second syntax for creating a named window is by supplying column names and types:

[context context_name] 
create window window_name.view_specifications [as] (column_name column_type 
  [,column_name column_type [,...])

The column_name is an identifier providing the event property name. The column_type is also required for each column. Valid column types are listed in Section 5.18.1, “Creating Variables: the Create Variable clause” and are the same as for variable types.

For attributes that are array-type append [] (left and right brackets).

The next statement creates a named window:

create window SecurityEvent.win:time(30 sec) 
    (ipAddress string, userId String, numAttempts int, properties String[])

Named window columns can hold events by declaring the column type as the event type name. Array-type in combination with event-type is also supported.

The next two statements declare an event type and create a named window with a column of the defined event type:

create schema SecurityData (name String, roles String[])
create window SecurityEvent.win:time(30 sec) 
    (ipAddress string, userId String, secData SecurityData, historySecData SecurityData[])

Whether the named window uses a Map or Object-array event representation for the rows can be specified as follows. If the create-window statement provides the @EventRepresentation(array=true) annotation the engine maintains named window rows as object array. If the statement provides the @EventRepresentation(array=false) annotation the engine maintains named window rows using Map objects. If neither annotation is provided, the engine uses the configured default event representation as discussed in Section 15.4.11.1, “Default Event Representation”.

The following EPL statement instructs the engine to represent FooWindow rows as object arrays:

@EventRepresentation(array=true) create window FooWindow.win:time(5 sec) as (string prop1)

The insert into clause inserts events into named windows. Your application must ensure that the column names and types match the declared column names and types of the named window to be inserted into.

For inserting into a named window and for simulateously checking if the inserted row already exists in the named window or for atomic update-insert operation on a named window, consider using on-merge as described in Section 5.15.12, “Triggered Upsert using the On-Merge Clause”. On-merge is similar to the SQL merge clause and provides what is known as an "Upsert" operation: Update existing events or if no existing event(s) are found then insert a new event, all in one atomic operation provided by a single EPL statement.

In this example we first create a named window using some of the columns of an OrderEvent event type:

create window OrdersWindow.win:keepall() as select symbol, volume, price from OrderEvent

The insert into the named window selects individual columns to be inserted:

insert into OrdersWindow(symbol, volume, price) select name, count, price from FXOrderEvent

An alternative form is shown next:

insert into OrdersWindow select name as symbol, vol as volume, price from FXOrderEvent

Following above statement, the engine enters every FXOrderEvent arriving into the engine into the named window 'OrdersWindow'.

The following EPL statements create a named window for an event type backed by a Java class and insert into the window any 'OrderEvent' where the symbol value is IBM:

create window OrdersWindow.win:time(30) as com.mycompany.OrderEvent
insert into OrdersWindow select * from com.mycompany.OrderEvent(symbol='IBM')

The last example adds one column named 'derivedPrice' to the 'OrderEvent' type by specifying a wildcard, and uses a user-defined function to populate the column:

create window OrdersWindow.win:time(30) as select *, price as derivedPrice from OrderEvent
insert into OrdersWindow select *, MyFunc.func(price, percent) as derivedPrice from OrderEvent

Event representations based on Java base classes or interfaces, and subclasses or implementing classes, are compatible as these statements show:

// create a named window for the base class
create window OrdersWindow.std:unique(name) as select * from ProductBaseEvent
// The ServiceProductEvent class subclasses the ProductBaseEvent class
insert into OrdersWindow select * from ServiceProductEvent
// The MerchandiseProductEvent class subclasses the ProductBaseEvent class
insert into OrdersWindow select * from MerchandiseProductEvent

To avoid duplicate events inserted in a named window and atomically check if a row already exists, use on-merge as outlined in Section 5.15.12, “Triggered Upsert using the On-Merge Clause”. An example:

on ServiceProductEvent as spe merge OrdersWindow as win
where win.id = spe.id when not matched then insert select *

Decorated events hold an underlying event and add additional properties to the underlying event, as described further in Section 5.10.4, “Decorated Events”.

Here we create a named window that decorates OrderEvent events by adding an additional property named priceTotal to each OrderEvent. A matching insert into statement is also part of the sample:

create window OrdersWindow.win:time(30) as select *, price as priceTotal from OrderEvent
insert into OrdersWindow select *, price * unit as priceTotal from ServiceOrderEvent

The property type of the additional priceTotal column is the property type of the existing price property of OrderEvent.

Columns in a named window may also hold an event itself. More information on the insert into clause providing event columns is in Section 5.10.5, “Event as a Property”.

The next sample creates a named window that specifies two columns: A column that holds an OrderEvent, and a column by name priceTotal. A matching insert into statement is also part of the sample:

create window OrdersWindow.win:time(30) as select this, price as priceTotal from OrderEvent
insert into OrdersWindow select order, price * unit as priceTotal  
from ServiceOrderEvent as order

Note that the this proprerty must exist on the event and must return the event class itself (JavaBean events only). The property type of the additional priceTotal column is the property type of the existing price property.

Your application can insert rows into a named window using on-demand (fire-and-forget, non-continuous) queries as described in Section 14.5, “On-Demand Fire-And-Forget Query Execution”.

The syntax for the insert into clause is as follows:

insert into window_name [(property_names)]
select value_expressions

The window_name is the name of the named window to insert events into.

After the named window name you can optionally provide a comma-separated list of event property names. When providing property names, the order of expressions in the select-clause must match the order of property names specified (select-clause property names are ignored). When not providing property names, the select-clause expressions must name the event properties to be inserted into by assigning a property name.

It follows the select keyword and a list of value expressions that provide values for the event properties of the event to be inserted into the named window.

The example code snippet inserts a new order event into the OrdersWindow named window:

String query = 
  "insert into OrdersWindow(orderId, symbol, price) select '001', 'GE', 100";
epService.getEPRuntime().executeQuery(query);

If you do not specify event property names, the select-clause expressions must name the event property names.

The following EPL inserts the same values as above but specifies property names as part of the select-clause expressions:

insert into OrdersWindow
select '001' as orderId, 'GE' as symbol, 100 as price

A named window can be referred to by any statement in the from clause of the statement. Filter criteria can also be specified. Additional views may be used onto named windows however such views cannot include data window views.

A statement selecting all events from a named window 'AllOrdersNamedWindow' is shown next. The named window must first be created via the create window clause before use.

select * from AllOrdersNamedWindow

The statement as above simply receives the unfiltered insert stream of the named window and reports that stream to its listeners. The iterator method returns returns all events in the named window, if any.

If your application desires to obtain the events removed from the named window, use the rstream keyword as this statement shows:

select rstream * from AllOrdersNamedWindow

The next statement derives an average price per symbol for the events held by the named window:

select symbol, avg(price) from AllOrdersNamedWindow group by symbol

A statement that consumes from a named window, like the one above, receives the insert and remove stream of the named window. The insert stream represents the events inserted into the named window. The remove stream represents the events expired from the named window data window and the events explicitly deleted via on-delete for on-demand (fire-and-forget) delete.

Your application may create a consuming statement such as above on an empty named window, or your application may create the above statement on an already filled named window. The engine provides correct results in either case: At the time of statement creation the Esper engine internally initializes the consuming statement from the current named window, also taking your declared filters into consideration. Thus, your statement deriving data from a named window does not start empty if the named window already holds one or more events. A consuming statement also sees the remove stream of an already populated named window, if any.

If you require a subset of the data in the named window, you can specify one or more filter expressions onto the named window as shown here:

select symbol, avg(price) from AllOrdersNamedWindow(sector='energy') group by symbol

By adding a filter to the named window, the aggregation and grouping as well as any views that may be declared onto to the named window receive a filtered insert and remove stream. The above statement thus outputs, continuously, the average price per symbol for all orders in the named window that belong to a certain sector.

A side note on variables in filters filtering events from named windows: The engine initializes consuming statements at statement creation time and changes aggregation state continuously as events arrive. If the filter criteria contain variables and variable values changes, then the engine does not re-evaluate or re-build aggregation state. In such a case you may want to place variables in the having clause which evaluates on already-built aggregation state.

The following example further declares a view into the named window. Such a view can be a plug-in view or one of the built-in views, but cannot be a data window view (with the exception of the std:groupwin grouped-window view which is allowed).

select * from AllOrdersNamedWindow(volume>0, price>0).mycompany:mypluginview()

Data window views cannot be used onto named windows since named windows post insert and remove streams for the events entering and leaving the named window, thus the expiry policy and batch behavior are well defined by the data window declared for the named window. For example, the following is not allowed and fails at time of statement creation:

// not a valid statement
select * from AllOrdersNamedWindow.win:time(30 sec)

The on select clause performs a one-time, non-continuous query on a named window every time a triggering event arrives or a triggering pattern matches. The query can consider all events in the named window, or only events that match certain criteria, or events that correlate with an arriving event or a pattern of arriving events.

The syntax for the on select clause is as follows:

on event_type[(filter_criteria)] [as stream_name]
[insert into insert_into_def]
select select_list
from window_name [as stream_name]
[where criteria_expression]
[group by grouping_expression_list]
[having grouping_search_conditions]
[order by order_by_expression_list]

The event_type is the name of the type of events that trigger the query against the named window. It is optionally followed by filter_criteria which are filter expressions to apply to arriving events. The optional as keyword can be used to assign an stream name. Patterns or named windows can also be specified in the on clause, see the samples in Section 5.15.10.1, “Using Patterns in the On Delete Clause” (for named windows only insert stream events trigger actions).

The insert into clause works as described in Section 5.10, “Merging Streams and Continuous Insertion: the Insert Into Clause”. The select clause is described in Section 5.3, “Choosing Event Properties And Events: the Select Clause”. For all clauses the semantics are equivalent to a join operation: The properties of the triggering event or events are available in the select clause and all other clauses.

The window_name in the from clause is the name of the named window to select events from. The as keyword is also available to assign a stream name to the named window. The as keyword is helpful in conjunction with wildcard in the select clause to select named window events via the syntax select streamname.* .

The optional where clause contains a criteria_expression that correlates the arriving (triggering) event to the events to be considered from the named window. The criteria_expression may also simply filter for events in the named window to be considered by the query.

The group by clause, the having clause and the order by clause are all optional and work as described in earlier chapters.

The similarities and differences between an on select clause and a regular or outer join are as follows:

  1. A join is evaluated when any of the streams participating in the join have new events (insert stream) or events leaving data windows (remove stream). A join is therefore bi-directional or multi-directional. However, the on select statement has one triggering event or pattern that causes the query to be evaluated and is thus uni-directional.

  2. The query within the on select statement is not continuous: It executes only when a triggering event or pattern occurs. Aggregation and groups are computed anew considering the contents of the named window at the time the triggering event arrives.

The iterator of the EPStatement object representing the on select clause returns the last batch of selected events in response to the last triggering event, or null if the last triggering event did not select any rows.

For correlated queries that correlate triggering events with events held by a named window, Esper internally creates efficient indexes to enable high performance querying of events. It analyzes the where clause to build one or more indexes for fast lookup in the named window based on the properties of the triggering event.

The next statement demonstrates the concept. Upon arrival of a QueryEvent event the statement selects all events in the 'OrdersNamedWindow' named window:

on QueryEvent select win.* from OrdersNamedWindow as win

The engine executes the query on arrival of a triggering event, in this case a QueryEvent. It posts the query results to any listeners to the statement, in a single invocation, as the new data array. By prefixing the wildcard (*) selector with the stream name, the select clause returns only events of the named window and does not also return triggering events.

The where clause filters and correlates events in the named window with the triggering event, as shown next:

on QueryEvent(volume>0) as query
select query.symbol, query.volume, win.symbol  from OrdersNamedWindow as win
where win.symbol = query.symbol

Upon arrival of a QueryEvent, if that event has a value for the volume property that is greater then zero, the engine executes the query. The query considers all events currently held by the 'OrdersNamedWindow' that match the symbol property value of the triggering QueryEvent event. The engine then posts query results to the statement's listeners.

Aggregation, grouping and ordering of results are possible as this example shows:

on QueryEvent as queryEvent
select symbol, sum(volume) from OrdersNamedWindow as win
group by symbol
having volume > 0
order by symbol

The above statement outputs the total volume per symbol for those groups where the sum of the volume is greater then zero, ordered by symbol ascending. The engine computes and posts the output based on the current contents of the 'OrdersNamedWindow' named window considering all events in the named window, since the query does not have a where clause.

When using wildcard (*) to select from streams in an on-select clause, each stream, that is the the triggering stream and the selected-upon named window, are selected, similar to a join. Therefore your wildcard select returns two columns: the triggering event and the selection result event, for each row.

on QueryEvent as queryEvent
select * from OrdersNamedWindow as win

The query above returns a queryEvent column and a win column for each event. If only a single stream's event is desired in the result, use select win.* instead.

To trigger an on-select when an update to the selected named window occurs or when the triggering event is the same event that is being inserted into the named window, specify the named window name as the event type.

The next query fires the select for every change to the named window OrdersNamedWindow:

on OrdersNamedWindow as trig 
select onw.symbol, sum(onw.volume) 
from OrdersNamedWindow as onw 
where onw.symbol = trig.symbol

An on update clause updates events held by a named window. The clause can be used to update all events, or only events that match certain criteria, or events that correlate with an arriving event or a pattern of arriving events.

For updating a named window and for simulateously checking if the updated row exists in the named window or for atomic update-insert operation on a named window, consider using on-merge as described in Section 5.15.12, “Triggered Upsert using the On-Merge Clause”. On-merge is similar to the SQL merge clause and provides what is known as an "Upsert" operation: Update existing events or if no existing event(s) are found then insert a new event, all in one atomic operation provided by a single EPL statement.

The syntax for the on update clause is as follows:

on event_type[(filter_criteria)] [as stream_name]
update window_name [as stream_name]
set mutation_expression [, mutation_expression [,...]]
[where criteria_expression]

The event_type is the name of the type of events that trigger an update of rows in a named window. It is optionally followed by filter_criteria which are filter expressions to apply to arriving events. The optional as keyword can be used to assign a name for use in expressions and the where clause. Patterns and named windows can also be specified in the on clause.

The window_name is the name of the named window to update events. The as keyword is also available to assign a name to the named window.

After the set keyword follows a list of comma-separated mutation_expression expressions. A mutation expression is any valid EPL expression. Subqueries may by part of expressions however aggregation functions and the prev or prior function may not be used in expressions.

The below table shows some typical mutation expessions:


The optional where clause contains a criteria_expression that correlates the arriving (triggering) event to the events to be updated in the named window. The criteria_expression may also simply filter for events in the named window to be updated.

The iterator of the EPStatement object representing the on update clause can also be helpful: It returns the last batch of updated events in response to the last triggering event, in any order, or null if the last triggering event did not update any rows.

Statements that reference the named window receive the new event in the insert stream and the event prior to the update in the remove stream.

Let's look at a couple of examples. In the simplest form, this statement updates all events in the named window 'AllOrdersNamedWindow' when any 'UpdateOrderEvent' event arrives, setting the price property to zero for all events currently held by the named window:

on UpdateOrderEvent update AllOrdersNamedWindow set price = 0

This example adds a where clause to the example above. Upon arrival of a triggering 'ZeroVolumeEvent', the statement updates prices on any orders that have a volume of zero or less:

on ZeroVolumeEvent update AllOrdersNamedWindow set price = 0 where volume <= 0

The next example shows a more complete use of the syntax, and correlates the triggering event with events held by the named window:

on NewOrderEvent(volume>0) as myNewOrders
update AllOrdersNamedWindow as myNamedWindow 
set price = myNewOrders.price
where myNamedWindow.symbol = myNewOrders.symbol

In the above sample statement, only if a 'NewOrderEvent' event with a volume greater then zero arrives does the statement trigger. Upon triggering, all events in the named window that have the same value for the symbol property as the triggering 'NewOrderEvent' event are then updated (their price property is set to that of the arriving event). The statement also showcases the as keyword to assign a name for use in the where expression.

For correlated queries (as above) that correlate triggering events with events held by a named window, Esper internally creates efficient indexes to enable high performance update of events.

Your application can subscribe a listener to your on update statements to determine update events. The statement post any events that are updated to all listeners attached to the statement as new data, and the events prior to the update as old data. Upon iteration, the statement provides the last update event, if any.

The following example shows the use of tags and a pattern. It sets the price value of orders to that of either a 'FlushOrderEvent' or 'OrderUpdateEvent' depending on which arrived:

on pattern [every ord=OrderUpdateEvent(volume>0) or every flush=FlushOrderEvent] 
update AllOrdersNamedWindow as win
set price = case when ord.price is null then flush.price else ord.price end
where ord.id = win.id or flush.id = win.id

When updating indexed properties use the syntax propertyName[index] = value with the index value being an integer number. When updating mapped properties use the syntax propertyName(key) = value with the key being a string value.

The engine executes assignments in the order they are listed. When performing multiple assignments, the engine takes the most recent named window event property value according to the last assignment, if any. To instruct the engine to use the initial property value before update, prefix the event property name with the literal initial.

The following statement illustrates:

on UpdateEvent as upd
update MyWindow as win
set field_a = 1, 
  field_b = win.field_a, // assigns the value 1 
  field_c = initial.field_a // assigns the field_a original value before update

The next example assumes that your application provides a user-defined function copyFields that receives 3 parameters: The update event, the new row and the initial state before-update row.

on UpdateEvent as upd update MyWindow as win set copyFields(win, upd, initial)

The following example assumes that your event type provides a method by name populateFrom that receives the update event as a parameter:

on UpdateEvent as upd update MyWindow as win set win.populateFrom(upd)

The following restrictions apply:

  1. Each property to be updated via assignment must be writable.

  2. For underlying event representations that are Java objects, a event object class must implement the java.io.Serializable interface as discussed in Section 5.21.1, “Immutability and Updates” and must provide setter methods for updated properties.

  3. When using an XML underlying event type, event properties in the XML document representation are not available for update.

  4. Nested properties are not supported for update. Revision event types and variant streams may also not be updated.

Your application can update named window rows using on-demand (fire-and-forget, non-continuous) queries as described in Section 14.5, “On-Demand Fire-And-Forget Query Execution”.

The syntax for the update clause is as follows:

update window_name [as stream_name]
set mutation_expression [, mutation_expression [,...]]
[where criteria_expression]

The window_name is the name of the named window to delete events from. The as keyword is also available to assign a name to the named window.

After the set keyword follows a comma-separated list of mutation expressions. For fire-and-forget queries the following restriction applies: Subqueries, aggregation functions and the prev or prior function may not be used in expressions. Mutation expressions are detailed in Section 5.15.8, “Updating Named Windows: the On Update clause”.

The optional where clause contains a criteria_expression that identifies events to be updated.

The example code snippet updates those rows of the named window that have a negative value for volume:

String query = "update AllOrdersNamedWindow set volume = 0 where volumne = 0";
epService.getEPRuntime().executeQuery(query);

To instruct the engine to use the initial property value before update, prefix the event property name with the literal initial.

An on delete clause removes events from a named window. The clause can be used to remove all events, or only events that match certain criteria, or events that correlate with an arriving event or a pattern of arriving events.

The syntax for the on delete clause is as follows:

on event_type[(filter_criteria)] [as stream_name]
delete from window_name [as stream_name]
[where criteria_expression]

The event_type is the name of the type of events that trigger removal from the named window. It is optionally followed by filter_criteria which are filter expressions to apply to arriving events. The optional as keyword can be used to assign a name for use in the where clause. Patterns and named windows can also be specified in the on clause as described in the next section.

The window_name is the name of the named window to delete events from. The as keyword is also available to assign a name to the named window.

The optional where clause contains a criteria_expression that correlates the arriving (triggering) event to the events to be removed from the named window. The criteria_expression may also simply filter for events in the named window to be removed from the named window.

The iterator of the EPStatement object representing the on delete clause can also be helpful: It returns the last batch of deleted events in response to the last triggering event, in any order, or null if the last triggering event did not remove any rows.

Let's look at a couple of examples. In the simplest form, this statement deletes all events from the named window 'AllOrdersNamedWindow' when any 'FlushOrderEvent' arrives:

on FlushOrderEvent delete from AllOrdersNamedWindow

This example adds a where clause to the example above. Upon arrival of a triggering 'ZeroVolumeEvent', the statement removes from the named window any orders that have a volume of zero or less:

on ZeroVolumeEvent delete from AllOrdersNamedWindow where volume <= 0

The next example shows a more complete use of the syntax, and correlates the triggering event with events held by the named window:

on NewOrderEvent(volume>0) as myNewOrders
delete from AllOrdersNamedWindow as myNamedWindow 
where myNamedWindow.symbol = myNewOrders.symbol

In the above sample statement, only if a 'NewOrderEvent' event with a volume greater then zero arrives does the statement trigger. Upon triggering, all events in the named window that have the same value for the symbol property as the triggering 'NewOrderEvent' event are then removed from the named window. The statement also showcases the as keyword to assign a name for use in the where expression.

For correlated queries (as above) that correlate triggering events with events held by a named window, Esper internally creates efficient indexes to enable high performance removal of events especially from named windows that hold large numbers of events.

Your application can subscribe a listener to your on delete statements to determine removed events. The statement post any events that are deleted from a named window to all listeners attached to the statement as new data. Upon iteration, the statement provides the last deleted event, if any.

Your application can delete rows from a named window using on-demand (fire-and-forget, non-continuous) queries as described in Section 14.5, “On-Demand Fire-And-Forget Query Execution”.

The syntax for the delete clause is as follows:

delete from window_name [as stream_name]
[where criteria_expression]

The window_name is the name of the named window to delete events from. The as keyword is also available to assign a name to the named window.

The optional where clause contains a criteria_expression that identifies events to be removed from the named window.

The example code snippet deletes from a named window all rows that have a negative value for volume:

String query = "delete from AllOrdersNamedWindow where volume <= 0";
epService.getEPRuntime().executeQuery(query);

The on merge clause is similar to the SQL merge clause. It provides what is known as an "Upsert" operation: Update existing events or if no existing event(s) are found then insert a new event, all in an atomic operation provided by a single EPL statement.

The syntax for the on merge clause is as follows:

on event_type[(filter_criteria)] [as stream_name]
merge [into] window_name [as stream_name]
[where criteria_expression]
  when [not] matched [and search_condition]
    then [
      insert [into streamname]
          [ (property_name [, property_name] [,...]) ] 
          select select_expression [, select_expression[,...]]
          [where filter_expression]
      |
      update set mutation_expression [, mutation_expression [,...]]
          [where filter_expression]
      |
      delete
          [where filter_expression]
    ]
    [then [insert|update|delete]] [,then ...]
  [when ...  then ... [...]] 

The event_type is the name of the type of events that trigger the merge. It is optionally followed by filter_criteria which are filter expressions to apply to arriving events. The optional as keyword can be used to assign a name for use in the where clause. Patterns and named windows can also be specified in the on clause as described in prior sections.

The window_name is the name of the named window to insert, update or delete events. The as keyword is also available to assign a name to the named window.

The optional where clause contains a criteria_expression that correlates the arriving (triggering) event to the events to be considered of the named window. We recommend specifying a criteria expression that is as specific as possible.

Following the where clause is one or more when matched or when not matched clauses in any order. Each may have an additional search condition associated.

After each when [not] matched follow one or more then clauses that each contain the action to take: Either an insert, update or delete keyword.

After when not matched only insert action(s) are available. After when matched any insert, update and delete action(s) are available.

After insert follows, optionally, the into keyword followed by the stream name or named window to insert-into. If no into and stream name is specified, the insert applies to the current named window. It follows an optional list of columns inserted. It follows the required select keyword and one or more select-clause expressions. The wildcard (*) is available in the select-clause as well. It follows an optional where-clause that may return Boolean false to indicate that the action should not be applied.

After update follows the set keyword and one or more mutation expressions. For mutation expressions please see Section 5.15.8, “Updating Named Windows: the On Update clause”. It follows an optional where-clause that may return Boolean false to indicate that the action should not be applied.

After delete follows an optional where-clause that may return Boolean false to indicate that the action should not be applied.

When according to the where-clause criteria_expression the engine finds no events in the named window that match the condition, the engine evaluates each when not matched clause. If the optional search condition returns true or no search condition was provided then the engine performs all of the actions listed after each then.

When according to the where-clause criteria_expression the engine finds one or more events in the named window that match the condition, the engine evaluates each when matched clause. If the optional search condition returns true or no search condition was provided the engine performs all of the actions listed after each then.

The engine executes when matched and when not matched in the order specified. If the optional search condition returns true or no search condition was specified then the engine takes the associated action (or multiple actions for multiple then keywords). When the block of actions completed the engine proceeds to the next matching event, if any. After completing all matching events the engine continues to the next triggering event if any.

In the first example we declare a schema that provides a product id and that holds a total price:

create schema ProductTotalRec as (productId string, totalPrice double)

We create a named window that holds a row for each unique product:

create window ProductWindow.std:unique(productId) as ProductTotalRec

The events for this example are order events that hold an order id, product id, price, quantity and deleted-flag declared by the next schema:

create schema OrderEvent as (orderId string, productId string, price double, 
    quantity int, deletedFlag boolean)

The following EPL statement utilizes on-merge to total up the price for each product based on arriving order events:

on OrderEvent oe
  merge ProductWindow pw
  where pw.productId = oe.productId
  when matched
    then update set totalPrice = totalPrice + oe.price
  when not matched 
    then insert select productId, price as totalPrice

In the above example, when an order event arrives, the engine looks up in the product named window the matching row or rows for the same product id as the arriving event. In this example the engine always finds no row or one row as the product named window is declared with a unique data window based on product id. If the engine finds a row in the named window, it performs the update action adding up the price as defined under when matched. If the engine does not find a row in the named window it performs the insert action as defined under when not matched, inserting a new row.

The insert keyword may be followed by a list of columns as shown in this EPL snippet:

// equivalent to the insert shown in the last 2 lines in above EPL
...when not matched 
    then insert(productId, totalPrice) select productId, price

The second example demonstrates the use of a select-clause with wildcard, a search condition and the delete keyword. It creates a named window that holds order events and employs on-merge to insert order events for which no corresponding order id was found, update quantity to the quantity provided by the last arriving event and delete order events that are marked as deleted:

create window OrderWindow.win:keepall() as OrderEvent
on OrderEvent oe
  merge OrderWindow pw
  where pw.orderId = oe.orderId
  when not matched 
    then insert select *
  when matched and oe.deletedFlag=true
    then delete
  when matched
    then update set pw.quantity = oe.quantity, pw.price = oe.price

In the above example the oe.deletedFlag=true search condition instructs the engine to take the delete action only if the deleted-flag is set.

You may specify multiple actions by providing multiple then keywords each followed by an action. Each of the insert, update and delete actions can itself have a where-clause as well. If a where-clause exists for an action, the engine evaluates the where-clause and applies the action only if the where-clause returns Boolean true.

This example specifies two update actions and uses the where-clause to trigger different update behavior depending on whether the order event price is less than zero. This example assumes that the host application defined a clearorder user-defined function, to demonstrate calling a user-defined function as part of the update mutation expressions:

on OrderEvent oe
  merge OrderWindow pw
  where pw.orderId = oe.orderId
  when matched
    then update set clearorder(pw) where oe.price < 0
    then update set pw.quantity = oe.quantity, pw.price = oe.price where oe.price >= 0

To insert events into another stream and not the named window, use insert into streamname.

In the next example each matched-clause contains two actions, one action to insert a log event and a second action to insert, delete or update:

on OrderEvent oe
  merge OrderWindow pw
  where pw.orderId = oe.orderId
  when not matched 
    then insert into LogEvent select 'this is an insert' as name
    then insert select *
  when matched and oe.deletedFlag=true
    then insert into LogEvent select 'this is a delete' as name
    then delete
  when matched
    then insert into LogEvent select 'this is a update' as name
    then update set pw.quantity = oe.quantity, pw.price = oe.price

While the engine evaluates and executes all actions listed under the same matched-clause in order, you may not rely on updated field values of an earlier action to trigger the where-clause of a later action. Similarly you should avoid simultaneous update and delete actions for the same match: the engine does not guarantee whether the update or the delete take final affect.

For correlated queries (as above) that correlate triggering events with events held by a named window, Esper internally creates efficient indexes to enable high performance update and removal of events especially from named windows that hold large numbers of events.

Your application can subscribe a listener to on merge statements to determine inserted, updated and removed events. Statements post any events that are inserted to, updated or deleted from a named window to all listeners attached to the statement as new data and removed data. Upon iteration, the statement provides the last inserted events, if any.

The following limitations apply to on-merge statements:

  1. Aggregation functions and the prev and prior operators are not available in conditions and the select-clause.

You may explicitly create an index on a named window. The engine considers explicitly-created as well as implicitly-allocated indexes in query planning and execution of the following types of usages of named windows:

  1. On-demand (fire-and-forget, non-continuous) queries as described in Section 14.5, “On-Demand Fire-And-Forget Query Execution”.

    On-select, on-merge, on-update, on-delete and on-insert.

    Subqueries against named windows.

    For joins (including outer joins) with named windows the engine considers the filter criteria listed in parenthesis using the syntax

    name_window_name(filter_criteria)

    for index access.

Please use the following syntax to create an explicit index on a named window:

create [unique] index index_name on named_window_name (property [hash|btree] 
    [, property] [hash|btree] [,...] )

The optional unique keyboard indicates that the property or properties uniquely identify rows. If unique is not specified the index allows duplicate rows.

The index_name is the name assigned to the index. The name uniquely identifies the index and is used in engine query plan logging.

The named_window_name is the name of an existing named window. If the named window has data already, the engine builds an index for the data in the named window.

The list of property names are the properties of events within the named window to include in the index. Following each property name you may specify the optional hash or btree keyword.

If you specify no keyword or the hash keyword for a property, the index will be a hash-based (unsorted) index in respect to that property. If you specify the btree keyword, the index will be a binary-tree-based sorted index in respect to that property. You may combine hash and btree properties for the same index. Specify btree for a property if you expect to perform numerical or string comparison using relational operators (<, >, >=, <=), the between or the in keyword for ranges and inverted ranges. Use hash (the default) instead of btree if you expect to perform exact comparison using =.

We list a few example EPL statements next that create a named window and create a single index:

// create a named window
create window UserProfileWindow.win:time(1 hour) select * from UserProfile
// create a non-unique index (duplicates allowed) for the user id property only
create index UserProfileIndex on UserProfileWindow(userId)

Next, execute an on-demand fire-and-forget query as shown below, herein we use the prepared version to demonstrate:

String query = "select * from UserProfileWindow where userId='Joe'";
EPOnDemandPreparedQuery prepared = epRuntime.prepareQuery(query);
// query performance excellent in the face of large number of rows
EPOnDemandQueryResult result = prepared.execute();
// ...later ...
prepared.execute();	// execute a second time

A unique index is generally preferable over non-unique indexes. If your data window declares a unique data window (std:unique, std:firstunique, including intersections and grouped unique data windows) it is not necessary to create a unique index unless index sharing is enabled, since the engine considers the unique data window declaration in query planning.

The engine enforces uniqueness (e.g. unique constraint) for unique indexes. If your application inserts a duplicate row the engine raises a runtime exception when processing the statement and discards the row. The default error handler logs such an exception and continues.

For example, if the user id together with the profile id uniquely identifies an entry into the named window, your application can create a unique index as shown below:

// create a unique index on user id and profile id
create unique index UserProfileIndex on UserProfileWindow(userId, profileId)

By default, the engine builds a hash code -based index useful for direct comparison via equals (=). Filter expressions that look for ranges or use in, between do not benefit from the hash-based index and should use the btree keyword. For direct comparison via equals (=) then engine does not use btree indexes.

The next example creates a composite index over two fields symbol and buyPrice:

// create a named window
create window TickEventWindow.win:time(1 hour) as (symbol string, buyPrice double)
// create a non-unique index 
create index idx1 on TickEventWindow(symbol hash, buyPrice btree)

A sample fire-and-forget query is shown below (this time the API calls are not shown):

// query performance excellent in the face of large number of rows
select * from TickEventWindow where symbol='GE' and buyPrice between 10 and 20

As outlined in Section 2.10, “Updating, Merging and Versioning Events”, revision event types process updates or new versions of events held by a named window.

A revision event type is simply one or more existing pre-configured event types whose events are related, as configured by static configuration, by event properties that provide same key values. The purpose of key values is to indicate that arriving events are related: An event amends, updates or adds properties to an earlier event that shares the same key values. No additional EPL is needed when using revision event types for merging event data.

Revision event types can be useful in these situations:

  1. Some of your events carry only partial information that is related to a prior event and must be merged together.

  2. Events arrive that add additional properties or change existing properties of prior events.

  3. Events may carry properties that have null values or properties that do no exist (for example events backed by Map or XML), and for such properties the earlier value must be used instead.

To better illustrate, consider a revision event type that represents events for creation and updates to user profiles. Lets assume the user profile creation events carry the user id and a full profile. The profile update events indicate only the user id and the individual properties that actually changed. The user id property shall serve as a key value relating profile creation events and update events.

A revision event type must be configured to instruct the engine which event types participate and what their key properties are. Configuration is described in Section 15.4.26, “Revision Event Type” and is not shown here.

Assume that an event type UserProfileRevisions has been configured to hold profile events, i.e. creation and update events related by user id. This statement creates a named window to hold the last 1 hour of current profiles per user id:

create window UserProfileWindow.win:time(1 hour) select * from UserProfileRevisions
insert into UserProfileWindow select * from UserProfileCreation
insert into UserProfileWindow select * from UserProfileUpdate

In revision event types, the term base event is used to describe events that are subject to update. Events that update, amend or add additional properties to base events are termed delta events. In the example, base events are profile creation events and delta events are profile update events.

Base events are expected to arrive before delta events. In the case where a delta event arrives and is not related by key value to a base event or a revision of the base event currently held by the named window the engine ignores the delta event. Thus, considering the example, profile update events for a user id that does not have an existing profile in the named window are not applied.

When a base or delta event arrives, the insert and remove stream output by the named window are the current and the prior version of the event. Let's come back to the example. As creation events arrive that are followed by update events or more creation events for the same user id, the engine posts the current version of the profile as insert stream (new data) and the prior version of the profile as remove stream (old data).

Base events are also implicitly delta events. That is, if multiple base events of the same key property values arrive, then each base event provides a new version. In the example, if multiple profile creation events arrive for the same user id then new versions of the current profile for that user id are output by the engine for each base event, as it does for delta events.

The expiry policy as specified by view definitions applies to each distinct key value, or multiple distinct key values for composite keys. An expiry policy re-evaluates when new versions arrive. In the example, user profile events expire from the time window when no creation or update event for a given user id has been received for 1 hour.

Tip

It usually does not make sense to configure a revision event type without delta event types. Use the unique data window (std:unique) or unique data window in intersection with other data windows instead (i.e. std:unique(field).win:time(1 hour)).

Several strategies are available for merging or overlaying events as the configuration chapter describes in greater detail.

Any of the Map, XML and JavaBean event representations as well as plug-in event representations may participate in a revision event type. For example, profile creation events could be JavaBean events, while profile update events could be java.util.Map events.

Delta events may also add properties to the revision event type. For example, one could add a new event type with security information to the revision event type and such security-related properties become available on the resulting revision event type.

The following restrictions apply to revision event types:

  • Nested properties are only supported for the JavaBean event representation. Nested properties are not individually versioned; they are instead versioned by the containing property.

  • Dynamic, indexed and mapped properties are only supported for nested properties and not as properties of the revision event type itself.

EPL allows declaring an event type via the create schema clause and also by means of the static or runtime configuration API addEventType functions. The term schema and event type has the same meaning in EPL.

Your application can declare an event type by providing the property names and types or by providing a class name. Your application may also declare a variant stream schema.

When using the create schema syntax to declare an event type, the engine automatically removes the event type when there are no started statements referencing the event type, including the statement that declared the event type. When using the configuration API, the event type stays cached even if there are no statements that refer to the event type and until explicitly removed via the runtime configuration API.

The synopsis of the create schema syntax providing property names and types is:

create [map | objectarray] schema schema_name [as] 
    (property_name property_type [,property_name property_type [,...])
  [inherits inherited_event_type[, inherited_event_type] [,...]]
  [starttimestamp timestamp_property_name]
  [endtimestamp timestamp_property_name]
  [copyfrom copy_type_name [, copy_type_name] [,...]]

The create keyword can be followed by map to instruct the engine to represent events of that type by the Map event representation, or objectarray to denote an Object-array event type. If neither the map or objectarray keywords are provided, the engine-wide default event representation applies.

After create schema follows a schema_name. The schema name is the event type name.

The property_name is an identifier providing the event property name. The property_type is also required for each property. Valid property types are listed in Section 5.18.1, “Creating Variables: the Create Variable clause” and in addition include:

  1. Any Java class name, fully-qualified or the simple class name if imports are configured.

  2. Add left and right square brackets [] to any type to denote an array-type event property.

  3. Use an event type name as a property type.

The optional inherits keywords is followed by a comma-separated list of event type names that are the supertypes to the declared type.

The optional starttimestamp keyword is followed by a property name. Use this to tell the engine that your event has a timestamp. The engine checks that the property name exists on the declared type and returns a date-time value. Declare a timestamp property if you want your events to implicitly carry a timestamp value for convenient use with interval algebra methods as a start timestamp.

The optional endtimestamp keyword is followed by a property name. Use this together with starttimestamp to tell the engine that your event has a duration. The engine checks that the property name exists on the declared type and returns a date-time value. Declare an endtimestamp property if you want your events to implicitly carry a duration value for convenient use with interval algebra methods.

The optional copyfrom keyword is followed by a comma-separate list of event type names. For each event type listed, the engine looks up that type and adds all event property definitions to the newly-defined type, in addition to those listed explicitly (if any).

A few example event type declarations follow:

// Declare type SecurityEvent
create schema SecurityEvent as (ipAddress string, userId String, numAttempts int)
			
// Declare type AuthorizationEvent with the roles property being an array of String 
// and the hostinfo property being a POJO object
create schema AuthorizationEvent(group String, roles String[], hostinfo com.mycompany.HostNameInfo)

// Declare type CompositeEvent in which the innerEvents property is an array of SecurityEvent
create schema CompositeEvent(group String, innerEvents SecurityEvent[])

// Declare type WebPageVisitEvent that inherits all properties from PageHitEvent
create schema WebPageVisitEvent(userId String) inherits PageHitEvent

// Declare a type with start and end timestamp (i.e. event with duration).
create schema RoboticArmMovement (robotId string, startts long, endts long) 
  starttimestamp startts endtimestamp endts
  
// Create a type that has all properties of SecurityEvent plus a userName property
create schema ExtendedSecurityEvent (userName string) copyfrom SecurityEvent

// Create a type that has all properties of SecurityEvent 
create schema SimilarSecurityEvent () copyfrom SecurityEvent

// Create a type that has all properties of SecurityEvent and WebPageVisitEvent plus a userName property
create schema WebSecurityEvent (userName string) copyfrom SecurityEvent, WebPageVisitEvent

To elaborate on the inherits keyword, consider the following two schema definitions:

create schema Foo as (string prop1)
create schema Bar() inherits Foo

Following above schema, Foo is a supertype or Bar and therefore any Bar event also fulfills Foo and matches where Foo matches. An EPL statement such as select * from Foo returns any Foo event as well as any event that is a subtype of Foo such as all Bar events. When your EPL queries don't use any Foo events there is no cost, thus inherits is generally an effective way to share properties between types. The start and end timestamp are also inherited from any supertype that has the timestamp property names defined.

The optional copyfrom keyword is for defining a schema based on another schema. This keyword causes the engine to copy property definitions: There is no inherits, extends, supertype or subtype relationship between the types listed.

To define an event type Bar that has the same properties as Foo:

create schema Foo as (string prop1)
create schema Bar() copyfrom Foo

To define an event type Bar that has the same properties as Foo and that adds its own property prop2:

create schema Foo as (string prop1)
create schema Bar(string prop2) copyfrom Foo

If neither the map or objectarray keywords are provided, and if the create-schema statement provides the @EventRepresentation(array=true) annotation the engine expects object array events. If the statement provides the @EventRepresentation(array=false) annotation the engine expects Map objects as events. If neither annotation is provided, the engine uses the configured default event representation as discussed in Section 15.4.11.1, “Default Event Representation”.

The following two EPL statements both instructs the engine to represent Foo events as object arrays. When sending Foo events into the engine use the sendEvent(Object[] data, String typeName) footprint.

create objectarray schema Foo as (string prop1)
@EventRepresentation(array=true) create schema Foo as (string prop1)

The next two EPL statements both instructs the engine to represent Foo events as Maps. When sending Foo events into the engine use the sendEvent(Map data, String typeName) footprint.

create map schema Foo as (string prop1)
@EventRepresentation(array=false) create schema Foo as (string prop1)

A variant stream is a predefined stream into which events of multiple disparate event types can be inserted. Please see Section 5.10.3, “Merging Disparate Types of Events: Variant Streams” for rules regarding property visibility and additional information.

The synopsis is:

create variant schema schema_name [as] eventtype_name|* [, eventtype_name|*] [,...]

Provide the variant keyword to declare a variant stream.

The '*' wildcard character declares a variant stream that accepts any type of event inserted into the variant stream.

Provide eventtype_name if the variant stream should hold events of the given type only. When using insert into to insert into the variant stream the engine checks to ensure the inserted event type or its supertypes match the required event type.

A few examples are shown below:

// Create a variant stream that accepts only LoginEvent and LogoutEvent event types
create variant schema SecurityVariant as LoginEvent, LogoutEvent

// Create a variant stream that accepts any event type
create variant schema AnyEvent as *

EPL offers a convenient syntax to splitting, routing or duplicating events into multiple streams, and for receiving unmatched events among a set of filter criteria.

For splitting a single event that acts as a container and expose child events as a property of itself consider the contained-event syntax as described in Section 5.20, “Contained-Event Selection”.

You may define a triggering event or pattern in the on-part of the statement followed by multiple insert into, select and where clauses.

The synopsis is:

[context context_name]
on event_type[(filter_criteria)] [as stream_name]
insert into insert_into_def select select_list [where condition]
[insert into insert_into_def select select_list [where condition]]
[insert into...]
[output first | all]

The event_type is the name of the type of events that trigger the split stream. It is optionally followed by filter_criteria which are filter expressions to apply to arriving events. The optional as keyword can be used to assign a stream name. Patterns and named windows can also be specified in the on clause.

Following the on-clause is one or more insert into clauses as described in Section 5.10, “Merging Streams and Continuous Insertion: the Insert Into Clause” and select clauses as described in Section 5.3, “Choosing Event Properties And Events: the Select Clause”.

Each select clause may be followed by a where clause containing a condition. If the condition is true for the event, the engine transforms the event according to the select clause and inserts it into the corresponding stream.

At the end of the statement can be an optional output clause. By default the engine inserts into the first stream for which the where clause condition matches if one was specified, starting from the top. If you specify the output all keywords, then the engine inserts into each stream (not only the first stream) for which the where clause condition matches or that do not have a where clause.

If, for a given event, none of the where clause conditions match, the statement listener receives the unmatched event. The statement listener only receives unmatched events and does not receive any transformed or inserted events. The iterator method to the statement returns no events.

You may specify an optional context name to the effect that the split-stream operates according to the context dimensional information as declared for the context. See Chapter 4, Context and Context Partitions for more information.

In the below sample statement, the engine inserts each OrderEvent into the LargeOrders stream if the order quantity is 100 or larger, or into the SmallOrders stream if the order quantity is smaller then 100:

on OrderEvent 
  insert into LargeOrders select * where orderQty >= 100
  insert into SmallOrders select *

The next example statement adds a new stream for medium-sized orders. The new stream receives orders that have an order quantity between 20 and 100:

on OrderEvent 
  insert into LargeOrders select orderId, customer where orderQty >= 100
  insert into MediumOrders select orderId, customer where orderQty between 20 and 100
  insert into SmallOrders select orderId, customer where orderQty > 0

As you may have noticed in the above statement, orders that have an order quantity of zero don't match any of the conditions. The engine does not insert such order events into any stream and the listener to the statement receives these unmatched events.

By default the engine inserts into the first insert into stream without a where clause or for which the where clause condition matches. To change the default behavior and insert into all matching streams instead (including those without a where clause), the output all keywords may be added to the statement.

The sample statement below shows the use of the output all keywords. The statement populates both the LargeOrders stream with large orders as well as the VIPCustomerOrders stream with orders for certain customers based on customer id:

on OrderEvent 
  insert into LargeOrders select * where orderQty >= 100
  insert into VIPCustomerOrders select * where customerId in (1001, 1002)
  output all

Since the output all keywords are present, the above statement inserts each order event into either both streams or only one stream or none of the streams, depending on order quantity and customer id of the order event. The statement delivers order events not inserted into any of the streams to the listeners and/or subscriber to the statement.

The following limitations apply to split-stream statements:

  1. Aggregation functions and the prev and prior operators are not available in conditions and the select-clause.

A variable is a scalar, object or event value that is available for use in all statements including patterns. Variables can be used in an expression anywhere in a statement as well as in the output clause for output rate limiting.

Variables must first be declared or configured before use, by defining each variable's type and name. Variables can be created via the create variable syntax or declared by runtime or static configuration. Variables can be assigned new values by using the on set syntax or via the setVariableValue methods on EPRuntime. The EPRuntime also provides method to read variable values.

A variable can be declared constant. A constant variable always has the initial value and cannot be assigned a new value. A constant variable can be used like any other variable and can be used wherever a constant is required. By declaring a variable constant you enable the Esper engine to optimize and perform query planning knowing that the variable value cannot change.

When declaring a class-type or an event type variable you may read or set individual properties within the same variable.

The engine guarantees consistency and atomicity of variable reads and writes on the level of context partition (this is a soft guarantee, see below). Variables are optimized for fast read access and are also multithread-safe.

Variables can also be removed, at runtime, by destroying all referencing statements including the statement that created the variable, or by means of the runtime configuration API.

The create variable syntax creates a new variable by defining the variable type and name. In alternative to the syntax, variables can also be declared in the runtime and engine configuration options.

The synopsis for creating a variable is as follows:

create [constant] variable variable_type [[]] variable_name [ = assignment_expression ]

Specify the optional constant keyword when the variable is a constant whose associated value cannot be altered. Your EPL design should prefer constant variables over non-constant variables.

The variable_type can be any of the following:

variable_type
	:  string
	|  char 
	|  character
	|  bool 
	|  boolean
	|  byte
	|  short 
	|  int 
	|  integer 
	|  long 
	|  double
	|  float
	|  object
	|  enum_class
	|  class_name
	|  event_type_name

All variable types accept null values. The object type is for an untyped variable that can be assigned any value. You can provide a class name (use imports) or a fully-qualified class name to declare a variable of that Java class type including an enumeration class. You can also supply the name of an event type to declare a variable that holds an event of that type.

Append [] to the variable type to declare an array variable. A limitation is that if your variable type is an event type then array is not allowed.

The variable_name is an identifier that names the variable. The variable name should not already be in use by another variable.

The assignment_expression is optional. Without an assignment expression the initial value for the variable is null. If present, it supplies the initial value for the variable.

The EPStatement object of the create variable statement provides access to variable values. The pull API methods iterator and safeIterator return the current variable value. Listeners to the create variable statement subscribe to changes in variable value: the engine posts new and old value of the variable to all listeners when the variable value is updated by an on set statement.

The example below creates a variable that provides a threshold value. The name of the variable is var_threshold and its type is long. The variable's initial value is null as no other value has been assigned:

create variable long var_threshold

This statement creates an integer-type variable named var_output_rate and initializes it to the value ten (10):

create variable integer var_output_rate = 10

The next statement declares a constant string-type variable:

create constant variable string const_filter_symbol = 'GE'

In addition to creating a variable via the create variable syntax, the runtime and engine configuration API also allows adding variables. The next code snippet illustrates the use of the runtime configuration API to create a string-typed variable:

epService.getEPAdministrator().getConfiguration()
  .addVariable("myVar", String.class, "init value");

The following example declares a constant that is an array of string:

create constant variable string[] const_filters = {'GE', 'MSFT'}

The next example declares a constant that is an array of enumeration values. It assumes the Color enumeration class was imported:

create constant variable Color[] const_colors = {Color.RED, Color.BLUE}

The engine removes the variable if the statement that created the variable is destroyed and all statements that reference the variable are also destroyed. The getVariableNameUsedBy and the removeVariable methods, both part of the runtime ConfigurationOperations API, provide use information and can remove a variable. If the variable was added via configuration, it can only be removed via the configuration API.

The on set statement assigns a new value to one or more variables when a triggering event arrives or a triggering pattern occurs. Use the setVariableValue methods on EPRuntime to assign variable values programmatically.

The synopsis for setting variable values is:

on event_type[(filter_criteria)] [as stream_name]
  set variable_name = expression [, variable_name = expression [,...]]

The event_type is the name of the type of events that trigger the variable assignments. It is optionally followed by filter_criteria which are filter expressions to apply to arriving events. The optional as keyword can be used to assign an stream name. Patterns and named windows can also be specified in the on clause.

The comma-separated list of variable names and expressions set the value of one or more variables. Subqueries may by part of expressions however aggregation functions and the prev or prior function may not be used in expressions.

All new variable values are applied atomically: the changes to variable values by the on set statement become visible to other statements all at the same time. No changes are visible to other processing threads until the on set statement completed processing, and at that time all changes become visible at once.

The EPStatement object provides access to variable values. The pull API methods iterator and safeIterator return the current variable values for each of the variables set by the statement. Listeners to the statement subscribe to changes in variable values: the engine posts new variable values of all variables to any listeners.

In the following example, a variable by name var_output_rate has been declared previously. When a NewOutputRateEvent event arrives, the variable is updated to a new value supplied by the event property 'rate':

on NewOutputRateEvent set var_output_rate = rate

The next example shows two variables that are updated when a ThresholdUpdateEvent arrives:

on ThresholdUpdateEvent as t 
  set var_threshold_lower = t.lower,
      var_threshold_higher = t.higher

The sample statement shown next counts the number of pattern matches using a variable. The pattern looks for OrderEvent events that are followed by CancelEvent events for the same order id within 10 seconds of the OrderEvent:

on pattern[every a=OrderEvent -> (CancelEvent(orderId=a.orderId) where timer:within(10 sec))]
  set var_counter = var_counter + 1

A variable name can be used in any expression and can also occur in an output rate limiting clause. This section presents examples and discusses performance, consistency and atomicity attributes of variables.

The next statement assumes that a variable named 'var_threshold' was created to hold a total price threshold value. The statement outputs an event when the total price for a symbol is greater then the current threshold value:

select symbol, sum(price) from TickEvent 
group by symbol 
having sum(price) > var_threshold

In this example we use a variable to dynamically change the output rate on-the-fly. The variable 'var_output_rate' holds the current rate at which the statement posts a current count to listeners:

select count(*) from TickEvent output every var_output_rate seconds

Variables are optimized towards high read frequency and lower write frequency. Variable reads do not incur locking overhead (99% of the time) while variable writes do incur locking overhead.

The engine softly guarantees consistency and atomicity of variables when your statement executes in response to an event or timer invocation. Variables acquire a stable value (implemented by versioning) when your statement starts executing in response to an event or timer invocation, and variables do not change value during execution. When one or more variable values are updated via on set statements, the changes to all updated variables become visible to statements as one unit and only when the on set statement completes successfully.

The atomicity and consistency guarantee is a soft guarantee. If any of your application statements, in response to an event or timer invocation, execute for a time interval longer then 15 seconds (default interval length), then the engine may use current variable values after 15 seconds passed, rather then then-current variable values at the time the statement started executing in response to an event or timer invocation.

The length of the time interval that variable values are held stable for the duration of execution of a given statement is by default 15 seconds, but can be configured via engine default settings.

Your application can declare an expression or script using the create expression clause. Such expressions or scripts become available globally to any EPL statement.

The synopsis of the create expression syntax is:

create expression expression_or_script

Use the create expression keywords and append the expression or scripts.

At the time your application creates the create expression statement the expression or script becomes globally visible.

At the time your application destroys the create expression statement the expression or script are no longer visible. Existing statements that use the global expression or script are unaffected.

The syntax and additional examples for declaring an expression is outlined in Section 5.2.8, “Expression Declaration”, which discusses declaring expressions that are visible within the same EPL statement i.e. visible locally only.

When using the create expression syntax to declare an expression the engine remembers the expression and allows the expression to be referenced in all other EPL statements.

The below EPL declares a globally visible expression that computes a mid-price:

create expression midPrice { in => (buy + sell) / 2 }

The next EPL returns mid-price for each event:

select midPrice(md) from MarketDataEvent as md

The expression name must be unique for global expressions. It is not possible to declare the same global expression twice with the same name.

Your application can declare an expression of the same name local to a given EPL statement as well as globally using create expression. The locally-declared expression overrides the globally declared expression.

The engine validates globally declared expressions at the time your application creates a statement that references the global expression. When a statement references a global expression, the engine uses that statement's type information to validate the global expressions. Global expressions can therefore be dynamically typed and type information does not need to be the same for all statements that reference the global expression.

This example shows a sequence of EPL, that can be created in the order shown, and that demonstrates expression validation at time of referral:

create expression minPrice {(select min(price) from OrderWindow)}
create window OrderWindow.win:time(30) as OrderEvent
insert into OrderWindow select * from OrderEvent
// Validates and incorporates the declared global expression
select minPrice() as minprice from MarketData

The syntax and additional examples for declaring scripts is outlined in Chapter 18, Script Support, which discusses declaring scripts that are visible within the same EPL statement i.e. visible locally only.

When using the create expression syntax to declare a script the engine remembers the script and allows the script to be referenced in all other EPL statements.

The below EPL declares a globally visible script in the JavaScript dialect that computes a mid-price:

create expression midPrice(buy, sell) [ (buy + sell) / 2 ]

The next EPL returns mid-price for each event:

select midPrice(buy, sell) from MarketDataEvent

The engine validates globally declared scripts at the time your application creates a statement that references the global script. When a statement references a global script, the engine uses that statement's type information to determine parameter types. Global scripts can therefore be dynamically typed and type information does not need to be the same for all statements that reference the global script.

The script name in combination with the number of parameters must be unique for global scripts. It is not possible to declare the same global script twice with the same name and number of parameters.

Your application can declare a script of the same name and number of parameters that is local to a given EPL statement as well as globally using create expression. The locally-declared script overrides the globally declared script.

Contained-event selection is for use when an event contains properties that are themselves events, or more generally when your application needs to split an event into multiple events. One example is when application events are coarse-grained structures and you need to perform bulk operations on the rows of the property graph in an event.

Use the contained-event selection syntax in a filter expression such as in a pattern, from clause, subselect, on-select and on-delete. This section provides the synopsis and examples.

To review, in the from clause a contained_selection may appear after the event stream name and filter criteria, and before any view specifications.

The synopsis for contained_selection is as follows:

[select select_expressions from] 
  contained_expression [@type(eventtype_name)] [as alias_name]
  [where filter_expression]

The select clause and select_expressions are optional and may be used to select specific properties of contained events.

The contained_expression is required and returns individual events. The expression can, for example, be an event property name that returns an event fragment, i.e. a property that can itself be represented as an event by the underlying event representation. Simple values such as integer or string are not fragments. The expression can also be any other expression such as a single-row function or a script that returns either an array or a java.util.Collection of events.

Provide the @type(name) annotation after the contained expression to name the event type of events returned by the expression. The annotation is optional and not needed when the contained-expression is an event property that returns a class or other event fragment.

The alias_name can be provided to assign a name to the expression result value rows.

The where clause and filter_expression is optional and may be used to filter out properties.

As an example event, consider a media order. A media order consists of order items as well as product descriptions. A media order event can be represented as an object graph (POJO event representation), or a structure of nested Maps (Map event representation) or a XML document (XML DOM or Axiom event representation) or other custom plug-in event representation.

To illustrate, a sample media order event in XML event representation is shown below. Also, a XML event type can optionally be strongly-typed with an explicit XML XSD schema that we don't show here. Note that Map and POJO representation can be considered equivalent for the purpose of this example.

Let us now assume that we have declared the event type MediaOrder as being represented by the root node <mediaorder> of such XML snip:

<mediaorder>
  <orderId>PO200901</orderId>
  <items>
    <item>
      <itemId>100001</itemId>
      <productId>B001</productId>
      <amount>10</amount>
      <price>11.95</price>
    </item>
  </items>
  <books>
    <book>
      <bookId>B001</bookId>
      <author>Heinlein</author>
      <review>
        <reviewId>1</reviewId>
        <comment>best book ever</comment>
      </review>
    </book>
    <book>
      <bookId>B002</bookId>
      <author>Isaac Asimov</author>
    </book>
  </books>
</mediaorder>

The next query utilizes the contained-event selection syntax to return each book:

select * from MediaOrder[books.book]

The result of the above query is one event per book. Output events contain only the book properties and not any of the mediaorder-level properties.

Note that, when using listeners, the engine delivers multiple results in one invocation of each listener. Therefore listeners to the above statement can expect a single invocation passing all book events within one media order event as an array.

To better illustrate the position of the contained-event selection syntax in a statement, consider the next two queries:

select * from MediaOrder(orderId='PO200901')[books.book]

The above query the returns each book only for media orders with a given order id. This query illustrates a contained-event selection and a view:

select count(*) from MediaOrder[books.book].std:unique(bookId)

The sample above counts each book unique by book id.

Contained-event selection can be staggered. When staggering multiple contained-event selections the staggered contained-event selection is relative to its parent.

This example demonstrates staggering contained-event selections by selecting each review of each book:

select * from MediaOrder[books.book][review]

Listeners to the query above receive a row for each review of each book. Output events contain only the review properties and not the book or media order properties.

The following is not valid:

// not valid
select * from MediaOrder[books.book.review]

The book property in an indexed property (an array or collection) and thereby requires an index in order to determine which book to use. The expression books.book[1].review is valid and means all reviews of the second (index 1) book.

The contained-event selection syntax is part of the filter expression and may therefore occur in patterns and anywhere a filter expression is valid.

A pattern example is below. The example assumes that a Cancel event type has been defined that also has an orderId property:

select * from pattern [c=Cancel -> books=MediaOrder(orderId = c.orderId)[books.book] ]

When used in a pattern, a filter with a contained-event selection returns an array of events, similar to the match-until clause in patterns. The above statement returns, in the books property, an array of book events.

The optional select clause provides control over which fields are available in output events. The expressions in the select-clause apply only to the properties available underneath the property in the from clause, and the properties of the enclosing event.

When no select is specified, only the properties underneath the selected property are available in output events.

In summary, the select clause may contain:

The next query's select clause selects each review for each book, and the order id as well as the book id of each book:

select * from MediaOrder[select orderId, bookId from books.book][select * from review]
// ... equivalent to ...
select * from MediaOrder[select orderId, bookId from books.book][review]]

Listeners to the statement above receive an event for each review of each book. Each output event has all properties of the review row, and in addition the bookId of each book and the orderId of the order. Thus bookId and orderId are found in each result event, duplicated when there are multiple reviews per book and order.

The above query uses wildcard (*) to select all properties from reviews. As has been discussed as part of the select clause, the wildcard (*) and property_alias.* do not copy properties for performance reasons. The wildcard syntax instead specifies the underlying type, and additional properties are added onto that underlying type if required. Only one wildcard (*) and property_alias.* (unless used with a column rename) may therefore occur in the select clause list of expressions.

All the following queries produce an output event for each review of each book. The next sample queries illustrate the options available to control the fields of output events.

The output events produced by the next query have all properties of each review and no other properties available:

select * from MediaOrder[books.book][review]

The following query is not a valid query, since the order id and book id are not part of the contained-event selection:

// Invalid select-clause: orderId and bookId not produced.
select orderId, bookId from MediaOrder[books.book][review]

This query is valid. Note that output events carry only the orderId and bookId properties and no other data:

select orderId, bookId from MediaOrder[books.book][select orderId, bookId from review]
//... equivalent to ...
select * from MediaOrder[select orderId, bookId from books.book][review]

This variation produces output events that have all properties of each book and only reviewId and comment for each review:

select * from MediaOrder[select * from books.book][select reviewId, comment from review]
// ... equivalent to ...
select * from MediaOrder[books.book as book][select book.*, reviewId, comment from review]

The output events of the next EPL have all properties of the order and only bookId and reviewId for each review:

select * from MediaOrder[books.book as book]
    [select mediaOrder.*, bookId, reviewId from review] as mediaOrder

This EPL produces output events with 3 columns: a column named mediaOrder that is the order itself, a column named book for each book and a column named review that holds each review:

insert into ReviewStream select * from MediaOrder[books.book as book]
  [select mo.* as mediaOrder, book.* as book, review.* as review from review as review] as mo
// .. and a sample consumer of ReviewStream...
select mediaOrder.orderId, book.bookId, review.reviewId from ReviewStream

Please note these limitations:

This section discusses contained-event selection in joins.

When joining within the same event it is not required that views are specified. Recall, in a join or outer join there must be views specified that hold the data to be joined. For self-joins, no views are required and the join executes against the data returned by the same event.

This query inner-joins items to books where book id matches the product id:

select book.bookId, item.itemId 
from MediaOrder[books.book] as book, 
      MediaOrder[items.item] as item 
where productId = bookId

Query results for the above query when sending the media order event as shown earlier are:

book.bookIditem.itemId
B001100001

The next example query is a left outer join. It returns all books and their items, and for books without item it returns the book and a null value:

select book.bookId, item.itemId 
from MediaOrder[books.book] as book 
  left outer join 
    MediaOrder[items.item] as item 
  on productId = bookId

Query results for the above query when sending the media order event as shown earlier are:

book.bookIditem.itemId
B001100001
B002null

A full outer join combines the results of both left and right outer joins. The joined table will contain all records from both tables, and fill in null values for missing matches on either side.

This example query is a full outer join, returning all books as well as all items, and filling in null values for book id or item id if no match is found:

select orderId, book.bookId,item.itemId 
from MediaOrder[books.book] as book 
  full outer join 
     MediaOrder[select orderId, * from items.item] as item 
  on productId = bookId 
order by bookId, item.itemId asc

As in all other continuous queries, aggregation results are cumulative from the time the statement was created.

The following query counts the cumulative number of items in which the product id matches a book id:

select count(*) 
from MediaOrder[books.book] as book, 
      MediaOrder[items.item] as item 
where productId = bookId

The unidirectional keyword in a join indicates to the query engine that aggregation state is not cumulative. The next query counts the number of items in which the product id matches a book id for each event:

select count(*) 
from MediaOrder[books.book] as book unidirectional, 
      MediaOrder[items.item] as item 
where productId = bookId

The update istream statement allows declarative modification of event properties of events entering a stream. Update is a pre-processing step to each new event, modifying an event before the event applies to any statements.

The synopsis of update istream is as follows:

update istream event_type [as stream_name]
  set property_name = set_expression [, property_name = set_expression] [,...]
  [where where_expression]

The event_type is the name of the type of events that the update applies to. The optional as keyword can be used to assign a name to the event type for use with subqueries, for example. Following the set keyword is a comma-separated list of property names and expressions that provide the event properties to change and values to set.

The optional where clause and expression can be used to filter out events to which to apply updates.

Listeners to an update statement receive the updated event in the insert stream (new data) and the event prior to the update in the remove stream (old data). Note that if there are multiple update statements that all apply to the same event then the engine will ensure that the output events delivered to listeners or subscribers are consistent with the then-current updated properties of the event (if necessary making event copies, as described below, in the case that listeners are attached to update statements). Iterating over an update statement returns no events.

As an example, the below statement assumes an AlertEvent event type that has properties named severity and reason:

update istream AlertEvent 
  set severity = 'High'
  where severity = 'Medium' and reason like '%withdrawal limit%'

The statement above changes the value of the severity property to "High" for AlertEvent events that have a medium severity and contain a specific reason text.

Update statements apply the changes to event properties before other statements receive the event(s) for processing, e.g. "select * from AlertEvent" receives the updated AlertEvent. This is true regardless of the order in which your application creates statements.

When multiple update statements apply to the same event, the engine executes updates in the order in which update statements are created. We recommend the @Priority EPL annotation to define a deterministic order of processing updates, especially in the case where update statements get created and destroyed dynamically or multiple update statements update the same fields. The update statement with the highest @Priority value applies last.

The update clause can be used on streams populated via insert into, as this example utilizing a pattern demonstrates:

insert into DoubleWithdrawalStream 
select a.id, b.id, a.account as account, 0 as minimum 
from pattern [a=Withdrawal -> b=Withdrawal(id = a.id)]
update istream DoubleWithdrawalStream set minimum = 1000 where account in (10002, 10003)

When using update with named windows, any changes to event properties apply before an event enters the named window.

Consider the next example (shown here with statement names in @Name EPL annotation, multiple EPL statements):

@Name("CreateWindow") create window MyWindow.win:time(30 sec) as AlertEvent

@Name("UpdateStream") update istream MyWindow set severity = 'Low' where reason = '%out of paper%'

@Name("InsertWindow") insert into MyWindow select * from AlertEvent

@Name("SelectWindow") select * from MyWindow

The UpdateStream statement specifies an update clause that applies to all events entering the named window. Note that update does not apply to events already in the named window at the time an application creates the UpdateStream statement, it only applies to new events entering the named window (after an application created the update statement).

Therefore, in the above example listeners to the SelectWindow statement as well as the CreateWindow statement receive the updated event, while listeners to the InsertWindow statement receive the original AlertEvent event (and not the updated event).

Subqueries can also be used in all expressions including the optional where clause.

This example demonstrates a correlated subquery in an assignment expression and also demonstrates the optional as keyword. It assigns the phone property of an AlertEvent event a new value based on the lookup within all unique PhoneEvent events (according to an empid property) correlating the AlertEvent property reporter with the empid property of PhoneEvent:

update istream AlertEvent as ae
  set phone = 
    (select phone from PhoneEvent.std:unique(empid) where empid = ae.reporter)

When updating indexed properties use the syntax propertyName[index] = value with the index value being an integer number. When updating mapped properties use the syntax propertyName(key) = value with the key being a string value.

When using update, please note these limitations:

The engine delivers all result events of a given statement to the statement's listeners and subscriber (if any) in a single invocation of each listener and subscriber's update method passing an array of result events. For example, a statement using a time-batch view may provide many result events after a time period passes, a pattern may provide multiple matching events or in a join the join cardinality could be multiple rows.

For statements that typically post multiple result events to listeners the for keyword controls the number of invocations of the engine to listeners and subscribers and the subset of all result events delivered by each invocation. This can be useful when your application listener or subscriber code expects multiple invocations or expects that invocations only receive events that belong together by some additional criteria.

The for keyword is a reserved keyword. It is followed by either the grouped_delivery keyword for grouped delivery or the discrete_delivery keyword for discrete delivery. The for clause is valid after any EPL select statement.

The synopsis for grouped delivery is as follows:

... for grouped_delivery (group_expression [, group_expression] [,...])

The group_expression expression list provides one or more expressions to apply to result events. The engine invokes listeners and subscribers once for each distinct set of values returned by group_expression expressions passing only the events for that group.

The synopsis for discrete delivery is as follows:

... for discrete_delivery

With discrete delivery the engine invokes listeners and subscribers once for each result event passing a single result event in each invocation.

Consider the following example without for-clause. The time batch data view collects RFIDEvent events for 10 seconds and posts an array of result events:

select * from RFIDEvent.win:time_batch(10 sec)

Let's consider an example event sequence as follows:


Without for-clause and after the 10-second time period passes, the engine delivers an array of 3 events in a single invocation to listeners and the subscriber.

The next example specifies the for-clause and grouped delivery by zone:

select * from RFIDEvent.win:time_batch(10 sec) for grouped_delivery (zone)

With grouped delivery and after the 10-second time period passes, the above statement delivers result events in two invocations to listeners and the subscriber: The first invocation delivers an array of two events that contains zone A events with id 1 and 3. The second invocation delivers an array of 1 event that contains a zone B event with id 2.

The next example specifies the for-clause and discrete delivery:

select * from RFIDEvent.win:time_batch(10 sec) for discrete_delivery

With discrete delivery and after the 10-second time period passes, the above statement delivers result events in three invocations to listeners and the subscriber: The first invocation delivers an array of 1 event that contains the event with id 1, the second invocation delivers an array of 1 event that contains the event with id 2 and the third invocation delivers an array of 1 event that contains the event with id 3.

Remove stream events are also delivered in multiple invocations, one for each group, if your statement selects remove stream events explicitly via irstream or rstream keywords.

The insert into for inserting events into a stream is not affected by the for-clause.

The delivery order respects the natural sort order or the explicit sort order as provided by the order by clause, if present.

The following are known limitations:

  1. The engine validates group_expression expressions against the output event type, therefore all properties specified in group_expression expressions must occur in the select clause.

Event patterns match when an event or multiple events occur that match the pattern's definition. Patterns can also be time-based.

Pattern expressions consist of pattern atoms and pattern operators:

  1. Pattern atoms are the basic building blocks of patterns. Atoms are filter expressions, observers for time-based events and plug-in custom observers that observe external events not under the control of the engine.

  2. Pattern operators control expression lifecycle and combine atoms logically or temporally.

The below table outlines the different pattern atoms available:


There are 4 types of pattern operators:

  1. Operators that control pattern sub-expression repetition: every, every-distinct, [num] and until

  2. Logical operators: and, or, not

  3. Temporal operators that operate on event order: -> (followed-by)

  4. Guards are where-conditions that control the lifecycle of subexpressions. Examples are timer:within, timer:withinmax and while-expression. Custom plug-in guards may also be used.

Pattern expressions can be nested arbitrarily deep by including the nested expression(s) in () round parenthesis.

Underlying the pattern matching is a state machine that transitions between states based on arriving events and based on time advancing. A single event or advancing time may cause a reaction in multiple parts of your active pattern state.

This is an example pattern expression that matches on every ServiceMeasurement events in which the value of the latency event property is over 20 seconds, and on every ServiceMeasurement event in which the success property is false. Either one or the other condition must be true for this pattern to match.

every (
  spike=ServiceMeasurement(latency>20000) 
  or 
  error=ServiceMeasurement(success=false)
)

In the example above, the pattern expression starts with an every operator to indicate that the pattern should fire for every matching events and not just the first matching event. Within the every operator in round brackets is a nested pattern expression using the or operator. The left hand of the or operator is a filter expression that filters for events with a high latency value. The right hand of the operator contains a filter expression that filters for events with error status. Filter expressions are explained in Section 6.4, “Filter Expressions In Patterns”.

The example above assigned the tags spike and error to the events in the pattern. The tags are important since the engine only places tagged events into the output event(s) that a pattern generates, and that the engine supplies to listeners of the pattern statement. The tags can further be selected in the select-clause of an EPL statement as discussed in Section 5.4.2, “Pattern-based Event Streams”.

Patterns can also contain comments within the pattern as outlined in Section 5.2.2, “Using Comments”.

Pattern statements are created via the EPAdministrator interface. The EPAdministrator interface allows to create pattern statements in two ways: Pattern statements that want to make use of the EPL select clause or any other EPL constructs use the createEPL method to create a statement that specifies one or more pattern expressions. EPL statements that use patterns are described in more detail in Section 5.4.2, “Pattern-based Event Streams”. Use the syntax as shown in below example.

EPAdministrator admin = EPServiceProviderManager.getDefaultProvider().getEPAdministrator();

String eventName = ServiceMeasurement.class.getName();

EPStatement myTrigger = admin.createEPL("select * from pattern [" +
  "every (spike=" + eventName + "(latency>20000) or error=" + eventName + "(success=false))]");

Pattern statements that do not need to make use of the EPL select clause or any other EPL constructs can use the createPattern method, as in below example.

EPStatement myTrigger = admin.createPattern(
  "every (spike=" + eventName + "(latency>20000) or error=" + eventName + "(success=false))");

Partially-completed patterns are incomplete matches that are not yet indicated by the engine because the complete pattern condition is not satisfied. Any given event can be part of multiple partially-completed patterns.

For example, consider the following pattern:

every a=A -> B and C(id=a.id)

Given this sequence of events:

A1{id='id1'}   A2{id='id2'}   B1  

According to the sequence above there are no matches. The pattern is partially completed waiting for C events. The combination {A1, B1} is waiting for a C{id='id1'} event before the pattern match is complete for that combination. The combination {A2, B1} is waiting for a C{id='id2'} event before the pattern match is complete for that combination.

Assuming event C1{id='id1') arrives the pattern outputs the combination {A1, B1, C1}. Assuming event C2{id='id2') arrives the pattern outputs the combination {A2, B1, C2}. Note that event B1 is part of both partially-completed patterns.

Use the @DiscardPartialsOnMatch pattern-level annotation to instruct the engine that when any matches occur to discard partially completed patterns that overlap in terms of the events that make up the match (or matches if there are multiple matches).

The same example using the @DiscardPartialsOnMatch pattern-level annotation is:

select * from pattern @DiscardPartialsOnMatch [every a=A -> B and C(id=a.id)]

When event C1{id='id1') arrives the pattern outputs the match combination {A1, B1, C1}. Upon indication of the match the engine discards all partially-completed patterns that refer to either of the A1, B1 and C1 events. Since event B1 is part of a partially-completed pattern waiting for C{id='id2'}, the engine discards that partially-completed pattern. Therefore when C2{id='id2'} arrives the engine outputs no matches.

When specifying both @DiscardPartialsOnMatch and @SuppressOverlappingMatches the engine discards the partially-completed patterns that overlap all matches including suppressed matches.

The operators at the top of this table take precedence over operators lower on the table.


If you are not sure about the precedence, please consider placing parenthesis () around your subexpressions. Parenthesis can also help make expressions easier to read and understand.

The following table outlines sample equivalent expressions, with and without the use of parenthesis for subexpressions.


The simplest form of filter is a filter for events of a given type without any conditions on the event property values. This filter matches any event of that type regardless of the event's properties. The example below is such a filter. Note that this event pattern would stop firing as soon as the first RfidEvent is encountered.

com.mypackage.myevents.RfidEvent

To make the event pattern fire for every RfidEvent and not just the first event, use the every keyword.

every com.mypackage.myevents.RfidEvent

The example above specifies the fully-qualified Java class name as the event type. Via configuration, the event pattern above can be simplified by using the name that has been defined for the event type.

every RfidEvent

Interfaces and superclasses are also supported as event types. In the below example IRfidReadable is an interface class, and the statement matches any event that implements this interface:

every org.myorg.rfid.IRfidReadable

The filtering criteria to filter for events with certain event property values are placed within parenthesis after the event type name:

RfidEvent(category="Perishable")

All expressions can be used in filters, including static method invocations that return a boolean value:

RfidEvent(com.mycompany.MyRFIDLib.isInRange(x, y) or (x<0 and y < 0))

Filter expressions can be separated via a single comma ','. The comma represents a logical AND between expressions:

RfidEvent(zone=1, category=10)
...is equivalent to...
RfidEvent(zone=1 and category=10)

The following set of operators are highly optimized through indexing and are the preferred means of filtering high-volume event streams:

  • equals =

  • not equals !=

  • comparison operators < , > , >=, <=

  • ranges

    • use the between keyword for a closed range where both endpoints are included

    • use the in keyword and round () or square brackets [] to control how endpoints are included

    • for inverted ranges use the not keyword and the between or in keywords

  • list-of-values checks using the in keyword or the not in keywords followed by a comma-separated list of values

At compile time as well as at run time, the engine scans new filter expressions for subexpressions that can be indexed. Indexing filter values to match event properties of incoming events enables the engine to match incoming events faster. The above list of operators represents the set of operators that the engine can best convert into indexes. The use of comma or logical and in filter expressions does not impact optimizations by the engine.

For more information on filters please see Section 5.4.1, “Filter-based Event Streams”. Contained-event selection on filters in patterns is further described in Section 5.20, “Contained-Event Selection”.

Filter criteria can also refer to events matching prior named events in the same expression. Below pattern is an example in which the pattern matches once for every RfidEvent that is preceded by an RfidEvent with the same asset id.

every e1=RfidEvent -> e2=RfidEvent(assetId=e1.assetId)

The syntax shown above allows filter criteria to reference prior results by specifying the event name tag of the prior event, and the event property name. The tag names in the above example were e1 and e2. This syntax can be used in all filter operators or expressions including ranges and the in set-of-values check:

every e1=RfidEvent -> 
  e2=RfidEvent(MyLib.isInRadius(e1.x, e1.y, x, y) and zone in (1, e1.zone))

An arriving event changes the truth value of all expressions that look for the event. Consider the pattern as follows:

every (RfidEvent(zone > 1) and RfidEvent(zone < 10))

The pattern above is satisfied as soon as only one event with zone in the interval [2, 9] is received.

An arriving event applies to all filter expressions for which the event matches. In other words, an arriving event is not consumed by any specify filter expression(s) but applies to all active filter expressions of all pattern sub-expressions.

You may provide the @consume annotation as part of a filter expression to control consumption of an arriving event. If an arriving event matches the filter expression marked with @consume it is no longer available to other filter expressions of the same pattern that also match the arriving event.

The @consume can include a level number in parenthesis. A higher level number consumes the event first. The default level number is 1. Multiple filter expressions with the same level number for @consume all match the event.

Consider the next sample pattern:

a=RfidEvent(zone='Z1') and b=RfidEvent(assetId='0001')

This pattern fires when a single RfidEvent event arrives that has zone 'Z1' and assetId '0001'. The pattern also matches when two RfidEvent events arrive, in any order, wherein one has zone 'Z1' and the other has assetId '0001'.

Mark a filter expression with @consume to indicate that if an arriving event matches multiple filter expressions that the engine prefers the marked filter expression and does not match any other filter expression.

This updated pattern statement uses @consume to indicate that a match against zone is preferred:

a=RfidEvent(zone='Z1')@consume and b=RfidEvent(assetId='0001')

This pattern no longer fires when a single RfidEvent arrives that has zone 'Z1' and assetId '0001', because when the first filter expression matches the pattern engine consumes the event. The pattern only matches when two RfidEvent events arrive in any order. One event must have zone 'Z1' and the other event must have a zone other then 'Z1' and an assetId '0001'.

The next sample pattern provides a level number for each @consume:

a=RfidEvent(zone='Z1')@consume(2) 
  or b=RfidEvent(assetId='0001')@consume(1) 
  or c=RfidEvent(category='perishable'))

The pattern fires when an RfidEvent arrives with zone 'Z1'. In this case the output event populates property 'a' but not properties 'b' and 'c'. The pattern also fires when an RfidEvent arrives with a zone other then 'Z1' and an asset id of '0001'. In this case the output event populates property 'b' but not properties 'a' and 'c'. The pattern also fires when an RfidEvent arrives with a zone other then 'Z1' and an asset id other then '0001' and a category of 'perishable'. In this case the output event populates property 'c' but not properties 'a' and 'b'.

The every operator indicates that the pattern sub-expression should restart when the subexpression qualified by the every keyword evaluates to true or false. Without the every operator the pattern sub-expression stops when the pattern sub-expression evaluates to true or false.

As a side note, please be aware that a single invocation to the UpdateListener interface may deliver multiple events in one invocation, since the interface accepts an array of values.

Thus the every operator works like a factory for the pattern sub-expression contained within. When the pattern sub-expression within it fires and thus quits checking for events, the every causes the start of a new pattern sub-expression listening for more occurrences of the same event or set of events.

Every time a pattern sub-expression within an every operator turns true the engine starts a new active subexpression looking for more event(s) or timing conditions that match the pattern sub-expression. If the every operator is not specified for a subexpression, the subexpression stops after the first match was found.

This pattern fires when encountering an A event and then stops looking.

A

This pattern keeps firing when encountering A events, and doesn't stop looking.

every A

When using every operator with the -> followed-by operator, each time the every operator restarts it also starts a new subexpression instance looking for events in the followed-by subexpression.

Let's consider an example event sequence as follows.

A1   B1   C1   B2   A2   D1   A3   B3   E1   A4   F1   B4


The examples show that it is possible that a pattern fires for multiple combinations of events that match a pattern expression. Each combination is posted as an EventBean instance to the update method in the UpdateListener implementation.

Let's consider the every operator in conjunction with a subexpression that matches 3 events that follow each other:

every (A -> B -> C)

The pattern first looks for A events. When an A event arrives, it looks for a B event. After the B event arrives, the pattern looks for a C event. Finally, when the C event arrives the pattern fires. The engine then starts looking for an A event again.

Assume that between the B event and the C event a second A2 event arrives. The pattern would ignore the A2 event entirely since it's then looking for a C event. As observed in the prior example, the every operator restarts the subexpression A -> B -> C only when the subexpression fires.

In the next statement the every operator applies only to the A event, not the whole subexpression:

every A -> B -> C

This pattern now matches for each A event that is followed by a B event and then a C event, regardless of when the A event arrives. Note that for each A event that arrives the pattern engine starts a new subexpression looking for a B event and then a C event, outputting each combination of matching events.

As the introduction of the every operator states, the operator starts new subexpression instances and can cause multiple matches to occur for a single arriving event.

New subexpressions also take a very small amount of system resources and thereby your application should carefully consider when subexpressions must end when designing patterns. Use the timer:within construct and the and not constructs to end active subexpressions. The data window onto a pattern stream does not serve to limit pattern sub-expression lifetime.

Lets look at a concrete example. Consider the following sequence of events arriving:

A1   A2   B1  

This pattern matches on arrival of B1 and outputs two events (an array of length 2 if using a listener). The two events are the combinations {A1, B1} and {A2, B1}:

every a=A -> b=B

The and not operators are used to end an active subexpression.

The next pattern matches on arrival of B1 and outputs only the last A event which is the combination {A2, B1}:

every a=A -> (b=B and not A)

The and not operators cause the subexpression looking for {A1, B?} to end when A2 arrives.

Similarly, in the pattern below the engine starts a new subexpression looking for a B event every 1 second. After 5 seconds there are 5 subexpressions active looking for a B event and 5 matches occur at once if a B event arrives after 5 seconds.

every timer:interval(1 sec) -> b=B

Again the and not operators can end subexpressions that are not intended to match any longer:

every timer:interval(1 sec) -> (b=B and not timer:interval(1 sec))
// equivalent to
every timer:interval(1 sec) -> (b=B where timer:within(1 sec))

In this example we consider a generic pattern in which the pattern must match for each A event followed by a B event and followed by a C event, in which both the B event and the C event must arrive within 1 hour of the A event. The first approach to the pattern is as follows:

every A  -> (B -> C) where timer:within(1 hour)

Consider the following sequence of events arriving:

A1   A2   B1   C1   B2   C2

First, the pattern as above never stops looking for A events since the every operator instructs the pattern to keep looking for A events.

When A1 arrives, the pattern starts a new subexpression that keeps A1 in memory and looks for any B event. At the same time, it also keeps looking for more A events.

When A2 arrives, the pattern starts a new subexpression that keeps A2 in memory and looks for any B event. At the same time, it also keeps looking for more A events.

After the arrival of A2, there are 3 subexpressions active:

In the pattern above, we have specified a 1-hour lifetime for subexpressions looking for B and C events. Thus, if no B and no C event arrive within 1 hour after A1, the first subexpression goes away. If no B and no C event arrive within 1 hour after A2, the second subexpression goes away. The third subexpression however stays around looking for more A events.

The pattern as shown above thus matches on arrival of C1 for combination {A1, B1, C1} and for combination {A2, B1, C1}, provided that B1 and C1 arrive within an hour of A1 and A2.

You may now ask how to match on {A1, B1, C1} and {A2, B2, C2} instead, since you may need to correlate on a given property.

The pattern as discussed above matches every A event followed by the first B event followed by the next C event, and doesn't specifically qualify the B or C events to look for based on the A event. To look for specific B and C events in relation to a given A event, the correlation must use one or more of the properties of the A event, such as the "id" property:

every a=A -> (B(id=a.id -> C(id=a.id)) where timer:within(1 hour)

The pattern as shown above thus matches on arrival of C1 for combination {A1, B1, C1} and on arrival of C2 for combination {A2, B2, C2}.

Similar to the every operator in most aspects, the every-distinct operator indicates that the pattern sub-expression should restart when the subexpression qualified by the every-distinct keyword evaluates to true or false. In addition, the every-distinct eliminates duplicate results received from an active subexpression according to its distinct-value expressions.

The synopsis for the every-distinct pattern operator is:

every-distinct(distinct_value_expr [, distinct_value_exp[...][, expiry_time_period])

Within parenthesis are one or more distinct_value_expr expressions that return the values by which to remove duplicates.

You may optionally specify an expiry_time_period time period. If present, the pattern engine expires and removes distinct key values that are older then the time period, removing their associated memory and allowing such distinct values to match again. When your distinct value expressions return an unlimited number of values, for example when your distinct value is a timestamp or auto-increment column, you should always specify an expiry time period.

When specifying properties in the distinct-value expression list, you must ensure that the event types providing properties are tagged. Only properties of event types within filter expressions that are sub-expressions to the every-distinct may be specified.

For example, this pattern keeps firing for every A event with a distinct value for its aprop property:

every-distinct(a.aprop) a=A

Note that the pattern above assigns the a tag to the A event and uses a.prop to identify the prop property as a value of the a event A.

A pattern that returns the first Sample event for each sensor, assuming sensor is a field that returns a unique id identifying the sensor that originated the Sample event, is:

every-distinct(s.sensor) s=Sample

The next pattern looks for pairs of A and B events and returns only the first pair for each combination of aprop of an A event and bprop of a B event:

every-distinct(a.aprop, b.bprop) (a=A and b=B)

The following pattern looks for A events followed by B events for which the value of the aprop of an A event is the same value of the bprop of a B event but only for each distinct value of aprop of an A event:

every-distinct(a.aprop) a=A -> b=B(bprop = a.aprop)

When specifying properties as part of distinct-value expressions, properties must be available from tagged event types in sub-expressions to the every-distinct.

The following patterns are not valid:

// Invalid: event type in filter not tagged
every-distinct(aprop) A
			
// Invalid: property not from a sub-expression of every-distinct
a=A -> every-distinct(a.aprop) b=B

When an active subexpression to every-distinct becomes permanently false, the distinct-values seen from the active subexpression are removed and the sub-expression within is restarted.

For example, the below pattern detects each A event distinct by the value of aprop.

every-distinct(a.aprop) (a=A and not B)

In the pattern above, when a B event arrives, the subexpression becomes permanently false and is restarted anew, detecting each A event distinct by the value of aprop without considering prior values.

When your distinct key is a timestamp or other non-unique property, specify an expiry time period.

The following example returns every distinct A event according to the timestamp property on the A event, retaining each timestamp value for 10 seconds:

every-distinct(a.timestamp, 10 seconds) a=A

In the example above, if for a given A event and its timestamp value the same timestamp value occurs again for another A event before 10 seconds passed, the A event is not a match. If 10 seconds passed the pattern indicates a second match.

You may not use every-distinct with a timer-within guard to expire keys: The expiry time notation as above is the recommended means to expire keys.

// This is not the same as above; It does not expire transaction ids and is not recommended
every-distinct(a.timestamp) a=A where timer:within(10 sec)

The repeat operator fires when a pattern sub-expression evaluates to true a given number of times. The synopsis is as follows:

[match_count] repeating_subexpr

The repeat operator is very similar to the every operator in that it restarts the repeating_subexpr pattern sub-expression up to a given number of times.

match_count is a positive number that specifies how often the repeating_subexpr pattern sub-expression must evaluate to true before the repeat expression itself evaluates to true, after which the engine may indicate a match.

For example, this pattern fires when the last of five A events arrives:

[5] A

Parenthesis must be used for nested pattern sub-expressions. This pattern fires when the last of a total of any five A or B events arrives:

[5] (A or B)

Without parenthesis the pattern semantics change, according to the operator precedence described earlier. This pattern fires when the last of a total of five A events arrives or a single B event arrives, whichever happens first:

[5] A or B

Tags can be used to name events in filter expression of pattern sub-expressions. The next pattern looks for an A event followed by a B event, and a second A event followed by a second B event. The output event provides indexed and array properties of the same name:

[2] (a=A -> b=B)

Using tags with repeat is further described in Section 6.5.4.6, “Tags and the Repeat Operator”.

Consider the following pattern that demonstrates the behavior when a pattern sub-expression becomes permanently false:

[2] (a=A and not C)

In the case where a C event arrives before 2 A events arrive, the pattern above becomes permanently false.

Lets add an every operator to restart the pattern and thus keep matching for all pairs of A events that arrive without a C event in between each pair:

every [2] (a=A and not C)

Since pattern matches return multiple A events, your select clause should use tag a as an array, for example:

select a[0].id, a[1].id from pattern [every [2] (a=A and not C)]

The repeat until operator provides additional control over repeated matching.

The repeat until operator takes an optional range, a pattern sub-expression to repeat, the until keyword and a second pattern sub-expression that ends the repetition. The synopsis is as follows:

[range] repeated_pattern_expr until end_pattern_expr

Without a range, the engine matches the repeated_pattern_expr pattern sub-expression until the end_pattern_expr evaluates to true, at which time the expression turns true.

An optional range can be used to indicate the minimum number of times that the repeated_pattern_expr pattern sub-expression must become true.

The optional range can also specify a maximum number of times that repeated_pattern_expr pattern sub-expression evaluates to true and retains tagged events. When this number is reached, the engine stops the repeated_pattern_expr pattern sub-expression.

The until keyword is always required when specifying a range and is not required if specifying a fixed number of repeat as discussed in the section before.

Similar to the Java && operator the and operator requires both nested pattern expressions to turn true before the whole expression turns true (a join pattern).

This pattern matches when both an A event and a B event arrive, at the time the last of the two events arrive:

A and B

This pattern matches on any sequence of an A event followed by a B event and then a C event followed by a D event, or a C event followed by a D and an A event followed by a B event:

(A -> B) and (C -> D)

Note that in an and pattern expression it is not possible to correlate events based on event property values. For example, this is an invalid pattern:

// This is NOT valid
a=A and B(id = a.id)

The above expression is invalid as it relies on the order of arrival of events, however in an and expression the order of events is not specified and events fulfill an and condition in any order. The above expression can be changed to use the followed-by operator:

// This is valid
a=A -> B(id = a.id)
// another example using 'and'...
a=A -> (B(id = a.id) and C(id = a.id))

Consider a pattern that looks for the same event:

A and A

The pattern above fires when a single A event arrives. The first arriving A event triggers a state transition in both the left and the right hand side expression.

In order to match after two A events arrive in any order, there are two options to express this pattern. The followed-by operator is one option and the repeat operator is the second option, as the next two patterns show:

A -> A
// ... or ...
[2] A

The not operator negates the truth value of an expression. Pattern expressions prefixed with not are automatically defaulted to true upon start, and turn permanently false when the expression within turns true.

The not operator is generally used in conjunction with the and operator or subexpressions as the below examples show.

This pattern matches only when an A event is encountered followed by a B event but only if no C event was encountered before either an A event and a B event, counting from the time the pattern is started:

(A -> B) and not C

Assume we'd like to detect when an A event is followed by a D event, without any B or C events between the A and D events:

A -> (D and not (B or C))

It may help your understanding to discuss a pattern that uses the or operator and the not operator together:

a=A -> (b=B or not C)

In the pattern above, when an A event arrives then the engine starts the subexpression B or not C. As soon as the subexpression starts, the not operator turns to true. The or expression turns true and thus your listener receives an invocation providing the A event in the property 'a'. The subexpression does not end and continues listening for B and C events. Upon arrival of a B event your listener receives a second invocation. If instead a C event arrives, the not turns permanently false however that does not affect the or operator (but would end an and operator).

To test for absence of an event, use timer:interval together with and not operators. The sample statement reports each 10-second interval during which no A event occurred:

every (timer:interval(10 sec) and not A)

In many cases the not operator, when used alone, does not make sense. The following example is invalid and will log a warning when the engine is started:

// not a sensible pattern
(not a=A) -> B(id=a.id)

The followed by -> operator specifies that first the left hand expression must turn true and only then is the right hand expression evaluated for matching events.

Look for an A event and if encountered, look for a B event. As always, A and B can itself be nested event pattern expressions.

A -> B

This is a pattern that fires when 2 status events indicating an error occur one after the other.

StatusEvent(status='ERROR') -> StatusEvent(status='ERROR')

A pattern that takes all A events that are not followed by a B event within 5 minutes:

every A -> (timer:interval(5 min) and not B)

A pattern that takes all A events that are not preceded by B within 5 minutes:

every (timer:interval(5 min) and not B -> A)

The followed-by -> operator can optionally be provided with an expression that limits the number of sub-expression instances of the right-hand side pattern sub-expression.

The synopsis for the followed-by operator with limiting expression is:

lhs_expression -[limit_expression]> rhs_expression

Each time the lhs_expression pattern sub-expression turns true the pattern engine starts a new rhs_expression pattern sub-expression. The limit_expression returns an integer value that defines a maximum number of pattern sub-expression instances that can simultaneously be present for the same rhs_expression.

When the limit is reached the pattern engine issues a com.espertech.esper.client.hook.ConditionPatternSubexpressionMax notification object to any condition handlers registered with the engine as described in Section 14.12, “Condition Handling” and does not start a new pattern sub-expression instance for the right-hand side pattern sub-expression.

For example, consider the following pattern which returns for every A event the first B event that matches the id field value of the A event:

every a=A -> b=B(id = a.id)

In the above pattern, every time an A event arrives (lhs) the pattern engine starts a new pattern sub-expression (rhs) consisting of a filter for the first B event that has the same value for the id field as the A event.

In some cases your application may want to limit the number of right-hand side sub-expressions because of memory concerns or to reduce output. You may add a limit expression returning an integer value as part of the operator.

This example employs the followed-by operator with a limit expression to indicate that maximally 2 filters for B events (the right-hand side pattern sub-expression) may be active at the same time:

every a=A -[2]> b=B(id = a.id)

Note that the limit expression in the example above is not a limit per value of id field, but a limit counting all right-hand side pattern sub-expression instances that are managed by that followed-by sub-expression instance.

If your followed-by operator lists multiple sub-expressions with limits, each limit applies to the immediate right-hand side. For example, the pattern below limits the number of filters for B events to 2 and the number of filters for C events to 3:

every a=A -[2]> b=B(id = a.id) -[3]> c=C(id = a.id)

Esper allows setting a maximum number of pattern sub-expressions in the configuration, applicable to all followed-by operators of all statements.

If your application has patterns in multiple EPL statements and all such patterns should count towards a total number of pattern sub-expression counts, you may consider setting a maximum number of pattern sub-expression instances, engine-wide, via the configuration described in Section 15.4.15.1, “Followed-By Operator Maximum Subexpression Count”.

When the limit is reached the pattern engine issues a notification object to any condition handlers registered with the engine as described in Section 14.12, “Condition Handling”. Depending on your configuration the engine can prevent the start of a new pattern sub-expression instance for the right-hand side pattern sub-expression, until pattern sub-expression instances end or statements are stopped or destroyed.

The notification object issued to condition handlers is an instance of com.espertech.esper.client.hook.ConditionPatternEngineSubexpressionMax. The notification object contains information which statement triggered the limit and the pattern counts per statement for all statements.

For information on static and runtime configuration, please consult Section 15.4.15.1, “Followed-By Operator Maximum Subexpression Count”. The limit can be changed and disabled or enabled at runtime via the runtime configuration API.

Guards are where-conditions that control the lifecycle of subexpressions. Custom guard functions can also be used. The section Chapter 17, Integration and Extension outlines guard plug-in development in greater detail.

The pattern guard where-condition has no relationship to the EPL where clause that filters sets of events.

Take as an example the following pattern expression:

MyEvent where timer.within(10 sec)

In this pattern the timer:within guard controls the subexpression that is looking for MyEvent events. The guard terminates the subexpression looking for MyEvent events after 10 seconds after start of the pattern. Thus the pattern alerts only once when the first MyEvent event arrives within 10 seconds after start of the pattern.

The every keyword requires additional discussion since it also controls subexpression lifecycle. Let's add the every keyword to the example pattern:

every MyEvent where timer.within(10 sec)

The difference to the pattern without every is that each MyEvent event that arrives now starts a new subexpression, including a new guard, looking for a further MyEvent event. The result is that, when a MyEvent arrives within 10 seconds after pattern start, the pattern execution will look for the next MyEvent event to arrive within 10 seconds after the previous one.

By placing parentheses around the every keyword and its subexpression, we can have the every under the control of the guard:

(every MyEvent) where timer.within(10 sec)

In the pattern above, the guard terminates the subexpression looking for all MyEvent events after 10 seconds after start of the pattern. This pattern alerts for all MyEvent events arriving within 10 seconds after pattern start, and then stops.

Guards do not change the truth value of the subexpression of which the guard controls the lifecycle, and therefore do not cause a restart of the subexpression when used with the every operator. For example, the next pattern stops returning matches after 10 seconds unless a match occurred within 10 seconds after pattern start:

every ( (A and B) where timer.within(10 sec) )

The timer:within guard acts like a stopwatch. If the associated pattern expression does not turn true within the specified time period it is stopped and permanently false.

The synopsis for timer:within is as follows:

timer:within(time_period_expression)

The time_period_expression is a time period (see Section 5.2.1, “Specifying Time Periods”) or an expression providing a number of seconds as a parameter. The interval expression may contain references to properties of prior events in the same pattern as well as variables and substitution parameters.

This pattern fires if an A event arrives within 5 seconds after statement creation.

A where timer:within (5 seconds)

This pattern fires for all A events that arrive within 5 seconds. After 5 seconds, this pattern stops matching even if more A events arrive.

(every A) where timer:within (5 seconds)

This pattern matches for any one A or B event in the next 5 seconds.

( A or B ) where timer:within (5 sec)

This pattern matches for any 2 errors that happen 10 seconds within each other.

every (StatusEvent(status='ERROR') -> StatusEvent(status='ERROR') where timer:within (10 sec))

The following guards are equivalent:

timer:within(2 minutes 5 seconds)
timer:within(125 sec)
timer:within(125)

The timer:withinmax guard is similar to the timer:within guard and acts as a stopwatch that additionally has a counter that counts the number of matches. It ends the subexpression when either the stopwatch ends or the match counter maximum value is reached.

The synopsis for timer:withinmax is as follows:

timer:withinmax(time_period_expression, max_count_expression)

The time_period_expression is a time period (see Section 5.2.1, “Specifying Time Periods”) or an expression providing a number of seconds.

The max_count_expression provides the maximum number of matches before the guard ends the subexpression.

Each parameter expression may also contain references to properties of prior events in the same pattern as well as variables and substitution parameters.

This pattern fires for every A event that arrives within 5 seconds after statement creation but only up to the first two A events:

(every A) where timer:withinmax (5 seconds, 2)

If the result of the max_count_expression is 1, the guard ends the subexpression after the first match and indicates the first match.

This pattern fires for the first A event that arrives within 5 seconds after statement creation:

(every A) where timer:withinmax (5 seconds, 1)

If the result of the max_count_expression is zero, the guard ends the subexpression upon the first match and does no indicate any matches.

This example receives every A event followed by every B event (as each B event arrives) until the 5-second subexpression timer ends or X number of B events have arrived (assume X was declared as a variable):

every A -> (every B) where timer:withinmax (5 seconds, X)

Observers observe time-based events for which the thread-of-control originates by the engine timer or external timer event. Custom observers can also be developed that observe timer events or other engine-external application events such as a file-exists check. The section Chapter 17, Integration and Extension outlines observer plug-in development in greater detail.

The timer:interval observer waits for the defined time before the truth value of the observer turns true. The observer takes a time period (see Section 5.2.1, “Specifying Time Periods”) as a parameter, or an expression that returns the number of seconds.

The observer may be parameterized by an expression that contains one or more references to properties of prior events in the same pattern, or may also reference variables, substitution parameters or any other expression returning a numeric value.

After an A event arrived wait 10 seconds then indicate that the pattern matches.

A -> timer:interval(10 seconds) 

The pattern below fires every 20 seconds.

every timer:interval(20 sec)

The next example pattern fires for every A event that is not followed by a B event within 60 seconds after the A event arrived. The B event must have the same "id" property value as the A event.

every a=A -> (timer:interval(60 sec) and not B(id=a.id)) 

Consider the next example, which assumes that the A event has a property waittime:

every a=A -> (timer:interval(a.waittime + 2) and not B(id=a.id))

In the above pattern the logic waits for 2 seconds plus the number of seconds provided by the value of the waittime property of the A event.

The timer:at observer is similar in function to the Unix “crontab” command. At a specified time the expression turns true. The at operator can also be made to pattern match at regular intervals by using an every operator in front of the timer:at operator.

The syntax is: timer:at (minutes, hours, days of month, months, days of week [, seconds [, time zone]]).

The value for seconds and time zone is optional. Each element allows wildcard * values. Ranges can be specified by means of lower bounds then a colon ‘:’ then the upper bound. The division operator */x can be used to specify that every xth value is valid. Combinations of these operators can be used by placing these into square brackets ([]).

The timer:at observer may also be parameterized by an expression that contains one or more references to properties of prior events in the same pattern, or may also reference variables, substitution parameters or any other expression returning a numeric value. The frequency division operator */x and parameters lists within brackets ([]) are an exception: they may only contain variables, substitution parameters or numeric values.

This expression pattern matches every 5 minutes past the hour.

every timer:at(5, *, *, *, *)

The below timer:at pattern matches every 15 minutes from 8am to 5:45pm (hours 8 to 17 at 0, 15, 30 and 45 minutes past the hour) on even numbered days of the month as well as on the first day of the month.

timer:at (*/15, 8:17, [*/2, 1], *, *)

The below table outlines the fields, valid values and keywords available for each field:


The keyword last used in the days-of-month field means the last day of the month (current month). To specify the last day of another month, a value for the month field has to be provided. For example: timer:at(*, *, last,2,*) is the last day of February.

The last keyword in the day-of-week field by itself simply means Saturday. If used in the day-of-week field after another value, it means "the last xxx day of the month" - for example "5 last" means "the last Friday of the month". So the last Friday of the current month will be: timer:at(*, *, *, *, 5 last). And the last Friday of June: timer:at(*, *, *, 6, 5 last).

The keyword weekday is used to specify the weekday (Monday-Friday) nearest the given day. Variant could include month like in: timer:at(*, *, 30 weekday, 9, *) which for year 2007 is Friday September 28th (no jump over month).

The keyword lastweekday is a combination of two parameters, the last and the weekday keywords. A typical example could be: timer:at(*, *, *, lastweekday, 9, *) which will define Friday September 28th (example year is 2007).

The time zone is a string-type value that specifies the time zone of the schedule. You must specify a value for seconds when specifying a time zone. Esper relies on the java.util.TimeZone to interpret the time zone value. Note that TimeZone does not validate time zone strings.

The following timer:at pattern matches at 5:00 pm Pacific Standard Time (PST):

timer:at (0, 17, *, *, *, *, 'PST')

Any expression may occur among the parameters. This example invokes a user-defined function computeHour to return an hour:

timer:at (0, computeHour(), *, *, *, *)

The following restrictions apply to crontab parameters:

  • It is not possible to specify both Days Of Month and Days Of Week.

Using match recognize patterns are defined in the familiar syntax of regular expressions.

The match recognize syntax presents an alternative way to specify pattern detection as compared to the EPL pattern language described in the previous chapter. A comparison of match recognize and EPL patterns is below.

The match recognize syntax is a proposal for incorporation into the SQL standard. It is thus subject to change as the standard evolves and finalizes (it has not finalized yet). Please consult row-pattern-recogniton-11-public for further information.

You may be familiar with regular expressions in the context of finding text of interest in a string, such as particular characters, words, or patterns of characters. Instead of matching characters, match recognize matches sequences of events of interest.

Esper can apply match-recognize patterns in real-time upon arrival of new events in a stream of events (also termed incrementally, streaming or continuous). Esper can also match patterns on-demand via the iterator pull-API, if specifying a named window or data window on a stream.

This section compares pattern detection via match recognize and via the EPL pattern language.

Table 7.1. Comparison Match Recognize to EPL Patterns

CategoryEPL PatternsMatch Recognize
PurposePattern detection in sequences of events.Same.
StandardsNot standardized, similar to Rapide pattern language.Proposal for incorporation into the SQL standard.
Real-time ProcessingYes.Yes.
On-Demand query via IteratorNo.Yes.
LanguageNestable expressions consisting of boolean AND, OR, NOT and time or arrival-based constructs such as -> (followed-by), timer:within and timer:interval.Regular expression consisting of variables each representing conditions on events.
Event TypesAn EPL pattern may react to multiple different types of events.The input is a single type of event (unless used with variant streams).
Data Window InteractionDisconnected, i.e. an event leaving a data window does not change pattern state.Connected, i.e. an event leaving a data window removes the event from match selection.
Semantic EvaluationTruth-value based: A EPL pattern such as (A and B) can fire when a single event arrives that satisfies both A and B conditions.Sequence-based: A regular expression (A B) requires at least two events to match.
Time Relationship Between EventsThe timer:within, timer:interval and NOT operator can expressively search for absence of events or other more complex timing relationships.Some support for detecting absence of events using the interval clause.
ExtensibilityCustom pattern objects, user-defined functions.User-defined functions, custom aggregation functions.
Memory UseLikely between 500 bytes to 2k per open sequence, depends on pattern.Likely between 100 bytes to 1k per open sequence, depends on pattern.

The synopsis is as follows:

match_recognize (
  [ partition by partition_expression [, partition_expression] [,...]  ]
  measures  measure_expression as col_name [, measure_expression as col_name ] [,...]
  [ all matches ]
  [ after match skip (past last row | to next row | to current row) ]
  pattern ( variable_regular_expr [, variable_regular_expr] [,...] )
  [ interval time_period [or terminated] ]
  define  variable as variable_condition [, variable as variable_condition]  [,...] 
)

The match_recognize keyword starts the match recognize definition and occurs right after the from clause in an EPL select statement. It is followed by parenthesis that surround the match recognize definition.

Partition by is optional and may be used to specify that events are to be partitioned by one or more event properties or expressions. If there is no Partition by then all rows of the table constitute a single partition. The regular expression applies to events in the same partition and not across partitions.

The measures clause defines columns that contain expressions over the pattern variables. The expressions can reference partition columns, singleton variables, aggregates as well as indexed properties on the group variables. Each measure_expression expression must be followed by the as keyword and a col_name column name.

The all matches keywords are optional and instructs the engine to find all possible matches. By default matches are ranked and the engine returns a single match following an algorithm to eliminate duplicate matches, as described below. When specifying all matches, matches may overlap and may start at the same row.

The after match skip keywords are optional and serve to determine the resumption point of pattern matching after a match has been found. By default the behavior is after match skip past last row. This means that after eliminating duplicate matches, the logic skips to resume pattern matching at the next event after the last event of the current match.

The pattern component is used to specify a regular expression. The regular expression is built from variable names, and may use the operators such as *, +, ?, *?, +?, ?? quantifiers and | alteration (concatenation is indicated by the absence of any operator sign between two successive items in a pattern).

With the optional interval keyword, time period and or terminated you can control how long the engine should wait for further events to arrive that may be part of a matching event sequence, before indicating a match (or matches) (not applicable to on-demand pattern matching).

Define is a mandatory component, used to specify the boolean condition that defines a variable name that is declared in the pattern. A variable name does not require a definition and if there is no definition, the default is a predicate that is always true. Such a variable name can be used to match any row.

For illustration, the examples in this chapter use the TemperatureSensorEvent event. Each event has 3 properties: the id property is a unique event id, the device is a sensor device number and the temp property is a temperature reading. An event described as "id=E1, device=1, temp=100" is a TemperatureSensorEvent event with id "E1" for device 1 with a reading of 100.

This example statement looks for two TemperatureSensorEvent events from the same device, directly following each other, that indicate a jump in temperature of 10 or more between the two events:

select * from TemperatureSensorEvent
match_recognize (
  partition by device
  measures A.id as a_id, B.id as b_id, A.temp as a_temp, B.temp as b_temp
  pattern (A B)
  define 
    B as Math.abs(B.temp - A.temp) >= 10
)

The partition by ensures that the regular expression applies to sequences of events from the same device.

The measures clause provides a list of properties or expressions to be selected from matching events. Each property name must be prefixed by the variable name.

In the pattern component the statement declares two variables: A and B. As a matter of convention, variable names are uppercase characters.

The define clause specifies no condition for variable A. This means that A defaults to true and any event matches A. Therefore, the pattern can start at any event.

The pattern A B indicates to look for a pattern in which an event that fulfills the condition for variable A is immediately followed by an event that fulfills the condition for variable B. A pattern thus defines the sequence (or sequences) of conditions that must be met for the pattern to fire.

Below table is an example sequence of events and output of the pattern:


At time 4000 when event with id E4 (or event E4 or just E4 for short) arrives the pattern matches and produces an output event. Matching then skips past the last event of the current match (E4) and begins at event E5 (the default skip clause is past last row). Therefore events E4 and E5 do not constitute a match.

At time 3000, events E1 and E3 do not constitute a match as E3 does not immediately follow E, since there is E2 in between.

At time 7000, event E7 does not constitute a match as it is from device 2 and thereby not in the same partition as prior events.

The pattern specifies the pattern to be recognized in the ordered sequence of events in a partition using regular expression syntax. Each variable name in a pattern corresponds to a boolean condition, which is specified later using the define component of the syntax. Thus the pattern can be regarded as implicitly declaring one or more variable names; the definition of those variable names appears later in the syntax. If a variable is not defined the variable defaults to true.

It is permitted for a variable name to occur more than once in a pattern, for example pattern (A B A).

The execution of match recognize is continuous and real-time by default. This means that every arriving event, or batch of events if using batching, evaluates against the pattern and matches are immediately indicated. Elimination of duplicate matches occurs between all matches of the arriving events (or batch of events) at a given time.

As an alternative, and if your application does not require continuous pattern evaluation, you may use the iterator API to perform on-demand matching of the pattern. For the purpose of indicating to the engine to not generate continuous results, specify the @Hint('iterate_only') hint.

When using one-or-more, zero-or-more or zero-or-one quantifiers (?, +, *, ??, +?, *?), the output of the real-time continuous query can differ from the output of the on-demand iterator execution: The continuous query will output a match (or multiple matches) as soon as matches are detected at a given time upon arrival of events (not knowing what further events may arrive). The on-demand execution, since it knows all possible events in advance, can determine the longest match(es). Thus elimination of duplicate matches can lead to different results between real-time and on-demand use.

If the all matches keywords are specified, then all matches are returned as the result and no elimination of duplicate matches as below occurs.

Otherwise matches to a pattern in a partition are ordered by preferment. Preferment is given to matches based on the following priorities:

After ranking matches by preferment, matches are chosen as follows:

Within define are listed the boolean conditions that defines a variable name that is declared in the pattern.

A variable name does not require a definition and if there is no definition, the default is a predicate that is always true. Such a variable name can be used to match any row.

The definitions of variable names may reference the same or other variable names as prior examples have shown.

If a variable in your condition expression is a singleton variable, then only individual columns may be referenced. If the variable is not matched by an event, a null value is returned.

If a variable in your condition expression is a group variable, then only indexed columns may be referenced. If the variable is not matched by an event, a null value is returned.

Aggregation functions are not allowed within expressions of the define clause. However define-clause expressions can utilize enumeration methods.

The prev function may be used in a define expression to access columns of the previous row of a variable name. If there is no previous row, the null value is returned.

The prev function can accept an optional non-negative integer argument indicating the offset to the previous rows. That argument must be a constant. In this case, the engine returns the property from the N-th row preceding the current row, and if the row doesn’t exist, it returns null.

This function can access variables currently defined, for example:

Y as Y.price < prev(Y.price, 2)

It is not legal to use prev with another variable then the one being defined:

// not allowed
Y as Y.price < prev(X.price, 2)

The prev function returns properties of events in the same partition. Also, it returns properties of events according to event order-of-arrival. When using data windows or deleting events from a named window, the remove stream does not remove events from the prev function.

The pattern looks for an event in which the temperature is greater or equal 100 and that, relative to that event, has an event preceding it by 2 events that also had a temperature greater or equal 100:

select * from TemperatureSensorEvent
match_recognize (
  partition by device
  measures A.id as a_id
  pattern (A)
  define 
	A as A.temp > 100 and prev(A.temp, 2) > 100)

An example sequence of events that matches the pattern above is:


The interval keyword and time period can be followed by or terminated keywords. When or-terminated is specified, the engine detects when a pattern state cannot match further and outputs matching event sequences collected so far that are otherwise only output at the end of the interval. This is not applicable to on-demand pattern matching.

Same as for interval alone, the interval timer starts are the arrival of the first event matching a sequence for a partition. Event arrival can terminate the interval and lead to immediate output as follows:

The next example looks for sensor events indicating a temperature of over 100, waiting for any number of additional events with a temperature of over 100 for 5 seconds or when the temperature falls to equal or below 100, whichever happens first:

select * from TemperatureSensorEvent
match_recognize (
partition by device
measures A.id as a_id, count(B.id) as count_b, first(B.id) as first_b, last(B.id) as last_b
pattern (A B*)
interval 5 seconds or terminated
define 
  A as A.temp > 100,
  B as B.temp > 100)

An example sequence of events that matches the pattern above is:


Note

Interval and Interval with or terminated make most sense for open-ended patterns such as, for example, pattern (A B*) or pattern (A B C+).

For patterns that terminate when a given event arrives, for example, pattern (A B), an Interval in combination with or terminated should not be specified and if specified have no effect on matching.

Esper arithmetic and logical operator precedence follows Java standard arithmetic and logical operator precedence.

The { and } curly braces are array definition operators following the Java array initialization syntax. Arrays can be useful to pass to user-defined functions or to select array data in a select clause.

Array definitions consist of zero or more expressions within curly braces. Any type of expression is allowed within array definitions including constants, arithmetic expressions or event properties. This is the syntax of an array definition:

{ [expression [,expression...]] }

Consider the next statement that returns an event property named actions. The engine populates the actions property as an array of java.lang.String values with a length of 2 elements. The first element of the array contains the observation property value and the second element the command property value of RFIDEvent events.

select {observation, command} as actions from RFIDEvent

The engine determines the array type based on the types returned by the expressions in the array definiton. For example, if all expressions in the array definition return integer values then the type of the array is java.lang.Integer[]. If the types returned by all expressions are compatible number types, such as integer and double values, the engine coerces the array element values and returns a suitable type, java.lang.Double[] in this example. The type of the array returned is Object[] if the types of expressions cannot be coerced or return object values. Null values can also be used in an array definition.

Arrays can come in handy for use as parameters to user-defined functions:

select * from RFIDEvent where Filter.myFilter(zone, {1,2,3})

You can use the dot operator to invoke a method on the result of an expression. The dot operator uses the dot (.) or period character.

The dot-operator is relevant with enumeration methods: Enumeration methods perform tasks such as transformation, filtering, aggregation, sequence-matching, sorting and others on subquery results, named windows, event properties or inputs that are or can be projected to a collection of events, scalar values or objects. See Chapter 10, EPL Reference: Enumeration Methods

Further the dot-operator is relevant to date-time methods. Date-time methods work on date-time values to add or subtract time periods, set or round calendar fields or query fields, among other tasks. See Chapter 11, EPL Reference: Date-Time Methods.

This section only describes the dot-operator in relation to property instance methods, the special get and size indexed-property methods and duck typing.

The synopsis for the dot operator is as follows

expression.method([parameter [,...]])[.method(...)][...]

The expression to evaluate by the dot operator is in parenthesis. After the dot character follows the method name and method parameters in parenthesis.

You may use the dot operator when your expression returns an object that you want to invoke a method on. The dot operator allows duck typing and convenient array and collection access methods.

This example statement invokes the getZones method of the RFID event class by referring to the stream name assigned in the from-clause:

select rfid.getZones() from RFIDEvent as rfid

The size() method can be used to return the array length or collection size. Use the get method to return the value at a given index for an array or collection.

The next statement selects array size and returns the last array element:

select arrayproperty.size() as arraySize, 
  arrayproperty.get((arrayproperty).size - 1) as lastInArray 
  from ProductEvent

The in keyword determines if a given value matches any value in a list. The syntax of the keyword is:

test_expression [not] in (expression [,expression...] )

The test_expression is any valid expression. The keyword is followed by a list of expressions to test for a match. The optional not keyword specifies that the result of the predicate be negated.

The result of an in expression is of type Boolean. If the value of test_expression is equal to any expression from the comma-separated list, the result value is true. Otherwise, the result value is false.

The next example shows how the in keyword can be applied to select certain command types of RFID events:

select * from RFIDEvent where command in ('OBSERVATION', 'SIGNAL')

The statement is equivalent to:

select * from RFIDEvent where command = 'OBSERVATION' or command = 'SIGNAL'

Expression may also return an array, a java.util.Collection or a java.util.Map. Thus event properties that are lists, sets or maps may provide values to compare against test_expression.

All expressions must be of the same type or a compatible type to test_expression. The in keyword may coerce number values to compatible types. If expression returns an array, then the component type of the array must be compatible, unless the component type of the array is Object.

If expression returns an array of component type Object, the operation compares each element of the array, applying equals semantics.

If expression returns a Collection, the operation determines if the collection contains the value returned by test_expression, applying contains semantics.

If expression returns a Map, the operation determines if the map contains the key value returned by test_expression, applying containsKey semantics.

Constants, arrays, Collection and Map expressions or event properties can be used combined.

For example, and assuming a property named 'mySpecialCmdList' exists that contains a list of command strings:

select * from RFIDEvent where command in ( 'OBSERVATION', 'SIGNAL', mySpecialCmdList)

When using prepared statements and substitution parameters with the in keyword, make sure to retain the parenthesis. Substitution values may also be arrays, Collection and Map values:

test_expression [not] in (? [,?...] )

Note that if there are no successes and at least one right-hand row yields null for the operator's result, the result of the any construct will be null, not false. This is in accordance with SQL's normal rules for Boolean combinations of null values.

The in keyword can be used to specify ranges, including open, half-closed, half-open and inverted ranges.

Ranges come in the following 4 varieties. The round () or square [] bracket indicate whether an endpoint is included or excluded. The low point and the high-point of the range are separated by the colon : character.

The following statement two statements are equivalent: Both statements select orders where the price is in the range of zero and 10000 (endpoints inclusive):

select * from OrderEvent where price in [0:10000]
select * from OrderEvent where price between 0 and 10000

The next statement selects order events where the price is greater then 100 and less-or-equal to 2000:

select * from OrderEvent where price in (100:2000]

Use the not in keywords to specify an inverted range.

The following statement selects an inverted range by selecting all order events where the price is less then zero or the price is greater or equal to 10000:

select * from OrderEvent where price not in (0:10000]

In case the value of low endpoint is less then the value of high endpoint the in operator reverses the range.

The following two statements are also equivalent:

select * from OrderEvent where price in [10000:0]
select * from OrderEvent where price >= 0 and price <= 1000

The any operator is true if the expression returns true for one or more of the values returned by a list of expressions including array, Collection and Map values.

The synopsis for the any keyword is as follows:

expression operator any (expression [,expression...] )

The left-hand expression is evaluated and compared to each expression result using the given operator, which must yield a Boolean result. The result of any is "true" if any true result is obtained. The result is "false" if no true result is found (including the special case where the expressions are collections that return no rows).

The operator can be any of the following values: =, !=, <>, <, <=, >, >=.

The some keyword is a synonym for any. The in construct is equivalent to = any.

Expression may also return an array, a java.util.Collection or a java.util.Map. Thus event properties that are lists, sets or maps may provide values to compare against.

All expressions must be of the same type or a compatible type. The any keyword coerces number values to compatible types. If expression returns an array, then the component type of the array must be compatible, unless the component type of the array is Object.

If expression returns an array, the operation compares each element of the array.

If expression returns a Collection, the operation determines if the collection contains the value returned by the left-hand expression, applying contains semantics. When using relationship operators <, <=, >, >= the operator applies to each element in the collection, and non-numeric elements are ignored.

If expression returns a Map, the operation determines if the map contains the key value returned by the left-hand expression, applying containsKey semantics. When using relationship operators <, <=, >, >= the operator applies to each key in the map, and non-numeric map keys are ignored.

Constants, arrays, Collection and Map expressions or event properties can be used combined.

The next statement demonstrates the use of the any operator:

select * from ProductOrder where category != any (categoryArray)

The above query selects ProductOrder event that have a category field and a category array, and returns only those events in which the category value is not in the array.

Note that if there are no successes and at least one right-hand row yields null for the operator's result, the result of the any construct will be null, not false. This is in accordance with SQL's normal rules for Boolean combinations of null values.

The all operator is true if the expression returns true for all of the values returned by a list of expressions including array, Collection and Map values.

The synopsis for the all keyword is as follows:

expression operator all (expression [,expression...] )

The left-hand expression is evaluated and compared to each expression result using the given operator, which must yield a Boolean result. The result of all is "true" if all rows yield true (including the special case where the expressions are collections that returns no rows). The result is "false" if any false result is found. The result is null if the comparison does not return false for any row, and it returns null for at least one row.

The operator can be any of the following values: =, !=, <>, <, <=, >, >=.

The not in construct is equivalent to != all.

Expression may also return an array, a java.util.Collection or a java.util.Map. Thus event properties that are lists, sets or maps may provide values to compare against.

All expressions must be of the same type or a compatible type. The all keyword coerces number values to compatible types. If expression returns an array, then the component type of the array must be compatible, unless the component type of the array is Object.

If expression returns an array, the operation compares each element of the array.

If expression returns a Collection, the operation determines if the collection contains the value returned by the left-hand expression, applying contains semantics. When using relationship operators <, <=, >, >= the operator applies to each element in the collection, and non-numeric elements are ignored.

If expression returns a Map, the operation determines if the map contains the key value returned by the left-hand expression, applying containsKey semantics. When using relationship operators <, <=, >, >= the operator applies to each key in the map, and non-numeric map keys are ignored.

Constants, arrays, Collection and Map expressions or event properties can be used combined.

The next statement demonstrates the use of the all operator:

select * from ProductOrder where category = all (categoryArray)

The above query selects ProductOrder event that have a category field and a category array, and returns only those events in which the category value matches all values in the array.

The new operator populates a new data structure by evaluating column names and assignment expressions. This is useful when an expression should return multiple results, for performing a transformation or inside enumeration method lambda expressions.

The synopsis for the new keyword is as follows:

new { column_name = [assignment_expression] [,column_name...] }

The result of the new-operator is a map data structure that contains column_name keys and values. If an assignment expression is provided for a column, the operator evaluates the expression and assigns the result to the column name. If no assignment expression is provided, the column name is assumed to be an event property name and the value is the event property value.

The next statement demonstrates the use of the new operator:

select new {category, price = 2*price} as priceInfo from ProductOrder

The above query returns a single property priceInfo for each arriving ProductOrder event. The property value is itself a map that contains two entries: For the key name category the value of the category property and for the key name price the value of the price property multiplied by two.

The next EPL is an example of the new operator within an expression definition and a case-statement (one EPL statement not multiple):

expression calcPrice {
  productOrder => case
    when category = 'fish' then new { sterialize = 'XRAY', priceFactor = 1.01 }
    when category = 'meat' then new { sterialize = 'UVL', priceFactor = 1 }
  end
}  

select calcPrice(po) as priceDetail from ProductOrder po

In above example the expression calcPrice returns both a sterialize string value and a priceFactor double value. The expression is evaluated as part of the select-clause and the map-type result placed in the priceDetail property pf output events.

When used within the case operator, the operator validates that the data structure is compatible between each case-when result in terms of column names and types. The default value for else in case is null.

Single-row functions return a single value for every single result row generated by your statement. These functions can appear anywhere where expressions are allowed.

Esper allows static Java library methods as single-row functions, and also features built-in single-row functions. In addition, Esper allows instance method invocations on named streams.

You may also register your own single-row function name with the engine so that your EPL statements become less cluttered. This is described in detail in Section 17.3, “Single-Row Function”. Single-row functions that return an object can be chained.

Esper auto-imports the following Java library packages:

  • java.lang.*

  • java.math.*

  • java.text.*

  • java.util.*

Thus Java static library methods can be used in all expressions as shown in below example:

select symbol, Math.round(volume/1000)
from StockTickEvent.win:time(30 sec)

In general, arbitrary Java class names have to be fully qualified (e.g. java.lang.Math) but Esper provides a mechanism for user-controlled imports of classes and packages as outlined in Section 15.4.6, “Class and package imports”.

The below table outlines the built-in single-row functions available.

Table 9.1. Syntax and results of single-row functions

Single-row FunctionResult
case value 
  when compare_value then result
  [when compare_value then result ...] 
  [else result] 
  end 

Returns result where the first value equals compare_value.

case 
  when condition then result
  [when condition then result ...] 
  [else result] 
  end

Returns the result for the first condition that is true.

cast(expression, type_name)

Casts the result of an expression to the given type.

coalesce(expression, expression [, expression ...])

Returns the first non-null value in the list, or null if there are no non-null values.

current_timestamp[()]

Returns the current engine time as a long millisecond value. Reserved keyword with optional parenthesis.

exists(dynamic_property_name)

Returns true if the dynamic property exists for the event, or false if the property does not exist.

instanceof(expression, type_name [, type_name ...])

Returns true if the expression returns an object whose type is one of the types listed.

istream()

Returns true if the event is part of the insert stream and false if the event is part of the remove stream.

max(expression, expression [, expression ...])

Returns the highest numeric value among the 2 or more comma-separated expressions.

min(expression, expression [, expression ...])

Returns the lowest numeric value among the 2 or more comma-separated expressions.

prev(expression, event_property)

Returns a property value or all properties of a previous event, relative to the event order within a data window, or according to an optional index parameter (N) the positional Nth-from-last value.

prevtail(expression, event_property)

Returns a property value or all properties of the first event in a data window relative to the event order within a data window, or according to an optional index parameter (N) the positional Nth-from-first value.

prevwindow(event_property)

Returns a single property value of all events or all properties of all events in a data window in the order that reflects the sort order of the data window.

prevcount(event_property)

Returns the count of events (number of data points) in a data window.

prior(integer, event_property)

Returns a property value of a prior event, relative to the natural order of arrival of events

typeof(expression)

If expression is a stream name, returns the event type name of the evaluated event, often used with variant streams. If expression is a property name or expression, returns the name of the expression result type.


The case control flow function has two versions. The first version takes a value and a list of compare values to compare against, and returns the result where the first value equals the compare value. The second version takes a list of conditions and returns the result for the first condition that is true.

The return type of a case expression is the compatible aggregated type of all return values.

The case expression is sometimes used with the new operator to return multiple results, see Section 8.13, “The 'new' Keyword”.

The example below shows the first version of a case statement. It has a String return type and returns the value 'one'.

select case myexpression when 1 then 'one' when 2 then 'two' else 'more' end from ...

The second version of the case function takes a list of conditions. The next example has a Boolean return type and returns the boolean value true.

select case when 1>0 then true else false end from ...

The cast function casts the return type of an expression to a designated type. The function accepts two parameters: The first parameter is the property name or expression that returns the value to be casted. The second parameter is the type to cast to.

Valid parameters for the second (type) parameter are:

  • Any of the Java built-in types: int, long, byte, short, char, double, float, string, BigInteger, BigDecimal, where string is a short notation for java.lang.String and BigInteger as well as BigDecimal are the classes in java.math. The type name is not case-sensitive. For example:

    cast(price, double)

  • The fully-qualified class name of the class to cast to, for example:

    cast(product, org.myproducer.Product)

The cast function is often used to provide a type for dynamic (unchecked) properties. Dynamic properties are properties whose type is not known at compile type. These properties are always of type java.lang.Object.

The cast function as shown in the next statement casts the dynamic "price" property of an "item" in the OrderEvent to a double value.

select cast(item.price?, double) from OrderEvent

The cast function returns a null value if the expression result cannot be casted to the desired type, or if the expression result itself is null.

The cast function adheres to the following type conversion rules:

  • For all numeric types, the cast function utilitzes java.lang.Number to convert numeric types, if required.

  • For casts to string or java.lang.String, the function calls toString on the expression result.

  • For casts to other objects including application objects, the cast function considers a Java class's superclasses as well as all directly or indirectly-implemented interfaces by superclasses .

The instanceof function returns a boolean value indicating whether the type of value returned by the expression is one of the given types. The first parameter to the instanceof function is an expression to evaluate. The second and subsequent parameters are Java type names.

The function determines the return type of the expression at runtime by evaluating the expression, and compares the type of object returned by the expression to the defined types. If the type of object returned by the expression matches any of the given types, the function returns true. If the expression returned null or a type that does not match any of the given types, the function returns false.

The instanceof function is often used in conjunction with dynamic (unchecked) properties. Dynamic properties are properties whose type is not known at compile type.

This example uses the instanceof function to select different properties based on the type:

select case when instanceof(item, com.mycompany.Service) then serviceName?
  when instanceof(item, com.mycompany.Product) then productName? end 
  from OrderEvent

The instanceof function returns false if the expression tested by instanceof returned null.

Valid parameters for the type parameter list are:

  • Any of the Java built-in types: int, long, byte, short, char, double, float, string, where string is a short notation for java.lang.String. The type name is not case-sensitive. For example, the next function tests if the dynamic "price" property is either of type float or type double:

    instanceof(price?, double, float)

  • The fully-qualified class name of the class to cast to, for example:

    instanceof(product, org.myproducer.Product)

The function considers an event class's superclasses as well as all the directly or indirectly-implemented interfaces by superclasses.

The prev function returns the property value or all event properties of a previous event. For data windows that introduce a sort order other then the order of arrival, such as the sorted data window and the time order data window, the function returns the event at the specified position.

The prev function is not an aggregation function and therefore does not return results per group when used with group by. Please consider the last, lastever or first aggregation functions instead as described in Section 9.2.2, “Event Aggregation Functions”. You must use an aggregation function instead of prev when querying a named window.

The first parameter to the prev function is an index parameter and denotes the i-th previous event, in the order established by the data window. If no index is provided, the default index is 1 and the function returns the previous event. The second parameter is a property name or stream name. If specifying a property name, the function returns the value for the previous event property value. If specifying a stream name, the function returns the previous event underlying object.

This example selects the value of the price property of the 2nd-previous event from the current Trade event:

select prev(2, price) from Trade.win:length(10)

By using the stream alias in the previous function, the next example selects the trade event itself that is immediately previous to the current Trade event

select prev(1, trade) from Trade.win:length(10) as trade

Since the prev function takes the order established by the data window into account, the function works well with sorted windows.

In the following example the statement selects the symbol of the 3 Trade events that had the largest, second-largest and third-largest volume.

select prev(0, symbol), prev(1, symbol), prev(2, symbol)
  from Trade.ext:sort(3, volume desc)

The i-th previous event parameter can also be an expression returning an Integer-type value. The next statement joins the Trade data window with an RankSelectionEvent event that provides a rank property used to look up a certain position in the sorted Trade data window:

select prev(rank, symbol) from Trade.ext:sort(10, volume desc), RankSelectionEvent unidirectional

Use the prev function in combination with a grouped data window to access a previous event per grouping criteria.

The example below returns the price of the previous Trade event for the same symbol, or null if for that symbol there is no previous Trade event:

select prev(1, price) from Trade.std:groupwin(symbol).win:length(2)

The prev function returns a null value if the data window does not currently hold the i-th previous event. The example below illustrates this using a time batch window. Here the prev function returns a null value for any events in which the previous event is not in the same batch of events. Note that the prior function as discussed below can be used if a null value is not the desired result.

select prev(1, symbol) from Trade.win:time_batch(1 min)

An alternative form of the prev function allows the index to not appear or appear after the property name if the index value is a constant and not an expression:

select prev(1, symbol) from Trade
// ... equivalent to ...
select prev(symbol) from Trade
// ... and ...
select prev(symbol, 1) from Trade

The combination of the prev function and std:groupwin view returns the property value for a previous event in the given data window group.

The following example returns for each event the current smallest price per symbol:

select symbol, prev(0, price) as topPricePerSymbol 
from Trade.std:groupwin(symbol).ext:sort(1, price asc)

The prevtail function returns the property value or all event properties of the positional-first event in a data window. For data windows that introduce a sort order other then the order of arrival, such as the sorted data window and the time order data window, the function returns the first event at the specified position.

The prevtail function is not an aggregation function and therefore does not return results per group when used with group by. Please consider the first, firstever or window aggregation functions instead as described in Section 9.2.2, “Event Aggregation Functions”. You must use an aggregation function instead of prevtail when querying a named window.

The first parameter is an index parameter and denotes the i-th from-first event in the order established by the data window. If no index is provided the default is zero and the function returns the first event in the data window. The second parameter is a property name or stream name. If specifying a property name, the function returns the value for the previous event property value. If specifying a stream name, the function returns the previous event underlying object.

This example selects the value of the price property of the first (oldest) event held in the length window:

select prevtail(price) from Trade.win:length(10)

By using the stream alias in the prevtail function, the next example selects the trade event itself that is the second event held in the length window:

select prevtail(1, trade) from Trade.win:length(10) as trade

Since the prevtail function takes the order established by the data window into account, the function works well with sorted windows.

In the following example the statement selects the symbol of the 3 Trade events that had the smallest, second-smallest and third-smallest volume.

select prevtail(0, symbol), prevtail(1, symbol), prevtail(2, symbol)
  from Trade.ext:sort(3, volume asc)

The i-th previous event parameter can also be an expression returning an Integer-type value. The next statement joins the Trade data window with an RankSelectionEvent event that provides a rank property used to look up a certain position in the sorted Trade data window:

select prevtail(rank, symbol) from Trade.ext:sort(10, volume asc), RankSelectionEvent unidirectional

The prev function returns a null value if the data window does not currently holds positional-first or the Nth-from-first event. For batch data windows the value returned is relative to the current batch.

The following example returns the first and second symbol value in the batch:

select prevtail(0, symbol), prevtail(1, symbol) from Trade.win:time_batch(1 min)

An alternative form of the prevtail function allows the index to not appear or appear after the property name if the index value is a constant and not an expression:

select prevtail(1, symbol) from Trade
// ... equivalent to ...
select prevtail(symbol) from Trade
// ... and ...
select prevtail(symbol, 1) from Trade

The combination of the prevtail function and std:groupwin view returns the property value for a positional first event in the given data window group.

Let's look at an example. This statement outputs the oldest price per symbol retaining the last 10 prices per symbol:

select symbol, prevtail(0, price) as oldestPrice
from Trade.std:groupwin(symbol).win:length(10)

The prevwindow function returns property values or all event properties for all events in a data window. For data windows that introduce a sort order other then the order of arrival, such as the sorted data window and the time order data window, the function returns the event data sorted in that order, otherwise it returns the events sorted by order of arrival with the newest arriving event first.

The prevwindow function is not an aggregation function and therefore does not return results per group when used with group by. Please consider the window aggregation function instead as described in Section 9.2.2, “Event Aggregation Functions”. You must use an aggregation function instead of prevwindow when querying a named window.

The single parameter is a property name or stream name. If specifying a property name, the function returns the value of the event property for all events held by the data window. If specifying a stream name, the function returns the event underlying object for all events held by the data window.

This example selects the value of the price property of all events held in the length window:

select prevwindow(price) from Trade.win:length(10)

By using the stream alias in the prevwindow function, the next example selects all trade events held in the length window:

select prevwindow(trade) from Trade.win:length(10) as trade

When used with a data window that introduces a certain sort order, the prevwindow function returns events sorted according to that sort order.

The next statement outputs for every arriving event the current 10 underying trade event objects that have the largest volume:

select prevwindow(trade) from Trade.ext:sort(10, volume desc) as trade

The prevwindow function returns a null value if the data window does not currently hold any events.

The combination of the prevwindow function and std:groupwin view returns the property value(s) for all events in the given data window group.

This example statement outputs all prices per symbol retaining the last 10 prices per symbol:

select symbol, prevwindow(price) from Trade.std:groupwin(symbol).win:length(10)

The prevcount function returns the number of events held in a data window.

The prevcount function is not an aggregation function and therefore does not return results per group when used with group by. Please consider the count(*) aggregation function instead as described in Section 9.2, “Aggregation Functions”. You must use an aggregation function instead of prevcount when querying a named window.

The single parameter is a property name or stream name of the data window to return the count for.

This example selects the number of data points for the price property held in the length window:

select prevcount(price) from Trade.win:length(10)

By using the stream alias in the prevcount function the next example selects the count of trade events held in the length window:

select prevcount(trade) from Trade.win:length(10) as trade

The combination of the prevcount function and std:groupwin view returns the count of events in the given data window group.

This example statement outputs the number of events retaining the last 10 events per symbol:

select symbol, prevcount(price) from Trade.std:groupwin(symbol).win:length(10)

The typeof function, when parameterized by a stream name, returns the event type name of the evaluated event which can be useful with variant streams. When parameterized by an expression or property name, the function returns the type name of the expression result or null if the expression result is null.

In summary, the function determines the return type of the expression at runtime by evaluating the expression and returns the type name of the expression result.

The typeof function is often used in conjunction with variant streams. A variant stream is a predefined stream into which events of multiple disparate event types can be inserted. The typeof function, when passed a stream name alias, returns the name of the event type of each event in the stream.

The following example elaborates on the use of variant streams with typeof. The first statement declares a variant stream SequencePatternStream:

create variant schema SequencePatternStream as *

The next statement inserts all order events and is followed by a statement to insert all product events:

insert into SequencePatternStream select * from OrderEvent;
insert into SequencePatternStream select * from PriceEvent;

This example statement returns the event type name for each event in the variant stream:

select typeof(sps) from SequencePatternStream as sps

The next example statement detects a pattern by utilizing the typeof function to find pairs of order event immediately followed by product event:

select * from SequencePatternStream match_recognize(
  measures A as a, B as b
  pattern (A B)
  define A as typeof(A) = "OrderEvent",
         B as typeof(B) = "ProductEvent"
  )

When passing a property name to the typeof function, the function evaluates whether the property type is event type (a fragment event type). If the property type is event type, the function returns the type name of the event in the property value or null if not provided. If the property type is not event type, the function returns the simple class name of the property value.

When passing an expression to the typeof function, the function evaluates the expression and returns the simple class name of the expression result value or null if the expression result value is null.

This example statement returns the simple class name of the value of the dynamic property prop of events in stream MyStream, or a null value if the property is not found for an event or the property value itself is null:

select typeof(prop?) from MyStream

When using subclasses or interface implementations as event classes or when using Map-event type inheritance, the function returns the event type name provided when the class or Map-type event was registered, or if the event type was not registered, the function returns the fully-qualified class name.

Aggregation functions are stateful and consider sets of events or value points. The group by clause is often used in conjunction with aggregation functions to group the result-set by one or more columns.

The EPL language extends the standard SQL aggregation functions by allowing filters and by further useful aggregation functions that can track a data window or compute event rates, for example. Your application may also add its own aggregation function as Section 17.5, “Aggregation Function” describes.

Aggregation values are always computed incrementally: Insert and remove streams result in aggregation value changes. The exceptions are on-demand queries and joins when using the unidirectional keyword. Aggregation functions are optimized to retain the minimal information necessary to compute the aggregated result, and to share aggregation state between eligible other aggregation functions in the same statement so that same-kind aggregation state is never held multiple times unless required.

Most aggregation functions can also be used with unbound streams when no data window is specified. A few aggregation functions require a data window or named window as documented below.

The SQL-standard aggregation functions are shown in below table.

Table 9.2. Syntax and results of SQL-standard aggregation functions

Aggregate FunctionResult
avedev([all|distinct] expression [, filter_expr])

Mean deviation of the (distinct) values in the expression, returning a value of double type.

The optional filter expression limits the values considered for computing the mean deviation.

avg([all|distinct] expression [, filter_expr])

Average of the (distinct) values in the expression, returning a value of double type.

The optional filter expression limits the values considered for computing the average.

count([all|distinct] expression [, filter_expr])

Number of the (distinct) non-null values in the expression, returning a value of long type.

The optional filter expression limits the values considered for the count.

count(* [, filter_expr])

Number of events, returning a value of long type.

The optional filter expression limits the values considered for the count.

max([all|distinct] expression)

fmax([all|distinct] expression, filter_expr)

Highest (distinct) value in the expression, returning a value of the same type as the expression itself returns.

Use fmax to provide a filter expression that limits the values considered for computing the maximum.

Consider using maxby instead if return values must include additional properties.

median([all|distinct] expression [, filter_expr])

Median (distinct) value in the expression, returning a value of double type. Double Not-a-Number (NaN) values are ignored in the median computation.

The optional filter expression limits the values considered for computing the median.

min([all|distinct] expression)

fmin([all|distinct] expression, filter_expr)

Consider using minby instead if return values must include additional properties.

Lowest (distinct) value in the expression, returning a value of the same type as the expression itself returns.

Use fmin to provide a filter expression that limits the values considered for computing the maximum.

stddev([all|distinct] expression [, filter_expr])

Standard deviation of the (distinct) values in the expression, returning a value of double type.

The optional filter expression limits the values considered for computing the standard deviation.

sum([all|distinct] expression [, filter_expr])

Totals the (distinct) values in the expression, returning a value of long, double, float or integer type depending on the expression.

The optional filter expression limits the values considered for computing the total.


If your application provides double-type values to an aggregation function, avoid using Not-a-Number (NaN) and infinity. Also when using double-type values, round-off errors (or rounding errors) may occur due to double-type precision. Consider rounding your result value to the desired precision.

Each of the aggregation functions above takes an optional filter expression as a parameter. The filter expression must return a boolean-type value and applies to the events considered for the aggregation. If a filter expression is provided, then only if the filter expression returns a value of true does the engine update the aggregation for that event or combination of events.

Consider the following example, which computes the quantity fraction of buy orders among all orders:

select sum(quantity, side='buy') / sum(quantity) as buy_fraction from Orders

Use the fmin and fmax aggregation functions instead of the min and max aggregation functions when providing a filter expression (the min and max functions are also single-row functions).

The next example computes the minimum quantity for buy orders and a separate minimum quantity for sell orders:

select fmin(quantity, side='buy'), fmin(quantity, side = 'sell') from Orders

The event aggregation functions return one or more events or event properties. When used with group by the event aggregation functions return one or more events or event properties per group.

The sorted and the window event aggregation functions require that a data window or named window is declared for the applicable stream. They cannot be used on unbound streams.

The below table summarizes the event aggregation functions available:


In connection with named windows, event aggregation functions can also be used in on-select, selects with named window in the from clause, subqueries against named windows and on-demand fire-and-forget queries.

The event aggregation functions are often useful in connection with enumeration methods and they can provide input events for enumeration. Please see Chapter 10, EPL Reference: Enumeration Methods for more information.

When comparing the last aggregation function to the prev function, the differences are as follows. The prev function is not an aggregation function and thereby not sensitive to the presence of group by. The prev function accesses data window contents directly and respects the sort order of the data window. The last aggregation function returns results based on arrival order and tracks data window contents in a separate shared data structure.

When comparing the first aggregation function to the prevtail function, the differences are as follows. The prevtail function is not an aggregation function and thereby not sensitive to the presence of group by. The prevtail function accesses data window contents directly and respects the sort order of the data window. The first aggregation function returns results based on arrival order and tracks data window contents in a separate shared data structure.

When comparing the window aggregation function to the prevwindow function, the differences are as follows. The prevwindow function is not an aggregation function and thereby not sensitive to the presence of group by. The prevwindow function accesses data window contents directly and respects the sort order of the data window. The window aggregation function returns results based on arrival order and tracks data window contents in a separate shared data structure.

When comparing the count aggregation function to the prevcount function, the differences are as follows. The prevcount function is not an aggregation function and thereby not sensitive to the presence of group by.

When comparing the last aggregation function to the nth aggregation function, the differences are as follows. The nth aggregation function does not consider out-of-order deletes (for example with on-delete and sorted windows) and does not revert to the prior expression value when the last event or nth-event was deleted from a data window. The last aggregation function tracks the data window and reflects out-of-order deletes.

From an implementation perspective, the first, last and window aggregation functions share a common data structure for each stream. The sorted, minby and maxby aggregation functions share a common data structure for each stream.

The synopsis for the first aggregation function is:

first(*|stream.*|value_expression [, index_expression])

The first aggregation function returns properties of the very first event. When used with group by, it returns properties of the first event for each group. When specifying an index expression, the function returns properties of the Nth-subsequent event to the first event, all according to order of arrival.

The first parameter to the function is required and defines the event properties or expression result to return. The second parameter is an optional index_expression that must return an integer value used as an index to evaluate the Nth-subsequent event to the first event.

You may specify the wildcard (*) character in which case the function returns the underlying event of the single selected stream. When selecting a single stream you may specify no parameter instead of wildcard. For joins and subqueries you must use the stream wildcard syntax below.

You may specify the stream name and wildcard (*) character in the stream.* syntax. This returns the underlying event for the specified stream.

You may specify a value_expression to evaluate for the first event. The value expression may not select properties from multiple streams.

The index_expression is optional. If no index expression is provided, the function returns the first event. If present, the function evaluates the index expression to determine the value for N, and evaluates the Nth-subsequent event to the first event. A value of zero returns the first event and a value of 1 returns the event subsequent to the first event. You may not specify event properties in the index expression.

The function returns null if there are no events or when the index is larger than the number of events held. When used with group by, it returns null if there are no events for that group or when the index is larger than the number of events held for that group.

To explain, consider the statement below which selects the underlying event of the first sensor event held by the length window of 2 events.

select first(*) from SensorEvent.win:length(2) 

Assume event E1, event E2 and event E3 are of type SensorEvent. When event E1 arrives the statement outputs the underlying event E1. When event E2 arrives the statement again outputs the underlying event E1. When event E3 arrives the statement outputs the underlying event E2, since event E1 has left the data window.

The stream wildcard syntax is useful for joins and subqueries. This example demonstrates a subquery that returns the first SensorEvent when a DoorEvent arrives:

select (select first(se.*) from SensorEvent.win:length(2) as se) from DoorEvent 

The following example shows the use of an index expression. The output value for f1 is the temperature property value of the first event, the value for f2 is the temperature property value of the second event:

select first(temperature, 0) as f1, first(temperature, 1) as f2
from SensorEvent.win:time(10 sec)

You may use dot-syntax to invoke a method on the first event. You may also append a property name using dot-syntax.

The synopsis for the last aggregation function is:

last(*|stream.*|value_expression [, index_expression])

The last aggregation function returns properties of the very last event. When used with group by, it returns properties of the last event for each group. When specifying an index expression, the function returns properties of the Nth-prior event to the last event, all according to order of arrival.

Similar to the first aggregation function described above, you may specify the wildcard (*) character, no parameter or stream name and wildcard (*) character or a value_expression to evaluate for the last event.

The index_expression is optional. If no index expression is provided, the function returns the last event. If present, the function evaluates the index expression to determine the value for N, and evaluates the Nth-prior event to the last event. A value of zero returns the last event and a value of 1 returns the event prior to the last event. You may not specify event properties in the index expression.

The function returns null if there are no events or when the index is larger than the number of events held. When used with group by, it returns null if there are no events for that group or when the index is larger than the number of events held for that group.

The next statement selects the underlying event of the first and last sensor event held by the time window of 10 seconds:

select first(*), last(*) from SensorEvent.win:time(10 sec) 

The statement shown next selects the last temperature (f1) and the prior-to-last temperature (f1) of sensor events in the last 10 seconds:

select last(temperature, 0) as f1, select last(temperature, 1) as f2
from SensorEvent.win:time(10 sec)

The synopsis for the maxby aggregation function is:

maxby(sort_criteria_expression [asc/desc][, sort_criteria_expression [asc/desc]...])

The maxby aggregation function returns the greatest of all events, compared by using criteria expressions. When used with group by, it returns the greatest of all events per group.

This example statement returns the sensor id and the temperature of the sensor event that had the highest temperature among all sensor events:

select maxby(temperature).sensorId, maxby(temperature).temperature from SensorEvent

The next EPL returns the sensor event that had the highest temperature and the sensor event that had the lowest temperature, per zone, among the last 10 seconds of sensor events:

select maxby(temperature), minby(temperature) from SensorEvent.win:time(10 sec) group by zone

Your EPL may specify multiple criteria expressions. If the sort criteria expression is descending please append the desc keyword.

The following EPL returns the sensor event with the highest temperature and if there are multiple sensor events with the highest temperature the query returns the sensor event that has the newest timestamp value:

select maxby(temperature asc, timestamp desc) from SensorEvent

Event properties that are listed in criteria expressions must refer to the same event stream and cannot originate from different event streams.

If your query does not define a data window and does not refer to a named window, the semantics of maxby are the same as maxbyever.

Esper provides the following additional aggregation functions beyond those in the SQL standard:

Table 9.4. Syntax and results of EPL aggregation functions

Aggregate FunctionResult
firstever(expression [, filter_expr])

The firstever aggregation function returns the very first value ever. When used with group by it returns the first value ever for that group.

When used with a data window, the result of the function does not change as data points leave a data window. Use the first or prevtail function to return values relative to a data window.

The optional filter expression limits the values considered for retaining the first-ever value.

The next example statement outputs the first price ever for sell orders:

select firstever(price, side='sell') from Order
lastever(expression [, filter_expr])

Returns the last value or last value per group, when used with group by.

This sample statement outputs the total price, the first price and the last price per symbol for the last 30 seconds of events and every 5 seconds:

select symbol, sum(price), lastever(price), firstever(price)
from StockTickEvent.win:time(30 sec) 
group by symbol
output every 5 sec

When used with a data window, the result of the function does not change as data points leave a data window (for example when all data points leave the data window). Use the last or prev function to return values relative to a data window.

The optional filter expression limits the values considered for retaining the last-ever value.

The next example statement outputs the last price (ever) for sell orders:

select lastever(price, side='sell') from Order
leaving()

Returns true when any remove stream data has passed, for use in the having clause to output only when a data window has filled.

The leaving aggregation function is useful when you want to trigger output after a data window has a remove stream data point. Use the output after syntax as an alternative to output after a time interval.

This sample statement uses leaving() to output after the first data point leaves the data window, ignoring the first datapoint:

select symbol, sum(price) 
from StockTickEvent.win:time(30 sec) 
having leaving()
nth(expression, N_index)

Returns the Nth oldest element; If N=0 returns the most recent value. If N=1 returns the value before the most recent value. If N is larger than the events held in the data window for this group, returns null.

A maximum N historical values are stored, so it can be safely used to compare recent values in large views without incurring excessive overhead.

As compared to the prev row function, this aggregation function works within the current group by group, see Section 3.7.2, “Output for Aggregation and Group-By”.

This statement outputs every 2 seconds the groups that have new data and their last price and the previous-to-last price:

select symbol, nth(price, 1), last(price) 
from StockTickEvent 
group by symbol
output last every 2 sec

rate(number_of_seconds)

Returns an event arrival rate per second over the provided number of seconds, computed based on engine time.

Returns null until events fill the number of seconds. Useful with output snapshot to output a current rate. This function footprint is for use without a data window onto the stream(s).

A sample statement to output, every 2 seconds, the arrival rate per second considering the last 10 seconds of events is shown here:

select rate(10) from StockTickEvent
output snapshot every 2 sec

The aggregation function retains an engine timestamp value for each arriving event.

rate(timestamp_property[, accumulator])

Returns an event arrival rate over the data window including the last remove stream event. The timestamp_property is the name of a long-type property of the event that provides a timestamp value.

The first parameter is a property name or expression providing millisecond timestamp values.

The optional second parameter is a property or expression for computing an accumulation rate: If a value is provided as a second parameter then the accumulation rate for that quantity is returned (e.g. turnover in dollars per second).

This footprint is designed for use with a data window and requires a data window declared onto the stream. Returns null until events start leaving the window.

This sample statement outputs event rate for each group (symbol) with fixed sample size of four events (and considering the last event that left). The timestamp event property must be part of the event for this to work.

select colour, rate(timestamp) as rate 
from StockTickEvent.std:groupwin(symbol).win:length(4) 
group by symbol

Built-in aggregation functions can be disabled via configuration (see Section 15.4.22.4, “Extended Built-in Aggregation Functions”). A custom aggregation function of the same name as a built-on function may be registered to override the built-in function.

A user-defined function (UDF) is a single-row function that can be invoked anywhere as an expression itself or within an expresson. The function must simply be a public static method that the classloader can resolve at statement creation time. The engine resolves the function reference at statement creation time and verifies parameter types.

You may register your own function name for the user-defined function. Please see the instructions in Section 17.3, “Single-Row Function” for registering a function name for a user-defined single-row function.

A single-row function that has been registered with a function name can simply be referenced as function_name(parameters) thus EPL statements can be less cluttered as no class name is required. The engine also optimizes evaluation of such registered single-row functions when used in filter predicate expressions as described in Section 17.3.4, “Single-Row Functions in Filter Predicate Expressions”.

An example EPL statement that utilizes the discount function is shown next (assuming that function has been registered).

select discount(quantity, price) from OrderEvent

When selecting from a single stream, use the wildcard (*) character to pass the underlying event:

select discount(*) from OrderEvent

Alternatively use the stream alias or EPL pattern tag to pass an event:

select discount(oe) from OrderEvent as oe

User-defined functions can be also be invoked on instances of an event: Please see Section 5.4.5, “Using the Stream Name” to invoke event instance methods on a named stream.

Note that user-defined functions (not single-row functions) are candidate for caching their return result if the parameters passed are constants and they are not used chained. Please see below for details and configuration.

The example below assumes a class MyClass that exposes a public static method myFunction accepting 2 parameters, and returing a numeric type such as double.

select 3 * com.mycompany.MyClass.myFunction(price, volume) as myValue 
from StockTick.win:time(30 sec)

User-defined functions also take array parameters as this example shows. The section on Section 8.5, “Array Definition Operator” outlines in more detail the types of arrays produced.

select * from RFIDEvent where com.mycompany.rfid.MyChecker.isInZone(zone, {10, 20, 30})

Java class names have to be fully qualified (e.g. java.lang.Math) but Esper provides a mechanism for user-controlled imports of classes and packages as outlined in Section 15.4.6, “Class and package imports”.

User-defined functions can return any value including null, Java objects or arrays. Therefore user-defined functions can serve to transform, convert or map events, or to extract information and assemble further events.

The following statement is a simple pattern that looks for events of type E1 that are followed by events of type E2. It assigns the tags "e1" and "e2" that the function can use to assemble a final event for output:

select MyLib.mapEvents(e1, e2) from pattern [every e1=E1 -> e2=E2]

User-defined functions may also be chained: If a user-defined function returns an object then the object can itself be the target of the next function call and so on.

Assume that there is a calculator function in the MyLib class that returns a class which provides the search method taking two parameters. The EPL that takes the result of the calculator function and that calls the search method on the result and returns its return value is shown below:

select MyLib.calculator().search(zonevariable, zone) from RFIDEvent]

A user-defined function should be implemented thread-safe.

The transpose function is only valid in the select-clause and indicates that the result of the parameter expression should become the underlying event object of the output event.

The transpose function takes a single expression as a parameter. The result object of the parameter expression is subject to transposing as described below.

The function can be useful with insert into to allow an object returned by an expression to become the event itself in the output stream.

Any expression returning a Java object can be used with the transpose function. Typical examples for expressions are a static method invocation, the result of an enumeration method, a plug-in single row function or a subquery.

The examples herein assume that a single-row function by name makeEvent returns an OrderEvent instance (a POJO object, not shown).

The following EPL takes the result object of the invocation of the makeEvent method (assumed to be an OrderEvent instance) and returns the OrderEvent instance as the underlying event of the output event:

select transpose(makeEvent(oi)) from OrderIndication oi

Your select-clause can select additional properties or expressions. In this case the output event underlying object is a pair of the expression result object and the additional properties.

The next EPL also selects the origin property of the order indication event. The output event is a pair of the OrderEvent instance and a map containing the property name and value of origin:

select origin, transpose(makeEvent(oi)) from OrderIndication oi

If the transpose function is not a top-level function, i.e. if it occurs within another expression or within any other clause then the select-clause, the function simply returns the expression result of the parameter expression.

EPL provides enumeration methods that work with lambda expressions to perform common tasks on subquery results, named windows, event properties or inputs that are or can be projected to a collection of events, scalar values or objects.

A lambda expression is an anonymous expression. Lambda expressions are useful for encapsulating user-defined expressions that are applied to each element in a collection. This section discusses built-in enumeration methods and their lambda expression parameters.

Lambda expressions use the lambda operator =>, which is read as "goes to". The left side of the lambda operator specifies the lambda expression input parameter(s) (if any) and the right side holds the expression. The lambda expression x => x * x is read "x goes to x times x.". Lambda expressions are also used for expression declaration as discussed in Section 5.2.8, “Expression Declaration”.

When writing lambdas, you do not have to specify a type for the input parameter(s) or output result(s) because the engine can infer all types based on the input and the expression body. So if you are querying an RFIDEvent, for example, then the input variable is inferred to be an RFIDEvent event, which means you have access to its properties and methods.

The term element in respect to enumeration methods means a single event, scalar value or object in a collection that is the input to an enumeraton method. The term collection means a sequence or group of elements.

The below table summarizes the built-in enumeration methods available:

Table 10.1. Enumeration Methods

MethodResult
aggregate(seed, accumulator lambda)

Aggregate elements by using seed as an initial accumulator value and applying an accumulator expression.

Section 10.6.1, “Aggregate”.

allof(predicate lambda)

Return true when all elements satisfy a condition.

Section 10.6.2, “AllOf”.

anyof(predicate lambda)

Return true when any element satisfies a condition.

Section 10.6.3, “AnyOf”.

average()

Computes the average of values obtained from numeric elements.

Section 10.6.4, “Average”.

average(projection lambda)

Computes the average of values obtained from elements by invoking a projection expression on each element.

Section 10.6.4, “Average”.

countof()

Returns the number of elements.

Section 10.6.5, “CountOf”.

countof(predicate lambda)

Returns the number of elements that satisfy a condition.

Section 10.6.5, “CountOf”.

distinctOf()

Returns distinct elements according to default hash and equals semantics.

Section 10.6.6, “DistinctOf”.

distinctOf(key-selector lambda)

Returns distinct elements according using the key function provided.

Section 10.6.6, “DistinctOf”.

except(source)

Produces the set difference of the two collections.

Section 10.6.7, “Except”.

firstof()

Returns the first element.

Section 10.6.8, “FirstOf”.

firstof(predicate lambda)

Returns the first element that satisfies a condition.

Section 10.6.8, “FirstOf”.

groupby(key-selector lambda)

Groups the elements according to a specified key-selector expression.

Section 10.6.9, “GroupBy”.

groupby(key-selector lambda, value-selector lambda)

Groups the elements according to a key-selector expression mapping each element to a value according to a value-selector.

Section 10.6.9, “GroupBy”.

intersect(source)

Produces the set intersection of the two collections.

Section 10.6.10, “Intersect”.

lastof()

Returns the last element.

Section 10.6.11, “LastOf”.

lastof(predicate lambda)

Returns the last element that satisfies a condition.

Section 10.6.11, “LastOf”.

leastFrequent()

Returns the least frequent value among a collection of values.

Section 10.6.12, “LeastFrequent”.

leastFrequent(transform lambda)

Returns the least frequent value returned by the transform expression when applied to each element.

Section 10.6.12, “LeastFrequent”.

max()

Returns the maximum value among a collection of elements.

Section 10.6.13, “Max”.

max(value-selector lambda)

Returns the maximum value returned by the value-selector expression when applied to each element.

Section 10.6.13, “Max”.

maxby(value-selector lambda)

Returns the element that provides the maximum value returned by the value-selector expression when applied to each element.

Section 10.6.14, “MaxBy”.

min()

Returns the minimum value among a collection of elements.

Section 10.6.13, “Max”.

min(value-selector lambda)

Returns the minimum value returned by the value-selector expression when applied to each element.

Section 10.6.15, “Min”.

minby(value-selector lambda)

Returns the element that provides the minimum value returned by the value-selector expression when applied to each element..

Section 10.6.16, “MinBy”.

mostFrequent()

Returns the most frequent value among a collection of values.

Section 10.6.17, “MostFrequent”.

mostFrequent(transform lambda)

Returns the most frequent value returned by the transform expression when applied to each element.

Section 10.6.17, “MostFrequent”.

orderBy()

Sorts the elements in ascending order.

Section 10.6.18, “OrderBy and OrderByDesc”.

orderBy(key-selector lambda)

Sorts the elements in ascending order according to a key.

Section 10.6.18, “OrderBy and OrderByDesc”.

orderByDesc()

Sorts the elements in descending order.

Section 10.6.18, “OrderBy and OrderByDesc”.

orderByDesc(key-selector lambda)

Sorts the elements in descending order according to a key.

Section 10.6.18, “OrderBy and OrderByDesc”.

reverse

Reverses the order of elements.

Section 10.6.19, “Reverse”.

selectFrom(transform lambda)

Transforms each element resulting in a collection of transformed elements.

Section 10.6.20, “SelectFrom”.

sequenceEqual(second)

Determines whether two collections are equal by comparing each element (equals semantics apply).

Section 10.6.21, “SequenceEqual”.

sumOf()

Computes the sum from a collection of numeric elements.

Section 10.6.22, “SumOf”.

sumOf(projection lambda)

Computes the sum by invoking a projection expression on each element.

Section 10.6.22, “SumOf”.

take(numElements)

Returns a specified number of contiguous elements from the start.

Section 10.6.23, “Take”.

takeLast(numElements)

Returns a specified number of contiguous elements from the end.

Section 10.6.24, “TakeLast”.

takeWhile(predicate lambda)

Returns elements from the start as long as a specified condition is true.

Section 10.6.25, “TakeWhile”.

takeWhile( (predicate, index) lambda)

Returns elements from the start as long as a specified condition is true, allowing each element's index to be used in the logic of the predicate expression.

Section 10.6.25, “TakeWhile”.

takeWhileLast(predicate)

Returns elements from the end as long as a specified condition is true.

Section 10.6.26, “TakeWhileLast”.

takeWhileLast( (predicate,index) lambda)

Returns elements from the end as long as a specified condition is true, allowing each element's index to be used in the logic of the predicate expression.

Section 10.6.26, “TakeWhileLast”.

toMap(key-selector lambda, value-selector lambda)

Returns a Map according to specified key selector and value-selector expressions.

Section 10.6.27, “ToMap”.

union(source)

Forms a union of the input elements with source elements.

Section 10.6.28, “Union”.

where(predicate lambda)

Filters elements based on a predicate.

Section 10.6.29, “Where”.

where( (predicate,index) lambda)

Filters elements based on a predicate, allowing each element's index to be used in the logic of the predicate expression.

Section 10.6.29, “Where”.


The examples in this section come out of the domain of location report (aka. RFID, asset tracking etc.) processing:

The examples use example single-row functions for computing the distance (distance) and for determining if a location falls within a rectangle (inrect) that are not provided by the EPL language. These example UDF functions are not enumeration methods and are used in EPL statements to provide a sensible example.

The Item event contains an assetId id, a (x,y) location, a luggage flag to indicate whether the item represents a luggage (true) or passenger (false), and the assetIdPassenger that holds the asset id of the associated passenger when the item is a piece of luggage.

The Item event is defined as follows (access methods not shown for brevity):

public class Item {
  String assetId;             // passenger or luggage asset id
  Location location;          // (x,y) location
  boolean luggage;            // true if this item is a luggage piece
  String assetIdPassenger;    // if the item is luggage, contains passenger associated
...

The LocationReport event contains a list of Item items for which it reports events.

The LocationReport event is defined as follows:

public class LocationReport {
  List<Item> items;
...

The Zone event contains a zone name and (x1, y1, x2, y2) rectangle.

The Zone event is defined as follows:

public class Zone {
  String name;
  Rectangle rectangle;
...

The Location object is a nested object to Item and provides the current (x,y) location:

public class Location {
  int x;
  int y;
...

The Rectangle object is a nested object to Zone and provides a zone rectangle(x1,y1,x2,y2):

public class Rectangle {
  int x1;
  int y1;
  int x2;
  int y2;
...

It is not necessary to use classes for event representation. The example above applies the same to Object-array, Map or XML underlying events.

For most enumeration methods the input can be any collection of events, scalar values or objects. For some enumeration methods limitations apply that are documented below. For example, the sumOf enumeration method requires a collection of numeric scalar values if used without parameters. If the input to sumOf is a collection of events or scalar values the enumeration method requires a lambda expression as parameter that yields the numeric value to use to compute the sum.

Many examples of this section operate on the collection returned by the event property items in the LocationReport event class. There are many other inputs yielding collections as listed below. Most examples herein use an event property as a input simply because the example can thus be brief and does not need to refer to a subquery or named window or other concept.

For enumeration methods that return a collection, for example where and orderBy, the engine outputs an implementation of the Collection interface that contains the selected value(s). The collection returned must be considered read-only. As Java does not allow resettable iterators, the Collection interface allows more flexibility to query size and navigate among collection elements. We recommend against down-casting a collection returned by the engine to a more specific subclass of the Collection interface.

For enumeration methods that return an element, for example firstOf, lastOf, minBy and maxBy the engine outputs the scalar value or the underlying event if operating on events. You may add an event property name after the enumeration method to return a property value.

Enumeration methods generally retain the order of elements provided by the collection.

The following restrictions apply to enumeration methods:

The input of data for built-in enumeration methods is a collection of scalar values, events or other objects. Input can originate from any of the following:

Subqueries can return the rows of another stream's data window or rows from a named window. By providing a where-clause the rows returned by a subquery can be correlated to data provided by stream(s) in the from-clause. See Section 5.11, “Subqueries”.

A subquery that selects (*) wildcard provides a collection of events as input. A subquery that selects a single value expression provides a collection of scalar values as input. Subqueries that selects multiple value expressions are not allowed as input to enumeration methods.

The following example uses a subquery to retrieve all zones for each location report item where the location falls within the rectangle of the zone. Please see a description of example events and functions above.

select assetId,
  (select * from Zone.std:unique(name)).where(z => inrect(z.rectangle, location)) as zones
from Item

You may place the subquery in an expression declaration to reuse the subquery in multiple places of the same EPL statement.

This sample EPL declares the same query as above in an expression declaration:

expression myquery {itm =>
  (select * from Zone.std:unique(name)).where(z => inrect(z.rectangle, itm.location))
}
select assetId, myquery(item) as subq, 
    myquery(item).where(z => z.zone = 'Z01') as assetItem
from Item as item

The above query also demonstrates how an enumeration method, in the example the where-method, can be run across the results returned by a subquery in an expression declaration.

Place a single column in the subquery select-clause to provide a collection of scalar values as input.

The next example selects all names of zones and orders the names returning an order collection of string names every 30 seconds:

select (select name from Zone.std:unique(name)).orderBy() as orderedZones
from pattern[every timer:interval(30)]

The next example utilizes a subquery that counts zone events per name and finds those that have a count greater then 1:

select (select name, count(*) as cnt from Zone.win:keepall() group by name)
  .where(v => cnt > 1) from LocationReport]

When the subquery selects a single column that is itself an event, the result of the subquery is a collection of events of that type and can provide input to enumeration methods.

For example:

create schema SettlementEvent (symbol string);
create schema PriceEvent (symbol string, price double);
create schema OrderEvent (orderId string, pricedata PriceEvent);
select (select pricedata from OrderEvent.std:unique(orderId))
  .anyOf(v => v.symbol = 'GE') as has_ge from SettlementEvent(symbol = 'GE')

Note that the engine can cache intermediate results thereby is not forced to re-evaluate the subquery for each occurrence in the select-clause.

Named windows are globally-visible data windows. See Section 5.15, “Creating and Using Named Windows”.

You may specify the named window name as input for an enumeration method and can optionally provide a correlation where-clause. The syntax is equivalent to a sub-query against a named window but much shorter.

Synopsis:

named-window-name[(correlation-expression)].enum-method-name(...)

When selecting all events in a named window you do not need the correlation-expression. To select a subset of data in the named window, specify a correlation-expression. From the perspective of best runtime performance, a correlation expression is preferred to reduce the number of rows returned.

The following example first declares a named window to hold the last zone event per zone name:

create window ZoneWindow.std:unique(name) as Zone

Then we create a statement to insert zone events that arrive to the named window:

insert into ZoneWindow select * from Zone

Finally this statement queries the named window to retrieve all zones for each location report item where the location falls within the rectangle of the zone:

select ZoneWindow.where(z => inrect(z.rectangle, location)) as zones from Item

If you have a filter or correlation expression, append the expression to the named window name and place in parenthesis.

This slightly modified query is the example above except that it adds a filter expression such that only zones with name Z1, Z2 or Z3 are considered:

select ZoneWindow(name in ('Z1', 'Z2', 'Z3')).where(z => inrect(z.rectangle, location)) as zones 
from Item

You may prefix property names provided by the named window with the name to disambiguate property names.

This sample query prefixed the name property and returns the count of matching zones:

select ZoneWindow(ZoneWindow.name in ('Z1', 'Z2', 'Z3')).countof()) as zoneCount
from Item

The engine internally interprets the shortcut syntax and creates a subquery from it. Thus all indexing and query planning for subqueries against named windows apply here as well.

Event aggregation functions return an event or multiple events. They are aggregation functions and as such sensitive to the presence of group by. See Section 9.2.2, “Event Aggregation Functions”.

You can use window, first or last event aggregation functions as input to an enumeration method. Specify the * wildcard as the parameter to the event aggregation function to provide a collection of events as input. Or specify a property name as the parameter to event aggregation function to provide a collection of scalar values as input.

You can use the sorted, maxby, minby, maxbyever or minbyever event aggregation functions as input to an enumeration method. Specify one or more criteria expressions that provide the sort order as parameters to the event aggregation function.

In this example query the window(*) aggregation function returns the last 10 seconds of item location reports for the same asset id as the incoming event. Among that last 10 seconds of events for the same asset id, the enumeration method returns those item location reports where the distance to center is less then 20, for each arriving Item event.

Sample query:

select window(*).where(p => distance(0, 0, p.location.x, p.location.y) < 20) as centeritems
from Item(type='P').win:time(10) group by assetId

The next sample query instead selects the asset id property of all events and returns an ordered collection:

select window(assetId).orderBy() as orderedAssetIds
from Item.win:time(10) group by assetId

The following example outputs the 5 highest prices per symbol among the last 10 seconds of stock ticks:

select sorted(price desc).take(5) as highest5PricesPerSymbol
from StockTick.win:time(10) group by symbol

The prev, prevwindow and prevtail single-row functions allow access into a stream's data window however are not aggregation functions and and as such not sensitive to the presence of group by. See Section 9.1.11, “The Previous-Window Function”.

When using any of the prev single-row functions as input to a built-in enumeration method you can specify the stream name as a parameter to the function or an event property. The input to the enumeration method is a collection of events if you specify the stream name, or a collection of scalar value if you specify an event property.

In this example query the prevwindow(stream) single-row function returns the last 10 seconds of item location reports, among which the enumeration method filters those item location reports where the distance to center is less then 20, for each Item event that arrived in the last 10 seconds considering passenger-type Item events only (see filter type = 'P').

Sample query:

select prevwindow(items)
    .where(p => distance(0, 0, p.location.x, p.location.y) < 20) as centeritems
from Item(type='P').win:time(10) as items

This sample query demonstrates the use of the prevwindow function to return a collection of scalar values (collection of asset id) as input to orderby:

select prevwindow(assetId).orderBy() as orderedAssetIds
from Item.win:time(10) as items

Your single-row or user-defined function can return either an array or any collection that implements either the Collection or Iterable interface. For arrays, the array component type and for collections, the collection or iterable generic type should be the class providing event properties.

As an example, assume a ZoneFactory class exists and a static method getZones() returns a list of zones to filter items, for example:

public class ZoneFactory {
  public static Iterable<Zone> getZones() {
    List<Zone> zones = new ArrayList<Zone>();
    zones.add(new Zone("Z1", new Rectangle(0, 0, 20, 20)));
    return zones;
  }
}

Import the class through runtime or static configuration, or add the method above as a plug-in single-row function.

The following query returns for each Item event all zones that the item belongs to:

select ZoneFactory.getZones().where(z => inrect(z.rectangle, item.location)) as zones
from Item as item

If the class and method were registered as a plug-in single-row function, you can leave the class name off, for example:

select getZones().where(z => inrect(z.rectangle, item.location)) as zones
from Item as item

Your single-row or user-defined function can also return an array, collection or iterable or scalar values.

For example, the static method getZoneNames() returns a list of zone names:

public static String[] getZoneNames() { 
  return new String[] { "Z1", "Z2"};
}

The following query returns zone names every 30 seconds and excludes zone Z1:

select getZoneNames().where(z => z != "Z1")
from pattern[every timer:interval(30)]

An enum type can also be a useful source for enumerable values.

The following sample Java declares an enum type EnumOfZones:

public enum EnumOfZones {
  ZONES_OUTSIDE(new String[] {"z1", "z2"}),
  ZONES_INSIDE(new String[] {"z3", "z4"})

  private final String[] zones;

  private EnumOfZones(String[] zones) {
	  this.zones = zones;
  }

  public String[] getZones() {
    return zones;
  }
}

A sample statement that utilizes the enum type is shown next:

select EnumOfZones.ZONES_OUTSIDE.getZones().anyOf(v => v = zone) from Item

The aggregate enumeration method takes an expression providing the initialization value (seed) and an accumulator lambda expression. The return value is the final accumulator value.

Via the aggregate method you may perform a calculation over elements. The method initializes the aggregated value by evaluating the expression provided in the first parameter. The method then calls the lambda expression of the second parameter once for each element in the input. The lambda expression receives the last aggregated value and the element from the input. The result of the expression replaces the previous aggregated value and returns the final result after completing all elements.

An expression example with scalar values:

{1, 2, 3}.aggregate(0, (result, value) => result + value)  // Returns 6

The example below aggregates price of each OrderEvent in the last 10 seconds computing a total price:

// Initialization value is zero.
// Aggregate by adding up the price.
select window(*).aggregate(0, (result, order) => result + order.price) as totalPrice
from OrderEvent.win:time(10)

In the query above, the initialization value is zero, result is used for the last aggregated value and order denotes the element that the expression adds the value of the price property.

This example aggregation builds a comma-separated list of all asset ids of all items:

select items.aggregate('', 
  (result, item) => result || (case when result='' then '' else ',' end) || item.assetId) as assets			
from LocationReport

In above query, the empty string '' represents the initialization value. The name result is used for the last aggregated value and the name item is used to denote the element.

The type value returned by the initialization expression must match to the type of value returned by the accumulator lambda expression.

If the input is null the method returns null. If the input is empty the method returns the initialization value.

The selectFrom enumeration method transforms each element resulting in a collection of transformed elements.

The enumeration method applies a transformation lambda expression to each element and returns the result of each transformation as a collection. Use the new operator to yield multiple values for each element, see Section 8.13, “The 'new' Keyword”.

The next EPL query returns a collection of asset ids:

select items.selectFrom(i => assetId) as itemAssetIds from LocationReport

This sample EPL query evaluates each item and returns the asset id as well as the distance from center for each item:

select items.selectFrom(i => 
  new {
    assetId, 
    distanceCenter = distance(i.location.x, i.location.y, 0, 0)
  } ) as itemInfo from LocationReport

If the input is null the method returns null. If the input is empty the method returns an empty collection.

EPL date-time methods work on date-time values to perform common tasks such as comparing times and time periods, adding or subtracting time periods, setting or rounding calendar fields and querying fields.

Date-time methods operate on:

  1. Any expression or event property that returns either a long-type millisecond value, a java.util.Calendar or java.util.Date including subclasses. Consider the built-in single-row function current_timestamp for use with date-time methods.

  2. Any event for which the event type declares a start timestamp property name and optionally also an end timestamp property name. Date-time methods operate on events by means of the stream-alias.method-name syntax.

The below table summarizes the built-in date-time methods available:

Table 11.1. Date-Time Methods

MethodResult
after(event or timestamp)

Returns true if an event happens after another event, or a timestamp is after another timestamp.

Section 11.4.5, “After”.

before(event or timestamp)

Returns true if an event happens before another event, or a timestamp is before another timestamp.

Section 11.4.6, “Before”.

between(timestamp, timestamp, boolean, boolean)

Returns true if a timestamp is between two timestamps.

Section 11.3.1, “Between”.

coincides(event or timestamp)

Returns true if an event and another event happen at the same time, or two timestamps are the same value.

Section 11.4.7, “Coincides”.

during(event or timestamp)

Returns true if an event happens during the occurrence of another event, or when a timestamps falls within the occurrence of an event.

Section 11.4.8, “During”.

finishes(event or timestamp)

Returns true if an event starts after another event starts and the event ends at the same time as the other event.

Section 11.4.9, “Finishes”.

finishedBy(event or timestamp)

Returns true if an event starts before another event starts and ends at the same time as the other event.

Section 11.4.10, “Finished By”.

format()

Formats the date-time returning a string.

Section 11.3.2, “Format”.

get(field)

Returns the value of the given date-time value field.

Section 11.3.3, “Get (By Field)”.

getMillisOfSecond()

getSecondOfMinute()

getMinuteOfHour()

getHourOfDay()

getDayOfWeek()

getDayOfMonth()

getDayOfYear()

getWeekyear()

getMonthOfYear()

getYear()

getEra()

Returns the value of the given date-time value field.

Section 11.3.4, “Get (By Name) ”.

includes(event or timestamp)

Returns true if the parameter event happens during the occurrence of the event, or when a timestamps falls within the occurrence of an event.

Section 11.4.11, “Includes”.

meets(event or timestamp)

Returns true if the event's end time is the same as another event's start time.

Section 11.4.12, “Meets”.

metBy(event or timestamp)

Returns true if the event's start time is the same as another event's end time.

Section 11.4.13, “Met By”.

minus(duration-millis)

Returns a date-time with the specified duration in long-type milliseconds taken away.

Section 11.3.5, “Minus”.

minus(time-period)

Returns a date-time with the specified duration in time-period syntax taken away.

Section 11.3.5, “Minus”.

overlaps(event or timestamp)

Returns true if the event starts before another event starts and finishes after the other event starts, but before the other event finishes (events have an overlapping period of time).

Section 11.4.14, “Overlaps”.

overlappedBy(event or timestamp)

Returns true if the parameter event starts before the input event starts and the parameter event finishes after the input event starts, but before the input event finishes (events have an overlapping period of time).

Section 11.4.15, “Overlapped By”.

plus(duration-millis)

Returns a date-time with the specified duration in long-type milliseconds added.

Section 11.3.6, “Plus”.

plus(time-period)

Returns a date-time with the specified duration in time-period syntax added.

Section 11.3.6, “Plus”.

roundCeiling(field)

Returns a date-time rounded to the highest whole unit of the date-time field.

Section 11.3.7, “RoundCeiling”.

roundFloor(field)

Returns a date-time rounded to the lowest whole unit of the date-time field.

Section 11.3.8, “RoundFloor”.

roundHalf(field)

Returns a date-time rounded to the nearest whole unit of the date-time field.

Section 11.3.9, “RoundHalf”.

set(field, value)

Returns a date-time with the specified field set to the value returned by a value expression.

Section 11.3.10, “Set (By Field)”.

starts(event or timestamp)

Returns true if an event and another event start at the same time and the event's end happens before the other event's end.

Section 11.4.16, “Starts”.

startedBy(event or timestamp)

Returns true if an event and another event start at the same time and the other event's end happens before the input event's end.

Section 11.4.17, “Started By”.

withDate(year,month,day)

Returns a date-time with the specified date, retaining the time fields.

Section 11.3.11, “WithDate”.

withMax(field)

Returns a date-time with the field set to the maximum value for the field.

Section 11.3.12, “WithMax”.

withMin(field)

Returns a date-time with the field set to the minimum value for the field.

Section 11.3.13, “WithMin”.

withTime(hour,minute,sec,msec)

Returns a date-time with the specified time, retaining the date fields.

Section 11.3.14, “WithTime”.

toCalendar()

Returns the Calendar object for this date-time value.

Section 11.3.15, “ToCalendar”.

toDate()

Returns the Date object for this date-time value.

Section 11.3.16, “ToDate”.

toMillisec()

Returns the long-type milliseconds value for this date-time value.

Section 11.3.17, “ToMillisec”.


The syntax for date-time methods is the same syntax as for any chained invocation:

input_val.datetime_method_name( [method_parameter [, method_parameter [,...]]])
	  .[ datetime_method_name(...) [...]] 

Following the input_val input value is the . (dot) operator and the datetime_method_name date-time method name. It follows in parenthesis a comma-separated list of method parameter expressions. Additional date-time methods can be chained thereafter.

The input value can be any expression or event property that returns a value of type long or java.util.Calendar or java.util.Date. If the input value is null, the expression result is also null.

The input value can also be an event. In this case the event type of the event must have the start timestamp property name defined and optionally also the end timestamp property name.

The following example EPL statement employs the withTime date-time method. This example returns the current engine time with the time-part set to 1 am:

select current_timestamp.withTime(1, 0, 0, 0) as time1am from MyEvent

As date-time methods can be chained, this EPL is equivalent:

select current_timestamp.set('hour', 1).set('min', 0).set('sec', 0).set('msec', 0) as time1am
from MyEvent

The statement above outputs in field time1am a long-type millisecond-value reflecting 1am on the same date as engine time. Since the input value is provided by the built-in current_timestamp function which returns current engine date as a long-type millisecond value the output is also a long-type millisecond value.

You may apply a date-time method to an event property.

Assume that the RFIDEvent event type has a Date-type property by name timeTaken. The following query rounds each time-taken value down to the nearest minute and outputs a Date-type value in column timeTakenRounded:

select timeTaken.roundFloor('min') as timeTakenRounded from RFIDEvent

You may apply a date-time method to events. This example assumes that the RFIDEvent and WifiEvent event types both have a timestamp property defined. The EPL compares the timestamps of the RFIDEvent and the WifiEvent:

select rfid.after(wifi) as isAfter 
from RFIDEvent.std:lastevent() rfid, WifiEvent.std:lastevent() wifi

For comparing date-time values and considering event duration (event start and end timestamps) we recommend any of the interval algebra methods. You may also compare millisecond values using the between or in ranges and inverted ranges or relational operators (> , <, >=, <=).

From a performance perspective, the date-time method evaluation ensures that for each unique chain of date-time methods only a single calendar objects is copied or created when necessary.

The between date-time method compares the input date-time value to the two date-time values passed in and returns true if the input value falls between the two parameter values.

The synopsis is:

input_val.between(range_start, range_end [, include_start, include_end])

The method takes either 2 or 4 parameters. The first two parameters range_start and range_end are expressions or properties that yield either a long-typed, Date-typed or Calendar-typed range start and end value.

The next two parameters include_start and include_end are optional. If not specified, the range start value and range end value are included in the range i.e. specify a closed range where both endpoints are included. If specified, the expressions must return a boolean-value indicating whether to include the range start value and range end value in the range.

The example below outputs true when the time-taken property value of the RFID event falls between the time-start property value and the time-end property value (closed range includes endpoints):

select timeTaken.between(timeStart, timeEnd) from RFIDEvent

The example below performs the same test as above but does not include endpoints (open range includes neither endpoint):

select timeTaken.between(timeStart, timeEnd, false, false) from RFIDEvent

If the range end value is less then the range start value, the algorithm reverses the range start and end value.

If the input date-time value or any of the parameter values evaluate to null the method returns a null result value.

The get date-time method returns the value of the given date-time value field.

The method takes a single string-constant field name as parameter. Please see Section 5.2.1, “Specifying Time Periods” for a list of recognized keywords (not case-sensitive).

The method returns the numeric value of the field within the date-time value. The value returned adheres to Calendar-class semantics: For example, the value for month starts at zero and has a maximum of 11.

The example below outputs the month value of the time-taken property value of the RFID event:

select timeTaken.get('month') as timeTakenMonth from RFIDEvent

The minus date-time method returns a date-time with the specified duration taken away.

The method has two versions: The first version takes the duration as a long-type millisecond value. The second version takes the duration as a time-period expression, see Section 5.2.1, “Specifying Time Periods”.

The example below demonstrates the time-period parameter to subtract two minutes from the time-taken property value of the RFID event:

select timeTaken.minus(2 minutes) as timeTakenMinus2Min from RFIDEvent

The next example is equivalent but passes a millisecond-value instead:

select timeTaken.minus(2*60*1000) as timeTakenMinus2Min from RFIDEvent

The plus date-time method returns a date-time with the specified duration added.

The method has two versions: The first version takes the duration as a long-type millisecond value. The second version takes the duration as a time-period expression, see Section 5.2.1, “Specifying Time Periods”.

The next example adds two minutes to the time-taken property value of the RFID event:

select timeTaken.plus(2 minutes) as timeTakenPlus2Min from RFIDEvent

The next example is equivalent but passes a millisecond-value instead:

select timeTaken.plus(2*60*1000) as timeTakenPlus2Min from RFIDEvent

The roundCeiling date-time method rounds to the highest whole unit of the date-time field.

The method takes a single string-constant field name as parameter. Please see Section 5.2.1, “Specifying Time Periods” for a list of recognized keywords (not case-sensitive).

The next example rounds-to-ceiling the minutes of the time-taken property value of the RFID event:

select timeTaken.roundCeiling('min') as timeTakenRounded from RFIDEvent

If the input time is 2002-05-30 09:01:23.050, for example, the output is 2002-05-30 09:02:00.000 (example timestamps are in format yyyy-MM-dd HH:mm:ss.SSS).

The roundFloor date-time method rounds to the lowest whole unit of the date-time field.

The method takes a single string-constant field name as parameter. Please see Section 5.2.1, “Specifying Time Periods” for a list of recognized keywords (not case-sensitive).

The next example rounds-to-floor the minutes of the time-taken property value of the RFID event:

select timeTaken.roundFloor('min') as timeTakenRounded from RFIDEvent

If the input time is 2002-05-30 09:01:23.050, for example, the output is 2002-05-30 09:01:00.000 (example timestamps are in format yyyy-MM-dd HH:mm:ss.SSS).

The roundFloor date-time method rounds to the nearest whole unit of the date-time field.

The method takes a single string-constant field name as parameter. Please see Section 5.2.1, “Specifying Time Periods” for a list of recognized keywords (not case-sensitive).

The next example rounds the minutes of the time-taken property value of the RFID event:

select timeTaken.roundHalf('min') as timeTakenRounded from RFIDEvent

The following table provides a few examples of the rounding (example timestamps are in format yyyy-MM-dd HH:mm:ss.SSS):


The set date-time method returns a date-time with the specified field set to the value returned by an expression.

The method takes a string-constant field name and an expression returning an integer-value as parameters. Please see Section 5.2.1, “Specifying Time Periods” for a list of recognized keywords (not case-sensitive).

The method returns the new date-time value with the field set to the provided value. Note that value adheres to Calendar-class semantics: For example, the value for month starts at zero and has a maximum of 11.

The example below outputs the time-taken with the value for month set to April:

select timeTaken.set('month', 3) as timeTakenMonth from RFIDEvent

The withMax date-time method returns a date-time with the field set to the maximum value for the field.

The method takes a string-constant field name as parameter. Please see Section 5.2.1, “Specifying Time Periods” for a list of recognized keywords (not case-sensitive).

The method returns the new date-time value with the specific date field set to the maximum value.

The example below outputs the time-taken property value with the second-part as 59 seconds:

select timeTaken.withMax('sec') as timeTakenMaxSec from RFIDEvent

The withMin date-time method returns a date-time with the field set to the minimum value for the field.

The method takes a string-constant field name as parameter. Please see Section 5.2.1, “Specifying Time Periods” for a list of recognized keywords (not case-sensitive).

The method returns the new date-time value with the specific date field set to the minimum value.

The example below outputs the time-taken property value with the second-part as 0 seconds:

select timeTaken.withMin('sec') as timeTakenMaxSec from RFIDEvent

Interval algebra methods compare start and end timestamps of events or timestamps in general.

When the expression input is only a timestamp value, such as a long-type value or a Date or Calendar object, the start and end timestamp represented by that value are the same timestamp value.

When expression input is an event stream alias, the engine determine the event type for the stream. If the event type declares a start timestamp property name, the engine uses that start timestamp property to determine the start timestamp for the event. If the event type also declares an end timestamp property name, the engine uses that end timestamp property to determine the end timestamp for the event (i.e. an event with duration). If an end timestamp property name is not declared, the start and end timestamp for each event is the same value and the event is considered to have zero duration (i.e. a point-in-time event).

Interval algebra methods all return Boolean-type value. When the input value start timestamp is null, or the end timestamp (if declared for the event type) is null or any of the start timestamp and end timestamp (if declared for the event type) values of the first parameter is null, the result value is null.

The after date-time method returns true if an event happens after another event, or a timestamp is after another timestamp.

The method compares the input value's start timestamp (a.startTimestamp) to the first parameter's end timestamp (b.endTimestamp) to determine whether A happens after B.

If used with one parameter, for example in a.after(b), the method returns true if A starts after B ends.

Sample EPL:

select * from A.std:lastevent() as a, B.std:lastevent() as b where a.after(b)
// Above matches when:
//   a.startTimestamp - b.endTimestamp > 0

If providing two parameters, for example in a.after(b, 5 sec), the method returns true if A starts at least 5 seconds after B ends.

Sample EPL:

select * from A.std:lastevent() as a, B.std:lastevent() as b where a.after(b, 5 sec)
// Above matches when:
//   a.startTimestamp - b.endTimestamp >= 5 seconds

If providing three parameters, for example in a.after(b, 5 sec, 10 sec), the method returns true if A starts at least 5 seconds but no more then 10 seconds after B ends.

Sample EPL:

select * from A.std:lastevent() as a, B.std:lastevent() as b where a.after(b, 5 sec, 10 sec)
// Above matches when:
//   5 seconds <= a.startTimestamp - b.endTimestamp <= 10 seconds

Negative values for the range are allowed. For example in a.after(b, -5 sec, -10 sec), the method returns true if A starts at least 5 seconds but no more then 10 seconds before B ends.

If the range low endpoint is greater than the range high endpoint, the engine automatically reverses them. Thus a.after(b, 10 sec, 5 sec) is the same semantics as a.after(b, 5 sec, 10 sec).

The before date-time method returns true if an event happens before another event, or a timestamp is before another timestamp.

The method compares the input value's end timestamp (a.endTimestamp) and the first parameter's start timestamp (b.startTimestamp) to determine whether A happens before B.

If used with one parameter, for example in a.before(b), the method returns true if A ends before B starts.

Sample EPL:

select * from A.std:lastevent() as a, B.std:lastevent() as b where a.before(b)
// Above matches when:
//   b.startTimestamp - a.endTimestamp > 0

If providing two parameters, for example in a.before(b, 5 sec), the method returns true if A ends at least 5 seconds before B starts.

Sample EPL:

select * from A.std:lastevent() as a, B.std:lastevent() as b where a.before(b, 5 sec)
// Above matches when:
//   b.startTimestamp - a.endTimestamp >= 5 seconds

If providing three parameters, for example in a.before(b, 5 sec, 10 sec), the method returns true if A ends at least 5 seconds but no more then 10 seconds before B starts.

Sample EPL:

select * from A.std:lastevent() as a, B.std:lastevent() as b where a.before(b, 5 sec, 10 sec)
// Above matches when:
//   5 seconds <= b.startTimestamp - a.endTimestamp <= 10 seconds

Negative values for the range are allowed. For example in a.before(b, -5 sec, -10 sec), the method returns true if A starts at least 5 seconds but no more then 10 seconds after B starts.

If the range low endpoint is greater than the range high endpoint, the engine automatically reverses them. Thus a.before(b, 10 sec, 5 sec) is the same semantics as a.before(b, 5 sec, 10 sec).

The coincides date-time method returns true if an event and another event happen at the same time, or two timestamps are the same value.

The method compares the input value's start and end timestamp with the first parameter's start and end timestamp and determines if they equal.

If used with one parameter, for example in a.coincides(b), the method returns true if the start timestamp of A and B are the same and the end timestamps of A and B are also the same.

Sample EPL:

select * from A.std:lastevent() as a, B.std:lastevent() as b where a.coincides(b)
// Above matches when:
//   a.startTimestamp = b.startTimestamp and a.endTimestamp = b.endTimestamp

If providing two parameters, for example in a.coincides(b, 5 sec), the method returns true if the difference between the start timestamps of A and B is equal to or less then 5 seconds and the difference between the end timestamps of A and B is also equal to or less then 5 seconds.

Sample EPL:

select * from A.std:lastevent() as a, B.std:lastevent() as b where a.coincides(b, 5 sec)
// Above matches when:
//   abs(a.startTimestamp - b.startTimestamp) <= 5 sec and 
//   abs(a.endTimestamp - b.endTimestamp) <= 5 sec

If providing three parameters, for example in a.coincides(b, 5 sec, 10 sec), the method returns true if the difference between the start timestamps of A and B is equal to or less then 5 seconds and the difference between the end timestamps of A and B is equal to or less then 10 seconds.

Sample EPL:

select * from A.std:lastevent() as a, B.std:lastevent() as b where a.coincides(b, 5 sec, 10 sec)
// Above matches when:
//   abs(a.startTimestamp - b.startTimestamp) <= 5 seconds and 
//   abs(a.endTimestamp - b.endTimestamp) <= 10 seconds

A negative value for interval parameters is not allowed. If your interval parameter is itself an expression that returns a negative value the engine logs a warning message and returns null.

The during date-time method returns true if an event happens during the occurrence of another event, or when a timestamps falls within the occurrence of an event..

The method determines whether the input value's start and end timestamp are during the first parameter's start and end timestamp. The symmetrical opposite is Section 11.4.11, “Includes”.

If used with one parameter, for example in a.during(b), the method returns true if the start timestamp of A is after the start timestamp of B and the end timestamp of A is before the end timestamp of B.

Sample EPL:

select * from A.std:lastevent() as a, B.std:lastevent() as b where a.during(b)
// Above matches when:
//   b.startTimestamp < a.startTimestamp <= a.endTimestamp < b.endTimestamp

If providing two parameters, for example in a.during(b, 5 sec), the method returns true if the difference between the start timestamps of A and B is equal to or less then 5 seconds and the difference between the end timestamps of A and B is also equal to or less then 5 seconds.

Sample EPL:

select * from A.std:lastevent() as a, B.std:lastevent() as b where a.during(b, 5 sec)
// Above matches when:
//   0 < a.startTimestamp - b.startTimestamp <= 5 sec and 
//   0 < a.endTimestamp - b.endTimestamp <= 5 sec

If providing three parameters, for example in a.during(b, 5 sec, 10 sec), the method returns true if the difference between the start timestamps of A and B and the difference between the end timestamps of A and B is between 5 and 10 seconds.

Sample EPL:

select * from A.std:lastevent() as a, B.std:lastevent() as b where a.during(b, 5 sec, 10 sec)
// Above matches when:
//   5 seconds <= a.startTimestamp - b.startTimestamp <= 10 seconds and 
//   5 seconds <= a.endTimestamp - b.endTimestamp <= 10 seconds

If providing five parameters, for example in a.during(b, 5 sec, 10 sec, 20 sec, 30 sec), the method returns true if the difference between the start timestamps of A and B is between 5 seconds and 10 seconds and the difference between the end timestamps of A and B is between 20 seconds and 30 seconds.

Sample EPL:

select * from A.std:lastevent() as a, B.std:lastevent() as b
  where a.during(b, 5 sec, 10 sec, 20 sec, 30 sec)
// Above matches when:
//   5 seconds <= a.startTimestamp - b.startTimestamp <= 10 seconds and 
//   20 seconds < a.endTimestamp - b.endTimestamp <= 30 seconds

The finishes date-time method returns true if an event starts after another event starts and the event ends at the same time as the other event.

The method determines whether the input value's start timestamp is after the first parameter's start timestamp and the end timestamp of the input value and the first parameter are the same. The symmetrical opposite is Section 11.4.10, “Finished By”.

If used with one parameter, for example in a.finishes(b), the method returns true if the start timestamp of A is after the start timestamp of B and the end timestamp of A and B are the same.

Sample EPL:

select * from A.std:lastevent() as a, B.std:lastevent() as b where a.finishes(b)
// Above matches when:
//   b.startTimestamp < a.startTimestamp and a.endTimestamp = b.endTimestamp

If providing two parameters, for example in a.finishes(b, 5 sec), the method returns true if the start timestamp of A is after the start timestamp of B and the difference between the end timestamps of A and B is equal to or less then 5 seconds.

Sample EPL:

select * from A.std:lastevent() as a, B.std:lastevent() as b where a.finishes(b, 5 sec)
// Above matches when:
//   b.startTimestamp < a.startTimestamp and 
//   abs(a.endTimestamp - b.endTimestamp ) <= 5 seconds

A negative value for interval parameters is not allowed. If your interval parameter is itself an expression that returns a negative value the engine logs a warning message and returns null.

The finishedBy date-time method returns true if an event starts before another event starts and the event ends at the same time as the other event.

The method determines whether the input value's start timestamp happens before the first parameter's start timestamp and the end timestamp of the input value and the first parameter are the same. The symmetrical opposite is Section 11.4.9, “Finishes”.

If used with one parameter, for example in a.finishedBy(b), the method returns true if the start timestamp of A is before the start timestamp of B and the end timestamp of A and B are the same.

Sample EPL:

select * from A.std:lastevent() as a, B.std:lastevent() as b where a.finishedBy(b)
// Above matches when:
//   a.startTimestamp < b.startTimestamp and a.endTimestamp = b.endTimestamp

If providing two parameters, for example in a.finishedBy(b, 5 sec), the method returns true if the start timestamp of A is before the start timestamp of B and the difference between the end timestamps of A and B is equal to or less then 5 seconds.

Sample EPL:

select * from A.std:lastevent() as a, B.std:lastevent() as b where a.finishedBy(b, 5 sec)
// Above matches when:
//   a.startTimestamp < b.startTimestamp and 
//   abs(a.endTimestamp - b.endTimestamp ) <= 5 seconds

The includes date-time method returns true if the parameter event happens during the occurrence of the input event, or when a timestamps falls within the occurrence of an event.

The method determines whether the first parameter's start and end timestamp are during the input value's start and end timestamp. The symmetrical opposite is Section 11.4.8, “During”.

If used with one parameter, for example in a.includes(b), the method returns true if the start timestamp of B is after the start timestamp of A and the end timestamp of B is before the end timestamp of A.

Sample EPL:

select * from A.std:lastevent() as a, B.std:lastevent() as b where a.includes(b)
// Above matches when:
//   a.startTimestamp < b.startTimestamp <= b.endTimestamp < a.endTimestamp 

If providing two parameters, for example in a.includes(b, 5 sec), the method returns true if the difference between the start timestamps of A and B is equal to or less then 5 seconds and the difference between the end timestamps of A and B is also equal to or less then 5 seconds.

Sample EPL:

select * from A.std:lastevent() as a, B.std:lastevent() as b where a.includes(b, 5 sec)
// Above matches when:
//   0 < b.startTimestamp - a.startTimestamp <= 5 sec and 
//   0 < a.endTimestamp - b.endTimestamp <= 5 sec

If providing three parameters, for example in a.includes(b, 5 sec, 10 sec), the method returns true if the difference between the start timestamps of A and B and the difference between the end timestamps of A and B is between 5 and 10 seconds.

Sample EPL:

select * from A.std:lastevent() as a, B.std:lastevent() as b where a.includes(b, 5 sec, 10 sec)
// Above matches when:
//   5 seconds <= a.startTimestamp - b.startTimestamp <= 10 seconds and 
//   5 seconds <= a.endTimestamp - b.endTimestamp <= 10 seconds

If providing five parameters, for example in a.includes(b, 5 sec, 10 sec, 20 sec, 30 sec), the method returns true if the difference between the start timestamps of A and B is between 5 seconds and 10 seconds and the difference between the end timestamps of A and B is between 20 seconds and 30 seconds.

Sample EPL:

select * from A.std:lastevent() as a, B.std:lastevent() as b
  where a.includes(b, 5 sec, 10 sec, 20 sec, 30 sec)
// Above matches when:
//   5 seconds <= a.startTimestamp - b.startTimestamp <= 10 seconds and 
//   20 seconds <= a.endTimestamp - b.endTimestamp <= 30 seconds

The overlaps date-time method returns true if the event starts before another event starts and finishes after the other event starts, but before the other event finishes (events have an overlapping period of time).

The method determines whether the input value's start and end timestamp indicate an overlap with the first parameter's start and end timestamp, such that A starts before B starts and A ends after B started but before B ends.

If used with one parameter, for example in a.overlaps(b), the method returns true if the start timestamp of A is before the start timestamp of B and the end timestamp of A and is before the end timestamp of B.

Sample EPL:

select * from A.std:lastevent() as a, B.std:lastevent() as b where a.overlaps(b)
// Above matches when:
//   a.startTimestamp < b.startTimestamp < a.endTimestamp < b.endTimestamp

If providing two parameters, for example in a.overlaps(b, 5 sec), the method returns true if, in addition, the difference between the end timestamp of A and the start timestamp of B is equal to or less then 5 seconds.

Sample EPL:

select * from A.std:lastevent() as a, B.std:lastevent() as b where a.overlaps(b, 5 sec)
// Above matches when:
//   a.startTimestamp < b.startTimestamp < a.endTimestamp < b.endTimestamp and 
//   0 <= a.endTimestamp - b.startTimestamp <= 5 seconds

If providing three parameters, for example in a.overlaps(b, 5 sec, 10 sec), the method returns true if, in addition, the difference between the end timestamp of A and the start timestamp of B is between 5 and 10 seconds.

Sample EPL:

select * from A.std:lastevent() as a, B.std:lastevent() as b where a.overlaps(b, 5 sec, 10 sec)
// Above matches when:
//   a.startTimestamp < b.startTimestamp < a.endTimestamp < b.endTimestamp and 
//   5 seconds <= a.endTimestamp - b.startTimestamp <= 10 seconds

The overlappedBy date-time method returns true if the parameter event starts before the input event starts and the parameter event finishes after the input event starts, but before the input event finishes (events have an overlapping period of time).

The method determines whether the input value's start and end timestamp indicate an overlap with the first parameter's start and end timestamp, such that B starts before A starts and B ends after A started but before A ends.

If used with one parameter, for example in a.overlappedBy(b), the method returns true if the start timestamp of B is before the start timestamp of A and the end timestamp of B and is before the end timestamp of A.

Sample EPL:

select * from A.std:lastevent() as a, B.std:lastevent() as b where a.overlappedBy(b)
// Above matches when:
//   b.startTimestamp < a.startTimestamp < b.endTimestamp < a.endTimestamp

If providing two parameters, for example in a.overlappedBy(b, 5 sec), the method returns true if, in addition, the difference between the end timestamp of B and the start timestamp of A is equal to or less then 5 seconds.

Sample EPL:

select * from A.std:lastevent() as a, B.std:lastevent() as b where a.overlappedBy(b, 5 sec)
// Above matches when:
//   b.startTimestamp < a.startTimestamp < b.endTimestamp < a.endTimestamp and 
//   0 <= b.endTimestamp - a.startTimestamp <= 5 seconds

If providing three parameters, for example in a.overlappedBy(b, 5 sec, 10 sec), the method returns true if, in addition, the difference between the end timestamp of B and the start timestamp of A is between 5 and 10 seconds.

Sample EPL:

select * from A.std:lastevent() as a, B.std:lastevent() as b where a.overlappedBy(b, 5 sec, 10 sec)
// Above matches when:
//   b.startTimestamp < a.startTimestamp < b.endTimestamp < a.endTimestamp and 
//   5 seconds <= b.endTimestamp - a.startTimestamp <= 10 seconds

This chapter outlines the views that are built into Esper. All views can be arbitrarily combined as many of the examples below show. The section on Chapter 3, Processing Model provides additional information on the relationship of views, filtering and aggregation. Please also see Section 5.4.3, “Specifying Views” for the use of views in the from clause with streams, patterns and named windows.

Esper organizes built-in views in namespaces and names. Views that provide sliding or tumbling data windows are in the win namespace. Other most commonly used views are in the std namespace. The ext namespace are views that order events. The stat namespace is used for views that derive statistical data.

Esper distinguishes between data window views and derived-value views. Data windows, or data window views, are views that retain incoming events until an expiry policy indicates to release events. Derived-value views derive a new value from event streams and post the result as events of a new type.

Two or more data window views can be combined. This allows a sets of events retained by one data window to be placed into a union or an intersection with the set of events retained by one or more other data windows. Please see Section 5.4.4, “Multiple Data Window Views” for more detail.

The keep-all data window counts as a data window but has no expiry policy: it retains all events received. The grouped-window declaration allocates a new data window per grouping criteria and thereby counts as a data window, but cannot appear alone.

The next table summarizes data window views:

Table 12.1. Built-in Data Window Views

ViewSyntaxDescription
Length Windowwin:length(size)Sliding length window extending the specified number of elements into the past.
Length Batch Windowwin:length_batch(size)Tumbling window that batches events and releases them when a given minimum number of events has been collected.
Time Windowwin:time(time period)Sliding time window extending the specified time interval into the past.
Externally-timed Windowwin:ext_timed(timestamp expression, time period)Sliding time window, based on the millisecond time value supplied by an expression.
Time Batch Windowwin:time_batch(time period[,optional reference point] [, flow control])Tumbling window that batches events and releases them every specified time interval, with flow control options.
Externally-timed Batch Windowwin:ext_timed_batch(timestamp expression, time period[,optional reference point])Tumbling window that batches events and releases them every specified time interval based on the millisecond value supplied by an expression.
Time-Length Combination Batch Windowwin:time_length_batch(time period, size [, flow control])Tumbling multi-policy time and length batch window with flow control options.
Time-Accumulating Windowwin:time_accum(time period)Sliding time window accumulates events until no more events arrive within a given time interval.
Keep-All Windowwin:keepall()The keep-all data window view simply retains all events.
Sorted Windowext:sort(size, sort criteria)Sorts by values returned by sort criteria expressions and keeps only the top events up to the given size.
Ranked Windowext:rank(unique criteria(s), size, sort criteria(s))Retains only the most recent among events having the same value for the criteria expression(s) sorted by sort criteria expressions and keeps only the top events up to the given size.
Time-Order Windowext:time_order(timestamp expression, time period)Orders events that arrive out-of-order, using an expression providing timestamps to be ordered.
Unique Windowstd:unique(unique criteria(s))Retains only the most recent among events having the same value for the criteria expression(s). Acts as a length window of size 1 for each distinct expression value.
Grouped Data Windowstd:groupwin(grouping criteria(s))Groups events into sub-views by the value of the specified expression(s), generally used to provide a separate data window per group.
Last Event Windowstd:lastevent()Retains the last event, acts as a length window of size 1.
First Event Windowstd:firstevent()Retains the very first arriving event, disregarding all subsequent events.
First Unique Windowstd:firstunique(unique criteria(s))Retains only the very first among events having the same value for the criteria expression(s), disregarding all subsequent events for same value(s).
First Length Windowwin:firstlength(size)Retains the first size events, disregarding all subsequent events.
First Time Windowwin:firsttime(time period)Retains the events arriving until the time interval has passed, disregarding all subsequent events.
Expiry Expression Windowwin:expr(expiry expression)Expire events based on the result of an expiry expression passed as a parameter.
Expiry Expression Batch Windowwin:expr_batch(expiry expression)Tumbling window that batches events and releases them based on the result of an expiry expression passed as a parameter.

The table below summarizes views that derive information from received events and present the derived information as an insert and remove stream of events that are typed specifically to carry the result of the computations:


All the views explained below are data window views, as are std:unique, std:firstunique, std:lastevent and std:firstevent.

This view is a moving (sliding) time window extending the specified time interval into the past based on the system time. Provide a time period (see Section 5.2.1, “Specifying Time Periods”) or an expression defining the number of seconds as a parameter:

win:time(time period)
win:time(seconds_interval_expression)

For the GE stock tick events in the last 1 second, calculate a sum of price.

select sum(price) from StockTickEvent(symbol='GE').win:time(1 sec)

The following time windows are equivalent specifications:

win:time(2 minutes 5 seconds)
win:time(125 sec)
win:time(125)
win:time(MYINTERVAL)  // MYINTERVAL defined as a variable

Similar to the time window, this view is a moving (sliding) time window extending the specified time interval into the past, but based on the millisecond time value supplied by a timestamp expression. The view takes two parameters: the expression to return long-typed timestamp values, and a time period or expression that provides a number of seconds:

win:ext_timed(timestamp_expression, time_period)
win:ext_timed(timestamp_expression, seconds_interval_expression)

The key difference comparing the externally-timed window to the regular time window is that the window slides not based on the engine time, but strictly based on the result of the timestamp expression when evaluated against the events entering the window.

The algorithm underlying the view compares the timestamp value returned by the expression when the oldest event arrived with the timestamp value returned by the expression for the newest arriving event on event arrival. If the time interval between the timestamp values is larger then the timer period parameter, then the algorithm removes all oldest events tail-first until the difference between the oldest and newest event is within the time interval. The window therefore slides only when events arrive and only considers each event's timestamp property (or other expression value returned) and not engine time.

This view holds stock tick events of the last 10 seconds based on the timestamp property in StockTickEvent.

select * from StockTickEvent.win:ext_timed(timestamp, 10 seconds)

The externally-timed data window expects strict ordering of the timestamp values returned by the timestamp expression. The view is not useful for ordering events in time order, please use the time-order view instead.

On a related subject, engine time itself can be entirely under control of the application as described in Section 14.8, “Controlling Time-Keeping”, allowing control over all time-based aspects of processing in one place.

This window view buffers events (tumbling window) and releases them every specified time interval in one update. The view takes a time period or an expression providing a number of seconds as a parameter, plus optional parameters described next.

win:time_batch(time_period [,optional_reference_point] [,flow_control])
win:time_batch(seconds_interval_expression [,optional_reference_point] [,flow_control])

The time batch window takes a second, optional parameter that serves as a reference point to batch flush times. If not specified, the arrival of the first event into the batch window sets the reference point. Therefore if the reference point is not specified and the first event arrives at time t1, then the batch flushes at time t1 plus time_period and every time_period thereafter.

Note that using this view means that the engine keeps events in memory until the time is up: Consider your event arrival rate and determine if this is the behavior you want. Use context declaration or output rate limiting such as output snapshot as an alternative.

The below example batches events into a 5 second window releasing new batches every 5 seconds. Listeners to updates posted by this view receive updated information only every 5 seconds.

select * from StockTickEvent.win:time_batch(5 sec)

By default, if there are no events arriving in the current interval (insert stream), and no events remain from the prior batch (remove stream), then the view does not post results to listeners. The view allows overriding this default behavior via flow control keywords.

The synopsis with flow control parameters is:

win:time_batch(time_period or seconds_interval_expr [,optional_reference_point] 
    [, "flow-control-keyword [, keyword...]"] )

The FORCE_UPDATE flow control keyword instructs the view to post an empty result set to listeners if there is no data to post for an interval. When using this keyword the irstream keyword should be used in the select clause to ensure the remove stream is also output. Note that FORCE_UPDATE is for use with listeners to the same statement and not for use with named windows. Consider output rate limiting instead.

The START_EAGER flow control keyword instructs the view to post empty result sets even before the first event arrives, starting a time interval at statement creation time. As when using FORCE_UPDATE, the view also posts an empty result set to listeners if there is no data to post for an interval, however it starts doing so at time of statement creation rather then at the time of arrival of the first event.

Taking the two flow control keywords in one sample statement, this example presents a view that waits for 10 seconds. It posts empty result sets after one interval after the statement is created, and keeps posting an empty result set as no events arrive during intervals:

select * from MyEvent.win:time_batch(10 sec, "FORCE_UPDATE, START_EAGER")

The optional reference point is provided as a long-value of milliseconds relative to January 1, 1970 and time 00:00:00.

The following example statement sets the reference point to 5 seconds and the batch size to 1 hour, so that each batch output is 5 seconds after each hour:

select * from OrderSummaryEvent.win:time_batch(1 hour, 5000L)

Similar to the time batch window, this view buffers events (tumbling) and releases them every specified time interval in one update, but based on the millisecond time value supplied by a timestamp expression. The view has two required parameters taking an expression that returns long-typed timestamp values and a time period or constant-value expression that provides a number of seconds:

win:ext_timed_batch(timestamp_expression, time_period [,optional_reference_point])
win:ext_timed_batch(timestamp_expression, seconds_interval_expression [,optional_reference_point])

The externally-timed batch window takes a third, optional parameter that serves as a reference point to batch flush times. If not specified, the arrival of the first event into the batch window sets the reference point. Therefore if the reference point is not specified and the first event arrives at time t1, then the batch flushes at time t1 plus time_period and every time_period thereafter.

The key difference comparing the externally-timed batch window to the regular time batch window is that the window tumbles not based on the engine time, but strictly based on the result of the timestamp expression when evaluated against the events entering the window.

The algorithm underlying the view compares the timestamp value returned by the expression when the oldest event arrived with the timestamp value returned by the expression for the newest arriving event on event arrival. If the time interval between the timestamp values is larger then the timer period parameter, then the algorithm posts the current batch of events. The window therefore posts batches only when events arrive and only considers each event's timestamp property (or other expression value returned) and not engine time.

Note that using this view means that the engine keeps events in memory until the time is up: Consider your event arrival rate and determine if this is the behavior you want. Use context declaration or output rate limiting such as output snapshot as an alternative.

The below example batches events into a 5 second window releasing new batches every 5 seconds. Listeners to updates posted by this view receive updated information only when event arrive with timestamps that indicate the start of a new batch:

select * from StockTickEvent.win:ext_timed_batch(timestamp, 5 sec)

The optional reference point is provided as a long-value of milliseconds relative to January 1, 1970 and time 00:00:00.

The following example statement sets the reference point to 5 seconds and the batch size to 1 hour, so that each batch output is 5 seconds after each hour:

select * from OrderSummaryEvent.win:ext_timed_batch(timestamp, 1 hour, 5000L)

The externally-timed data window expects strict ordering of the timestamp values returned by the timestamp expression. The view is not useful for ordering events in time order, please use the timeorder view instead.

On a related subject, engine time itself can be entirely under control of the application as described in Section 14.8, “Controlling Time-Keeping”, allowing control over all time-based aspects of processing in one place.

This data window view is a combination of time and length batch (tumbling) windows. Similar to the time and length batch windows, this view batches events and releases the batched events when either one of the following conditions occurs, whichever occurs first: the data window has collected a given number of events, or a given time interval has passed.

The view parameters take 2 forms. The first form accepts a time period or an expression providing a number of seconds, and an expression for the number of events:

win:time_length_batch(time_period, number_of_events_expression)
win:time_length_batch(seconds_interval_expression, number_of_events_expression)

The next example shows a time-length combination batch window that batches up to 100 events or all events arriving within a 1-second time interval, whichever condition occurs first:

 select * from MyEvent.win:time_length_batch(1 sec, 100)

In this example, if 100 events arrive into the window before a 1-second time interval passes, the view posts the batch of 100 events. If less then 100 events arrive within a 1-second interval, the view posts all events that arrived within the 1-second interval at the end of the interval.

By default, if there are no events arriving in the current interval (insert stream), and no events remain from the prior batch (remove stream), then the view does not post results to listeners. This view allows overriding this default behavior via flow control keywords.

The synopsis of the view with flow control parameters is:

win:time_length_batch(time_period or seconds_interval_expression, number_of_events_expression, 
    "flow control keyword [, keyword...]")

The FORCE_UPDATE flow control keyword instructs the view to post an empty result set to listeners if there is no data to post for an interval. The view begins posting no later then after one time interval passed after the first event arrives. When using this keyword the irstream keyword should be used in the select clause to ensure the remove stream is also output.

The START_EAGER flow control keyword instructs the view to post empty result sets even before the first event arrives, starting a time interval at statement creation time. As when using FORCE_UPDATE, the view also posts an empty result set to listeners if there is no data to post for an interval, however it starts doing so at time of statement creation rather then at the time of arrival of the first event.

Taking the two flow control keywords in one sample statement, this example presents a view that waits for 10 seconds or reacts when the 5th event arrives, whichever comes first. It posts empty result sets after one interval after the statement is created, and keeps posting an empty result set as no events arrive during intervals:

 select * from MyEvent.win:time_length_batch(10 sec, 5, "FORCE_UPDATE, START_EAGER")

The expr view applies an expiry expression and removes events from the data window when the expression returns false.

Use this view to implement rolling and dynamically shrinking or expanding time, length or other windows. Rolling can, for example, be controlled based on event properties of arriving events, based on aggregation values or based on the return result of user-defined functions. Use this view to accumulate events until a value changes or other condition occurs based on arriving events or change of a variable value.

The synopsis is:

win:expr(expiry_expression)

The expiry expression can be any expression including expressions on event properties, variables, aggregation functions or user-defined functions. The view applies this expression to the oldest event(s) currently in the view, as described next.

When a new event arrives or when a variable value referenced by the expiry expression changes then the view applies the expiry expression starting from the oldest event in the data window. If the expiry expression returns false for the oldest event, the view removes the event from the data window. The view then applies the expression to the next oldest event. If the expiry expression returns true for the oldest event, no further evaluation takes place and the view indicates any new and expired events through insert and remove stream.

By using variables in the expiry expression it is possible to change the behavior of the view dynamically at runtime. When one or more variables used in the expression are updated the view evaluates the expiry expression starting from the oldest event.

Aggregation functions, if present in the expiry expression, are continuously updated as events enter and leave the data window. Use the grouped data window with this window to compute aggregations per group.

The engine makes the following built-in properties available to the expiry expression:


This EPL declares an expiry expression that retains the last 2 events:

select * from MyEvent.win:expr(current_count <= 2)

The following example implements a dynamically-sized length window by means of a SIZE variable. As the SIZE variable value changes the view retains the number of events according to the current value of SIZE:

create variable int SIZE = 1000
select * from MyEvent.win:expr(current_count <= SIZE)

The next EPL retains the last 2 seconds of events:

select * from MyEvent.win:expr(oldest_timestamp > newest_timestamp - 2000)

The following example implements a dynamically-sized time window. As the SIZE millisecond variable value changes the view retains a time interval accordingly:

create variable long SIZE = 1000
select * from MyEvent.win:expr(newest_timestamp - oldest_timestamp < SIZE)

The following example declares a KEEP variable and flushes all events from the data window when the variable turns false:

create variable boolean KEEP = true
select * from MyEvent.win:expr(KEEP)

The next example specifies a rolling window that removes the oldest events from the window until the total price of all events in the window is less then 1000:

select * from MyEvent.win:expr(sum(price) < 1000)

This example retains all events that have the same value of the flag event property. When the flag value changes, the data window expires all events with the old flag value and retains only the most recent event of the new flag value:

select * from MyEvent.win:expr(newest_event.flag = oldest_event.flag)

The expr_batch view buffers events (tumbling window) and releases them when a given expiry expression returns true.

Use this view to implement dynamic or custom batching behavior, such as for dynamically shrinking or growing time, length or other batches, for batching based on event properties of arriving events, aggregation values or for batching based on a user-defined function.

The synopsis is:

win:expr_batch(expiry_expression, [include_triggering_event])

The expiry expression can be any expression including expressions on event properties, variables, aggregation functions or user-defined functions. The view applies this expression to arriving event(s), as described next.

The optional second parameter include_triggering_event defines whether to include the event that triggers the batch in the current batch (true, the default) or in the next batch (false).

When a new event arrives or when a variable value referenced by the expiry expression changes or when events get removed from the data window then the view applies the expiry expression. If the expiry expression returns true the data window posts the collected events as the insert stream and the last batch of events as remove stream.

By using variables in the expiry expression it is possible to change the behavior of the view dynamically at runtime. When one or more variables used in the expression are updated the view evaluates the expiry expression as well.

Aggregation functions, if present in the expiry expression, are continuously updated as events enter the data window and reset when the engine posts a batch of events. Use the grouped data window with this window to compute aggregations per group.

The engine makes the following built-in properties available to the expiry expression:


This EPL declares an expiry expression that posts event batches consisting of 2 events:

select * from MyEvent.win:expr_batch(current_count >= 2)

The following example implements a dynamically-sized length batch window by means of a SIZE variable. As the SIZE variable value changes the view accomulates and posts the number of events according to the current value of SIZE:

create variable int SIZE = 1000
select * from MyEvent.win:expr_batch(current_count >= SIZE)

The following example accumulates events until an event arrives that has a value of postme for property myvalue:

select * from MyEvent.win:expr_batch(myvalue = 'postme')

The following example declares a POST variable and posts a batch of events when the variable turns true:

create variable boolean POST = false
select * from MyEvent.win:expr_batch(POST)

The next example specifies a tumbling window that posts a batch of events when the total price of all events in the window is greater then 1000:

select * from MyEvent.win:expr_batch(sum(price) > 1000)

Specify the second parameter as false when you want the triggering event not included in the current batch.

This example batches all events that have the same value of the flag event property. When the flag value changes, the data window releases the batch of events collected for the old flag value. The data window collects the most recent event and the future arriving events of the same new flag value:

select * from MyEvent.win:expr_batch(newest_event.flag != oldest_event.flag, false)

This view groups events into sub-views by the value returned by the specified expression or the combination of values returned by a list of expressions. The view takes a single expression to supply the group criteria values, or a list of expressions as parameters, as the synopsis shows:

std:groupwin(grouping_expression [, grouping_expression ...])

The grouping_expression expression(s) return one or more group keys, by which the view creates sub-views for each distinct group key. Note that the expression should not return an unlimited number of values: the grouping expression should not return a time value or otherwise unlimited key.

An expression may return a null value. The engine treats a null value as any other value. An expression can also return a custom application object, whereby the application class should implement the hashCode and equals methods.

Use group by instead of the grouped data window to control how aggregations are grouped.

A grouped data window with a length window of 1 is equivalent to the unique data window std:unique. The std:unique data window is the preferred notation:

select * from StockTickEvent.std:unique(symbol)	// Prefer this
// ... equivalent to ...
select * from StockTickEvent.std:groupwin(symbol).win:length(1)

This example computes the total price for the last 5 events considering the last 5 events per each symbol, aggregating the price across all symbols (since no group by clause is specified the aggregation is across all symbols):

select symbol, sum(price) from StockTickEvent.std:groupwin(symbol).win:length(5)

The @Hint("reclaim_group_aged=age_in_seconds") hint instructs the engine to discard grouped data window state that has not been updated for age_in_seconds seconds. The optional @Hint("reclaim_group_freq=sweep_frequency_in_seconds") can be specified in addition to control the frequency at which the engine sweeps data window state. If the hint is not specified, the frequency defaults to the same value as age_in_seconds. Use the hints when your group criteria returns a changing or unlimited number of values. By default and without hints the view does not reclaim or remove data windows for group criteria values.

The updated sample statement with both hints:

// Remove data window views for symbols not updated for 10 seconds or more and sweep every 30 seconds
@Hint('reclaim_group_aged=10,reclaim_group_freq=30')
select symbol, sum(price) from StockTickEvent.std:groupwin(symbol).win:length(5)

Reclaim executes when an event arrives and not in the timer thread. In the example above reclaim can occur up to 40 seconds of engine time after the newest event arrives. Reclaim may affect iteration order for the statement and iteration order becomes indeterministic with reclaim.

To compute the total price for the last 5 events considering the last 5 events per each symbol and outputting a price per symbol, add the group by clause:

select symbol, sum(price) from StockTickEvent.std:groupwin(symbol).win:length(5) group by symbol

The std:groupwin grouped-window view can also take multiple expressions that provide values to group by. This example computes the total price for each symbol and feed for the last 10 events per symbol and feed combination:

select sum(price) from StockTickEvent.std:groupwin(symbol, feed).win:length(10)

The order in which the std:groupwin grouped-window view appears within sub-views of a stream controls the data the engine derives from events for each group. The next 2 statements demonstrate this using a length window.

Without the std:groupwin declaration query the same query returns the total price per symbol for only the last 10 events across all symbols. Here the engine allocates only one length window for all events:

select sum(price) from StockTickEvent.win:length(10)

We have learned that by placing the std:groupwin grouped-window view before other views, these other views become part of the grouped set of views. The engine dynamically allocates a new view instance for each subview, every time it encounters a new group key such as a new value for symbol. Therefore, in std:groupwin(symbol).win:length(10) the engine allocates a new length window for each distinct symbol. However in win:length(10) alone the engine maintains a single length window.

The std:groupwin can be used with multiple data window views to achieve a grouped intersection or union policy.

The next query retains the last 4 events per symbol and only those events that are also not older then 10 seconds:

select * from StockTickEvent.std:groupwin(symbol).win:length(4).win:time(10)

Last, we consider a grouped data window for two group criteria. Here, the query results are total price per symbol and feed for the last 100 events per symbol and feed.

select sum(price) from StockTickEvent.std:groupwin(symbol, feed).win:length(100)

Note

A note on grouped time windows: When using grouped-window with time windows, note that whether the engine retains 5 minutes of events or retains 5 minutes of events per group, the result is the same from the perspective of retaining events as both policies retain, considering all groups, the same set of events. Therefore please specify the time window alone (ungrouped).

For example:

// Use this:
select sum(price) from StockTickEvent.win:time(1 minute)

// is equivalent to (don't use this):
// select sum(price) from StockTickEvent.std:groupwin(symbol).win:time(1 minute)

// Use the group-by clause for grouping aggregation by symbol.

For advanced users: There is an optional view that can control how the std:groupwin grouped-window view gets evaluated and that view is the std:merge view. The merge view can only occur after a std:groupwin grouped-window view in a view chain and controls at what point in the view chain the merge of the data stream occurs from view-instance-per-criteria to single view.

Compare the following statements:

select * from Market.std:groupwin(ticker).win:length(1000000)
    .stat:weighted_avg(price, volume).std:merge(ticker)
// ... and ...
select * from Market.std:groupwin(ticker).win:length(1000000).std:merge(ticker)
    .stat:weighted_avg(price, volume)

If your statement does not specify the optional std:merge view, the semantics are the same as the first statement.

The first statement, in which the merge-view is added to the end (same as no merge view), computes weighted average per ticker, considering, per-ticker, the last 1M Market events for each ticker. The second statement, in which the merge view is added to the middle, computes weighted average considering, per-ticker, the last 1M Market events, computing the weighted average for all such events using a single view rather then multiple view instances with one view per ticker.

This view posts the number of events received from a stream or view plus any additional event properties or expression values listed as parameters. The synopsis is:

std:size([expression, ...] [ * ])

The view posts a single long-typed property named size. The view posts the prior size as old data, and the current size as new data to update listeners of the view. Via the iterator method of the statement the size value can also be polled (read). The view only posts output events when the size count changes and does not stay the same.

As optional parameters the view takes a list of expressions that the view evaluates against the last arriving event and provides along the size field. You may also provide the * wildcard selector to have the view output all event properties.

An alternative to receiving a data window event count is the prevcount function. Compared to the std:size view the prevcount function requires a data window while the std:size view does not. The related count(...) aggregation function provides a count per group when used with group by.

When combined with a data window view, the size view reports the current number of events in the data window in the insert stream and the prior number of events in the data window as the remove stream. This example reports the number of tick events within the last 1 minute:

select size from StockTickEvent.win:time(1 min).std:size()

To select additional event properties you may add each event property to output as a parameter to the view.

The next example selects the symbol and feed event properties in addition to the size property:

select size, symbol, feed from StockTickEvent.win:time(1 min).std:size(symbol, feed)

This example selects all event properties in addition to the size property:

select * from StockTickEvent.win:time(1 min).std:size(*)

The size view is also useful in conjunction with a std:groupwin grouped-window view to count the number of events per group. The EPL below returns the number of events per symbol.

select size from StockTickEvent.std:groupwin(symbol).std:size()

When used without a data window, the view simply counts the number of events:

select size from StockTickEvent.std:size()

All views can be used with pattern statements as well. The next EPL snippet shows a pattern where we look for tick events followed by trade events for the same symbol. The size view counts the number of occurrences of the pattern.

select size from pattern[every s=StockTickEvent -> TradeEvent(symbol=s.symbol)].std:size()

The statistics views can be used combined with data window views or alone. Very similar to aggregation functions, these views aggregate or derive information from an event stream. As compared to aggregation functions, statistics views can post multiple derived fields including properties from the last event that was received. The derived fields and event properties are available for querying in the where-clause and are often compared to prior values using the prior function.

Statistics views accept one or more primary value expressions and any number of optional additional expressions that return values based on the last event received.

This view calculates univariate statistics on a numeric expression. The view takes a single value expression as a parameter plus any number of optional additional expressions to return properties of the last event. The value expression must return a numeric value:

stat:uni(value_expression [,expression, ...] [ * ])

After the value expression you may optionally list additional expressions or event properties to evaluate for the stream and return their value based on the last arriving event. You may also provide the * wildcard selector to have the view output all event properties.


The below example selects the standard deviation on price for stock tick events for the last 10 events.

select stddev from StockTickEvent.win:length(10).stat:uni(price)

To add properties from the event stream you may simply add all additional properties as parameters to the view.

This example selects all of the derived values, based on the price property, plus the values of the symbol and feed event properties:

select * from StockTickEvent.win:length(10).stat:uni(price, symbol, feed)

The following example selects all of the derived values plus all event properties:

select * from StockTickEvent.win:length(10).stat:uni(price, symbol, *)

This view calculates regression and related intermediate results on the values returned by two expressions. The view takes two value expressions as parameters plus any number of optional additional expressions to return properties of the last event. The value expressions must return a numeric value:

stat:linest(value_expression, value_expression [,expression, ...] [ * ])

After the two value expressions you may optionally list additional expressions or event properties to evaluate for the stream and return their value based on the last arriving event. You may also provide the * wildcard selector to have the view output all event properties.


The next example calculates regression and returns the slope and y-intercept on price and offer for all events in the last 10 seconds.

select slope, YIntercept from StockTickEvent.win:time(10 seconds).stat:linest(price, offer)

To add properties from the event stream you may simply add all additional properties as parameters to the view.

This example selects all of the derived values, based on the price and offer properties, plus the values of the symbol and feed event properties:

select * from StockTickEvent.win:time(10 seconds).stat:linest(price, offer, symbol, feed)

The following example selects all of the derived values plus all event properties:

select * from StockTickEvent.win:time(10 seconds).stat:linest(price, offer, *)

This view returns the weighted average given an expression returning values to compute the average for and an expression returning weight. The view takes two value expressions as parameters plus any number of optional additional expressions to return properties of the last event. The value expressions must return numeric values:

stat:weighted_avg(value_expression_field, value_expression_weight [,expression, ...] [ * ])

After the value expression you may optionally list additional expressions or event properties to evaluate for the stream and return their value based on the last arriving event. You may also provide the * wildcard selector to have the view output all event properties.


A statement that derives the volume-weighted average price for the last 3 seconds for a given symbol is shown below:

select average 
from StockTickEvent(symbol='GE').win:time(3 seconds).stat:weighted_avg(price, volume)

To add properties from the event stream you may simply add all additional properties as parameters to the view.

This example selects all of the derived values, based on the price and volume properties, plus the values of the symbol and feed event properties:

select *
from StockTickEvent.win:time(3 seconds).stat:weighted_avg(price, volume, symbol, feed)

The next example selects all of the derived values plus the values of all event properties:

select *
from StockTickEvent.win:time(3 seconds).stat:weighted_avg(price, volume, *)

Aggregation functions could instead be used to compute the weighted average as well. The next example also posts weighted average per symbol considering the last 3 seconds of stock tick data:

select symbol, sum(price*volume)/sum(volume)
from StockTickEvent.win:time(3 seconds) group by symbol

The following example computes weighted average keeping a separate data window per symbol considering the last 5 events of each symbol:

select symbol, average
from StockTickEvent.std:groupwin(symbol).win:length(5).stat:weighted_avg(price, volume)

The views in this set are data windows that order events according to a criteria.

This view sorts by values returned by the specified expression or list of expressions and keeps only the top (or bottom) events up to the given size.

This view retains all events in the stream that fall into the sort range. Use the ranked window as described next to retain events per unique key(s) and sorted.

The syntax is as follows:

ext:sort(size_expression, 
    sort_criteria_expression [asc/desc][, sort_criteria_expression [asc/desc]...]) 

An expression may be followed by the optional asc or desc keywords to indicate that the values returned by that expression are sorted in ascending or descending sort order.

The view below retains only those events that have the highest 10 prices considering all events (and not only the last event per symbol, see rank below) and reports a total price:

select sum(price) from StockTickEvent.ext:sort(10, price desc)

The following example sorts events first by price in descending order, and then by symbol name in ascending (alphabetical) order, keeping only the 10 events with the highest price (with ties resolved by alphabetical order of symbol).

select * from StockTickEvent.ext:sort(10, price desc, symbol asc)

The sorted window is often used with the prev, prevwindow or prevtail single-row functions to output properties of events at a certain position or to output the complete data window according to sort order.

Use the grouped window to retain a separate sort window for each group. For example, the views std:groupwin(market).ext:sort(10, price desc) instruct the engine to retain, per market, the highest 10 prices.

This view retains only the most recent among events having the same value for the criteria expression(s), sorted by sort criteria expressions and keeps only the top events up to the given size.

This view is similar to the sorted window in that it keeps only the top (or bottom) events up to the given size, however the view also retains only the most recent among events having the same value(s) for the specified uniqueness expression(s).

The syntax is as follows:

ext:rank(unique_expression [, unique_expression ...],
    size_expression, 
    sort_criteria_expression [asc/desc][, sort_criteria_expression [asc/desc]...]) 

Specify the expressions returning unique key values first. Then specify a constant value that is the size of the ranked window. Then specify the expressions returning sort criteria values. The sort criteria expressions may be followed by the optional asc or desc keywords to indicate that the values returned by that expression are sorted in ascending or descending sort order.

The view below retains only those events that have the highest 10 prices considering only the last event per symbol and reports a total price:

select sum(price) from StockTickEvent.ext:rank(symbol, 10, price desc)

The following example retains, for the last event per market and symbol, those events that sort by price and quantity ascending into the first 10 ranks:

select * from StockTickEvent.ext:rank(market, symbol, 10, price, quantity)

The ranked window is often used with the prev, prevwindow or prevtail single-row functions to output properties of events at a certain position or to output the complete data window according to sort order.

This example outputs every 5 seconds the top 10 events according to price descending and considering only the last event per symbol:

select prevwindow(*) from StockTickEvent.ext:rank(symbol, 10, price desc)
  output snapshot every 5 seconds limit 1  // need only 1 row

Use the grouped window to retain a separate rank for each group. For example, the views std:groupwin(market).ext:rank(symbol, 10, price desc) instruct the engine to retain, per market, the highest 10 prices considering the last event per symbol.

This view orders events that arrive out-of-order, using timestamp-values provided by an expression, and by comparing that timestamp value to engine system time.

The syntax for this view is as follows.

ext:time_order(timestamp_expression, time_period)
ext:time_order(timestamp_expression, seconds_interval_expression)

The first parameter to the view is the expression that supplies timestamp values. The timestamp is expected to be a long-typed millisecond value that denotes an event's time of consideration by the view (or other expression). This is typically the time of arrival. The second parameter is a number-of-seconds expression or the time period specifying the time interval that an arriving event should maximally be held, in order to consider older events arriving at a later time.

Since the view compares timestamp values to engine time, the view requires that the timestamp values and current engine time are both following the same clock. Therefore, to the extend that the clocks that originated both timestamps differ, the view may produce inaccurate results.

As an example, the next statement uses the arrival_time property of MyTimestampedEvent events to order and release events by arrival time:

insert rstream into ArrivalTimeOrderedStream
select rstream * from MyTimestampedEvent.ext:time_order(arrival_time, 10 sec)

In the example above, the arrival_time property holds a long-typed timestamp value in milliseconds. On arrival of an event, the engine compares the timestamp value of each event to the tail-time of the window. The tail-time of the window is, in this example, 10 seconds before engine time (continuously sliding). If the timestamp value indicates that the event is older then the tail-time of the time window, the event is released immediately in the remove stream. If the timestamp value indicates that the event is newer then the tail-time of the window, the view retains the event until engine time moves such that the event timestamp is older then tail-time.

The examples thus holds each arriving event in memory anywhere from zero seconds to 10 seconds, to allow for older events (considering arrival time timestamp) to arrive. In other words, the view holds an event with an arrival time equal to engine time for 10 seconds. The view holds an event with an arrival time that is 2 seconds older then engine time for 8 seconds. The view holds an event with an arrival time that is 10 or more seconds older then engine time for zero seconds, and releases such (old) events immediately into the remove stream.

The insert stream of this sliding window consists of all arriving events. The remove stream of the view is ordered by timestamp value: The event that has the oldest timestamp value is released first, followed by the next newer events. Note the statement above uses the rstream keyword in both the insert into clause and the select clause to select ordered events only. It uses the insert into clause to makes such ordered stream available for subsequent statements to use.

It is up to your application to populate the timestamp property into your events or use a sensible expression that returns timestamp values for consideration by the view. The view also works well if you use externally-provided time via timer events.

Data flows in Esper EPL have the following purposes:

  1. Support for data flow programming and flow-based programming.

  2. Declarative and runtime manageable integration of Esper input and output adapters that may be provided by EsperIO or by an application.

  3. Remove the need to use an event bus achieving dataflow-only visibility of events and event types for performance gains.

Data flow operators communicate via streams of either underlying event objects or wrapped events. Underlying event objects are POJO, Map, Object-array or DOM/XML. Wrapped events are represented by EventBean instances that associate type information to underlying event objects.

For more information on data flow programming or flow-based programming please consult the Wikipedia FBP Article.

Esper offers a number of useful built-in operators that can be combined in a graph to program a data flow. In addition EsperIO offers prebuilt operators that act as sources or sinks of events. An application can easily create and use its own data flow operators.

Using data flows an application can provide events to the data flow operators directly without using an engine's event bus. Not using an event bus (as represented by EPRuntime.sendEvent) can achieve performance gains as the engine does not need to match events to statements and the engine does not need to wrap underlying event objects in EventBean instances.

Data flows also allow for finer-grained control over threading, synchronous and asynchronous operation.

Note

Data flows are new in release 4.6 and may be subject to evolutionary change.

Your application declares a data flow using create dataflow dataflow-name. Declaring the data flow causes the EPL compiler to validate the syntax and some aspects of the data flow graph of operators. Declaring the data flow does not actually instantiate or execute a data flow. Resolving event types and instantiating operators (as required) takes place at time of data flow instantiation.

After your application has declared a data flow, it can instantiate the data flow and execute it. A data flow can be instantiated as many times as needed and each data flow instance can only be executed once.

The example EPL below creates a data flow that, upon execution, outputs the text Hello World to console and then ends.

create dataflow HelloWorldDataFlow
  BeaconSource -> helloworld.stream { text: 'hello world' , iterations: 1}
  LogSink(helloworld.stream) {}

The sample data flow above declares a BeaconSource operator parameterized by the "hello world" text and 1 iteration. The -> keyword reads as produces streams. The BeaconSource operator produces a single stream named helloworld.stream. The LogSink operator receives this stream and prints it unformatted.

The next program code snippet declares the data flow to the engine:

String epl = "create dataflow HelloWorldDataFlow\n" +
  "BeaconSource -> helloworldStream { text: 'hello world' , iterations: 1}\n" +
  "LogSink(helloworldStream) {}";
epService.getEPAdministrator().createEPL(epl);

After declaring a data flow to an engine, your application can then instantiate and execute the data flow.

The following program code snippet instantiates the data flow:

EPDataFlowInstance instance =
  epService.getEPRuntime().getDataFlowRuntime().instantiate("HelloWorldDataFlow");

A data flow instance is represented by an EPDataFlowInstance object.

The next code snippet executes the data flow instance:

instance.run();

By using the run method of EPDataFlowInstance the engine executes the data flow using the same thread (blocking execute) and returns when the data flow completes. A data flow completes when all operators receive final markers.

The hello world data flow simply prints an unformatted Hello World string to console. Please check the built-in operator reference for BeaconSource and LogSink for more options.

The synopsis for declaring a data flow is:

create dataflow name
	[schema_declarations]
	[operator_declarations]
	

After create dataflow follows the data flow name and a mixed list of event type (schema) declarations and operator declarations.

Schema declarations define an event type. Specify any number of create schema clauses as part of the data flow declaration followed by a comma character to end each schema declaration. The syntax for create schema is described in Section 5.16, “Declaring an Event Type: Create Schema”.

All event types that are defined as part of a data flow are private to the data flow and not available to other EPL statements. To define event types that are available across data flows and other EPL statements, use a create schema EPL statement, runtime or static configuration.

Annotations as well as expression declarations and scripts can also be pre-pended to the data flow declaration.

For each operator, declare the operator name, input streams, output streams and operator parameters.

The syntax for declaring a data flow operator is:

operator_name [(input_streams)]  [-> output_streams] {
  [parameter_name : parameter_value_expr] [, ...]
}

The operator name is an identifier that identifies an operator.

If the operator accepts input streams then those may be listed in parenthesis after the operator name, see Section 13.2.2.2, “Declaring Input Streams”.

If the operator can produce output streams then specify -> followed by a list of output stream names and types. See Section 13.2.2.3, “Declaring Output Streams”.

Following the input and output stream declaration provide curly brackets ({}) containing operator parameters. See Section 13.2.2.4, “Declaring Operator Parameters”.

An operator that receives no input streams, produces no output streams and has no parameters assigned to it is shown in this EPL example data flow:

create dataflow MyDataFlow
  MyOperatorSimple {}

The next EPL shows a data flow that consists of an operator MyOperator that receives a single input stream myInStream and produces a single output stream myOutStream holding MyEvent events. The EPL configures the operator parameter myParameter with a value of 10:

create dataflow MyDataFlow
  create schema MyEvent as (id string, price double),
  MyOperator(myInStream) -> myOutStream<MyEvent> {
    myParameter : 10
  }

The next sections outline input stream, output stream and parameter assignment in greater detail.

In case the operator produces output streams, list the output streams after the -> keyword. Multiple output streams can be listed separated by comma. We use the term output port to mean the ordinal number of the output stream in the order the output streams are listed.

The sample EPL below declares an operator that produces two output streams my.out.one and my.out.two.

create dataflow MyDataFlow
  MyOperator -> my.out.one, my.out.two {}

Each output stream can be assigned optional type information within less/greater-then (<>). Type information is required if the operator cannot deduce the output type from the input type and the operator does not declare explicit output type(s). The event type name can either be an event type defined within the same data flow or an event type defined in the engine.

This EPL example declares an RFIDSchema event type based on an object-array event representation and associates the output stream rfid.stream with the RFIDSchema type. The stream rfid.stream therefore carries object-array (Object[]) typed objects according to schema RFIDSchema:

create dataflow MyDataFlow
  create objectarray schema RFIDSchema (tagId string, locX double, locY double),
  MyOperator -> rfid.stream<RFIDSchema> {}

The keyword eventbean is reserved: Use eventbean<type-name> to indicate that a stream carries EventBean instances of the given type instead of the underlying event object.

This EPL example declares an RFIDSchema event type based on an object-array event representation and associates the output stream rfid.stream with the event type, such that the stream rfid.stream carries EventBean objects:

create dataflow MyDataFlow
  create objectarray schema RFIDSchema (tagId string, locX double, locy double),
  MyOperator -> rfid.stream<eventbean<RFIDSchema>> {}

Use questionmark (?) to indicate that the type of events is not known in advance.

In the next EPL the stream my.stream carries EventBean instances of any type:

create dataflow MyDataFlow
  MyOperator -> my.stream<eventbean<?>> {}

Operators can receive constants, objects, EPL expressions and complete EPL statements as parameters. All parameters are listed within curly brackets ({}) after input and output stream declarations. Curly brackets are required as a separator even if the operator has no parameters.

The syntax for parameters is:

name : value_expr [,...]

The parameter name is an identifier that is followed by the colon (:) or equals (=) character and a value expression. A value expression can be any expression, system property, JSON notation object or EPL statement. Parameters are separated by comma character.

The next EPL demonstrates operator parameters that are scalar values:

create dataflow MyDataFlow
  MyOperator {
    stringParam : 'sample',
    secondString : "double-quotes are fine",
    intParam : 10
  }

Operator parameters can be any EPL expression including expressions that use variables. Subqueries, aggregations and the prev and prior functions cannot be applied here.

The EPL shown below lists operator parameters that are expressions:

create dataflow MyDataFlow
  MyOperator {
    intParam : 24*60*60,
    threshold : var_threshold	// a variable defined in the engine
  }

To obtain the value of a system property, the special systemProperties property name is reserved for access to system properties.

The following EPL sets operator parameters to a value obtained from a system property:

create dataflow MyDataFlow
  MyOperator {
    someSystemProperty : systemProperties('mySystemProperty') 
  }

Any JSON value can also be used as a value. Use square brackets [] for JSON arrays. Use curly brackets {} to hold nested Map or other object values. Provide the special class property to instantiate a given instance by class name. The engine populates the respective array, Map or Object as specified in the JSON parameter value.

The below EPL demonstrates operator parameters that are JSON values:

create dataflow MyDataFlow
  MyOperator {
    myStringArray: ['a', "b"],
    myMapOrObject: {
      a : 10,
      b : 'xyz',
    },
    myInstance: {
      class: 'com.myorg.myapp.MyImplementation',
      myValue : 'sample'
    }
  }

The special parameter name select is reserved for use with EPL select statements. Please see the Select built-in operator for an example.

The below table summarizes the built-in data flow operators (Esper only) available:


The below table summarizes the built-in EsperIO data flow operators. Please see the EsperIO documentation and source for more information.


The BeaconSource operator generates events and populates event properties.

The BeaconSource operator does not accept any input streams and has no input ports.

The BeaconSource operator must have a single output stream. When the BeaconSource operator completed generating events according to the number of iterations provided or when it is cancelled it outputs a final marker to the output stream.

Parameters for the BeaconSource operator are all optional parameters:


Event properties to be populated can simply be added to the parameters.

If your declaration provides an event type for the output stream then BeaconSource will populate event properties of the underlying events. If no event type is specified, BeaconSource creates an anonymous object-array event type to carry the event properties that are generated and associates this type with its output stream.

Examples are:

create dataflow MyDataFlow
  create schema SampleSchema(tagId string, locX double),	// sample type			
			
  // BeaconSource that produces empty object-array events without delay 
  // or interval until cancelled.
  BeaconSource -> stream.one {}
  
  // BeaconSource that produces one RFIDSchema event populating event properties
  // from a user-defined function "generateTagId" and the provided values.
  BeaconSource -> stream.two<SampleSchema> {
    iterations : 1,
    tagId : generateTagId(),
    locX : 10
  }
  
  // BeaconSource that produces 10 object-array events populating
  // the price property with a random value.
  BeaconSource -> stream.three {
    iterations : 1,
    interval : 10, // every 10 seconds
    initialDelay : 5, // start after 5 seconds
    price : Math.random() * 100
  }

The EPStatementSource operator maintains a subscription to the results of one or more EPL statements. The operator produces the statement output events.

The EPStatementSource operator does not accept any input streams and has no input ports.

The EPStatementSource operator must have a single output stream. It does not generate a final or other marker.

Either the statement name or the statement filter parameter is required:


If a statement name is provided, the operator subscribes to output events of the statement if the statement exists or when it gets created at a later point in time.

If a statement filter is provided instead, the operator subscribes to output events of all statements that currently exist and pass the filter pass method or that get created at a later point in time and pass the filter pass method.

The collector can be specified to transform output events. If no collector is specified the operator submits the underlying events of the insert stream received from the statement. The collector object must implement the interface EPDataFlowIRStreamCollector.

Examples are:

create dataflow MyDataFlow
  create schema SampleSchema(tagId string, locX double),	// sample type			
			
  // Consider only the statement named MySelectStatement when it exists.
  // No transformation.
  EPStatementSource -> stream.one<eventbean<?>> {
    statementName : 'MySelectStatement'
  }
  
  // Consider all statements that match the filter object provided.
  // No transformation.
  EPStatementSource -> stream.two<eventbean<?>> {
    statementFilter : {
      class : 'com.mycompany.filters.MyStatementFilter'
    }
  }
  
  // Consider all statements that match the filter object provided.
  // With collector that performs transformation.
  EPStatementSource -> stream.two<SampleSchema> {
    collector : {
      class : 'com.mycompany.filters.MyCollector'
    },
    statementFilter : {
      class : 'com.mycompany.filters.MyStatementFilter'
    }
  }

The EventBusSource operator receives events from the event bus and produces an output stream of the events received. With the term event bus we mean any event visible to the engine either because the application send the event via EPRuntime.sendEvent or because statements populated streams as a result of insert into.

The EventBusSource operator does not accept any input streams and has no input ports.

The EventBusSource operator must have a single output stream. It does not generate a final or other marker. The event type declared for the output stream is the event type of events received from the event bus.

All parameters to EventBusSource are optional:


The collector can be specified to transform output events. If no collector is specified the operator submits the underlying events of the stream received from the event bus. The collector object must implement the interface EPDataFlowEventBeanCollector.

The filter is an expression that the event bus compiles and efficiently matches even in the presence of a large number of event bus sources. The filter expression must return a boolean-typed value, returning true for those events that the event bus passes to the operator.

Examples are:

create dataflow MyDataFlow

  // Receive all SampleSchema events from the event bus.
  // No transformation.
  EventBusSource -> stream.one<SampleSchema> {}
  
  // Receive all SampleSchema events with tag id '001' from the event bus.
  // No transformation.
  EventBusSource -> stream.one<SampleSchema> {
    filter : tagId = '001'
  }

  // Receive all SampleSchema events from the event bus.
  // With collector that performs transformation.
  EventBusSource -> stream.two<SampleSchema> {
    collector : {
      class : 'com.mycompany.filters.MyCollector'
    },
  }

The Select operator is configured with an EPL select statement. It applies events from input streams to the select statement and outputs results either continuously or when the final marker arrives.

The Select operator accepts one or more input streams.

The Select operator requires a single output stream.

The Select operator requires the select parameter, all other parameters are optional:


Set the optional iterate flag to false (the default) to have the operator output results continuously. Set the iterate flag to true to indicate that the operator outputs results only when the final marker arrives. If iterate is true then output rate limiting clauses are not supported.

The select parameter is required and provides an EPL select statement within parenthesis. For each input port the statement should list the input stream name or the alias name in the from clause. Only filter-based streams are allowed in the from clause and patterns or named windows are not supported. Also not allowed are the insert into clause, the irstream keyword and subselects.

The Select operator determines the event type of output events based on the select clause. It is not necessary to declare an event type for the output stream.

Examples are:

create dataflow MyDataFlow
  create schema SampleSchema(tagId string, locX double),	// sample type			
  BeaconSource -> instream<SampleSchema> {}  // sample stream
  BeaconSource -> secondstream<SampleSchema> {}  // sample stream
  
  // Simple continuous count of events
  Select(instream) -> outstream {
    select: (select count(*) from instream)
  }
  
  // Demonstrate use of alias
  Select(instream as myalias) -> outstream {
    select: (select count(*) from myalias)
  }
  
  // Output only when the final marker arrives
  Select(instream as myalias) -> outstream {
    select: (select count(*) from myalias),
    iterate: true
  }

  // Same input port for the two sample streams
  Select( (instream, secondstream) as myalias) -> outstream {
    select: (select count(*) from myalias)
  }

  // A join with multiple input streams,
  // joining the last event per stream forming pairs
  Select(instream, secondstream) -> outstream {
    select: (select a.tagId, b.tagId 
        from instream.std:lastevent() as a, secondstream.std:lastevent() as b)
  }
  
  // A join with multiple input streams and using aliases.
  Select(instream as S1, secondstream as S2) -> outstream {
    select: (select a.tagId, b.tagId 
        from S1.std:lastevent() as a, S2.std:lastevent() as b)
  }

This section outlines the steps to declare, instantiate, execute and cancel or complete data flows.

The com.espertech.esper.client.dataflow.EPDataFlowRuntime available via getDataFlowRuntime on EPRuntime manages declared data flows.

Use the instantiate method on EPDataFlowRuntime to instantiate a data flow after it has been declared. Pass the data flow name and optional instantiation options to the method. A data flow can be instantiated any number of times.

A data flow instance is represented by an instance of EPDataFlowInstance. Each instance has a state as well as methods to start, run, join and cancel as well as methods to obtain execution statistics.

Various optional arguments including operator parameters can be passed to instantiate via the EPDataFlowInstantiationOptions object as explained in more detail below.

The following code snippet instantiates the data flow:

EPDataFlowInstance instance =
  epService.getEPRuntime().getDataFlowRuntime().instantiate("HelloWorldDataFlow");

The engine does not track or otherwise retain data flow instances in memory. It is up to your application to retain data flow instances as needed.

Each data flow instance associates to a state. The start state is EPDataFlowState.INSTANTIATED. The end state is either COMPLETED or CANCELLED.

The following table outlines all states:


After your application instantiated a data flow instance it can execute the data flow instance using either the start, run or startCaptive methods.

Use the start method to have the engine allocate a thread for each source operator. Execution is non-blocking. Use the join method to have one or more threads join a data flow instance execution.

Use the run method to have the engine use the current thread to execute the single source operator. Multiple source operators are not allowed when using run.

Use the startCaptive method to have the engine return all Runnable instances and emitters, for the purpose of having complete control over execution. The engine allocates no threads and does not perform any logic for the data flow unless your application employs the Runnable instances and emitters returned by the method.

The next code snippet executes the data flow instance as a blocking call:

instance.run();

By using the run method of EPDataFlowInstance the engine executes the data flow instance using the same thread (blocking execute) and returns when the data flow instance completes. A data flow instance completes when all operators receive final markers.

The hello world data flow simply prints an unformatted Hello World string to console. The BeaconSource operator generates a final marker when it finishes the 1 iteration. The data flow instance thus transitions to complete after the LogSink operator receives the final marker, and the thread invoking the run method returns.

The next code snippet executes the data flow instance as a non-blocking call:

instance.start();

Use the cancel method to cancel execution of a running data flow instance:

instance.cancel();

Use the join method to join execution of a running data flow instance, causing the joining thread to block until the data flow instance either completes or is cancelled:

instance.join();

The EPDataFlowInstantiationOptions object that can be passed to the instantiate method may be used to customize the operator graph, operator parameters and execution of the data flow instance.

Passing runtime parameters to data flow operators is easiest using the addParameterURI method. The first parameter is the data flow operator name and the operator parameter name separated by the slash character. The second parameter is the value object.

For example, in order to pass the file name to the FileSource operator at runtime, use the following code:

EPDataFlowInstantiationOptions options = new EPDataFlowInstantiationOptions();
options.addParameterURI("FileSource/file", filename);
EPDataFlowInstance instance = epService.getEPRuntime().getDataFlowRuntime()
    .instantiate("MyFileReaderDataFlow",options);
instance.run();

The optional operatorProvider member takes an implementation of the EPDataFlowOperatorProvider interface. The engine invokes this provider to obtain operator instances.

The optional parameterProvider member takes an implementation of the EPDataFlowOperatorParameterProvider interface. The engine invokes this provider to obtain operator parameter values. The values override the values provided via parameter URI above.

The optional exceptionHandler member takes an implementation of the EPDataFlowExceptionHandler interface. The engine invokes this provider to when exceptions occur.

The optional dataFlowInstanceId can be assigned any string value for the purpose of identifying the data flow instance.

The optional dataFlowInstanceUserObject can be assigned any object value for the purpose of associating a user object to the data flow instance.

Set the operatorStatistics flag to true to obtain statistics for operator execution.

Set the cpuStatistics flag to true to obtain CPU statistics for operator execution.

Use the startCaptive method on a EPDataFlowInstance data flow instance when your application requires full control over threading. This method returns an EPDataFlowInstanceCaptive instance that contains a list of java.lang.Runnable instances that represent each source operator.

The special Emitter operator can occur in a data flow. This emitter can be used to inject events into the data flow without writing a new operator. Emitter takes a single name parameter that provides the name of the emitter and that is returned in a map of emitters by EPDataFlowInstanceCaptive.

The example EPL below creates a data flow that uses emitter.

create dataflow HelloWorldDataFlow
  create objectarray schema SampleSchema(text string),	// sample type		
	
  Emitter -> helloworld.stream<SampleSchema> { name: 'myemitter' }
  LogSink(helloworld.stream) {}

Your application may obtain the Emitter instance and sends events directly into the output stream. This feature is only supported in relationship with startCaptive since the engine does not allocate any threads or run source operators.

The example code snippet below obtains the emitter instance and send events directly into the data flow instance:

EPDataFlowInstance instance =
      epService.getEPRuntime().getDataFlowRuntime().instantiate("HelloWorldDataFlow", options);
EPDataFlowInstanceCaptive captiveStart = instance.startCaptive();
Emitter emitter = captiveStart.getEmitters().get("myemitter");
emitter.submit(new Object[] {"this is some text"});

When emitting DOM XML events please emit the root element obtained from document.getDocumentElement().

The following example is a rolling top words count implemented as a data flow, over a 30 second time window and providing the top 3 words every 2 seconds:

create dataflow RollingTopWords
  create objectarray schema WordEvent (word string),
  
  Emitter -> wordstream<WordEvent> {name:'a'} {} // Produces word stream
  
  Select(wordstream) -> wordcount { // Sliding time window count per word
    select: (select word, count(*) as wordcount 
          from wordstream.win:time(30) group by word)
  }

  Select(wordcount) -> wordranks { // Rank of words
    select: (select window(*) as rankedWords 
          from wordcount.ext:sort(3, wordcount desc) 
          output snapshot every 2 seconds)
  }
  
  LogSink(wordranks) {}

The next example implements a bargain index computation that separates a mixed trade and quote event stream into a trade and a quote stream, computes a vwap and joins the two streams to compute an index:

create dataflow VWAPSample
  create objectarray schema TradeQuoteType as (type string, ticker string, price double, volume long, askprice double, asksize long),
  
  MyObjectArrayGraphSource -> TradeQuoteStream<TradeQuoteType> {}
  
  Filter(TradeQuoteStream) -> TradeStream {
    filter: type = "trade"
  }
  
  Filter(TradeQuoteStream) -> QuoteStream {
    filter: type = "quote"
  }
  
  Select(TradeStream) -> VwapTrades {
    select: (select ticker, sum(price * volume) / sum(volume) as vwap, 
          min(price) as minprice
          from TradeStream.std:groupwin(ticker).win:length(4) group by ticker)
  }
  
  Select(VwapTrades as T, QuoteStream as Q) -> BargainIndex {
    select: 
      (select case when vwap > askprice then asksize * (Math.exp(vwap - askprice)) else 0.0d end as index
      from T.std:unique(ticker) as t, Q.std:lastevent() as q
      where t.ticker = q.ticker)
  }
  
  LogSink(BargainIndex) {}

The final example is a word count data flow, in which three custom operators tokenize, word count and aggregate. The custom operators in this example are discussed next.

create dataflow WordCount
  MyLineFeedSource -> LineOfTextStream {}
  MyTokenizerCounter(LineOfTextStream) -> SingleLineCountStream {}
  MyWordCountAggregator(SingleLineCountStream) -> WordCountStream {}
  LogSink(WordCountStream) {}

This section discusses how to implement classes that serve as operators in a data flow. The section employs the example data flow as shown earlier.

This example data flow has operators MyLineFeedSource, MyTokenizerCounter and MyWordCountAggregator that are application provided operators:

create dataflow WordCount
  MyLineFeedSource -> LineOfTextStream {}
  MyTokenizerCounter(LineOfTextStream) -> SingleLineCountStream {}
  MyWordCountAggregator(SingleLineCountStream) -> WordCountStream {}
  LogSink(WordCountStream) {}

In order to resolve application operators, add the package or operator class to imports:

// Sample code adds 'package.*' to simply import the package.
epService.getEPAdministrator().getConfiguration()
  .addImport(MyTokenizerCounter.class.getPackage().getName() + ".*");

The implementation class must implement the DataFlowSourceOperator interface.

The implementation for the sample MyLineFeedSource with comments is:

// The OutputTypes annotation can be used to specify the type of events 
// that are output by the operator.
// If provided, it is not necessary to declare output types in the data flow.
// The event representation is object-array.
@OutputTypes(value = {
    @OutputType(name = "line", typeName = "String")
    })

// Provide the DataFlowOpProvideSignal annotation to indicate that 
// the source operator provides a final marker.
@DataFlowOpProvideSignal

public class MyLineFeedSource implements DataFlowSourceOperator {

     // Use the DataFlowContext annotation to indicate the field that receives the emitter.
     // The engine provides the emitter. 
    @DataFlowContext
    private EPDataFlowEmitter dataFlowEmitter;

    // Mark a parameter using the DataFlowOpParameter annotation
    @DataFlowOpParameter
    private String myStringParameter;

    private final Iterator<String> lines;

    public MyLineFeedSource(Iterator<String> lines) {
        this.lines = lines;
    }

    // Invoked by the engine at time of data flow instantiation.
    public DataFlowOpInitializeResult initialize(DataFlowOpInitializateContext context) throws Exception {
        return null;	// can return type information here instead 
    }

    // Invoked by the engine at time of data flow instante execution.
    public void open(DataFlowOpOpenContext openContext) {
      // attach to input
    }

    // Invoked by the engine in a tight loop. 
    // Submits the events which contain lines of text.
    public void next() {
        // read and submit events
        if (lines.hasNext()) {
            dataFlowEmitter.submit(new Object[] {lines.next()});
        }
        else {
            dataFlowEmitter.submitSignal(new EPDataFlowSignalFinalMarker() {});
        }
    }

    // Invoked by the engine at time of cancellation or completion.
    public void close(DataFlowOpCloseContext openContext) {
      // detach from input
    }
}
14.1. API Overview
14.2. The Service Provider Interface
14.3. The Administrative Interface
14.3.1. Creating Statements
14.3.2. Receiving Statement Results
14.3.3. Setting a Subscriber Object
14.3.4. Adding Listeners
14.3.5. Using Iterators
14.3.6. Managing Statements
14.3.7. Runtime Configuration
14.4. The Runtime Interface
14.4.1. Event Sender
14.4.2. Receiving Unmatched Events
14.5. On-Demand Fire-And-Forget Query Execution
14.5.1. On-Demand Query Single Execution
14.5.2. On-Demand Query Prepared Unparameterized Execution
14.5.3. On-Demand Query Prepared Parameterized Execution
14.6. Event and Event Type
14.6.1. Event Type Metadata
14.6.2. Event Object
14.6.3. Query Example
14.6.4. Pattern Example
14.7. Engine Threading and Concurrency
14.7.1. Advanced Threading
14.7.2. Processing Order
14.8. Controlling Time-Keeping
14.8.1. Controlling Time Using Time Span Events
14.8.2. Additional Time-Related APIs
14.9. Time Resolution
14.10. Service Isolation
14.10.1. Overview
14.10.2. Example: Suspending a Statement
14.10.3. Example: Catching up a Statement from Historical Data
14.10.4. Isolation for Insert-Into
14.10.5. Isolation for Named Windows
14.10.6. Runtime Considerations
14.11. Exception Handling
14.12. Condition Handling
14.13. Statement Object Model
14.13.1. Building an Object Model
14.13.2. Building Expressions
14.13.3. Building a Pattern Statement
14.13.4. Building a Select Statement
14.13.5. Building a Create-Variable and On-Set Statement
14.13.6. Building Create-Window, On-Delete and On-Select Statements
14.14. Prepared Statement and Substitution Parameters
14.15. Engine and Statement Metrics Reporting
14.15.1. Engine Metrics
14.15.2. Statement Metrics
14.16. Event Rendering to XML and JSON
14.16.1. JSON Event Rendering Conventions and Options
14.16.2. XML Event Rendering Conventions and Options
14.17. Plug-in Loader
14.18. Interrogating EPL Annotations
14.19. Context Partition Selection
14.19.1. Selectors
14.20. Context Partition Administration
14.21. Test and Assertion Support
14.21.1. EPAssertionUtil Summary
14.21.2. SupportUpdateListener Summary
14.21.3. Usage Example

Esper has the following primary interfaces:

For EPL introductory information please see Section 5.1, “EPL Introduction” and patterns are described at Section 6.1, “Event Pattern Overview”.

The JavaDoc documentation is also a great source for API information.

The EPServiceProvider interface represents an engine instance. Each instance of an Esper engine is completely independent of other engine instances and has its own administrative and runtime interface.

An instance of the Esper engine is obtained via static methods on the EPServiceProviderManager class. The getDefaultProvider method and the getProvider(String providerURI) methods return an instance of the Esper engine. The latter can be used to obtain multiple instances of the engine for different provider URI values. The EPServiceProviderManager determines if the provider URI matches all prior provider URI values and returns the same engine instance for the same provider URI value. If the provider URI has not been seen before, it creates a new engine instance.

The code snipped below gets the default instance Esper engine. Subsequent calls to get the default engine instance return the same instance.

EPServiceProvider epService = EPServiceProviderManager.getDefaultProvider();

This code snippet gets an Esper engine for the provider URI RFIDProcessor1. Subsequent calls to get an engine with the same provider URI return the same instance.

EPServiceProvider epService = EPServiceProviderManager.getProvider("RFIDProcessor1");

Since the getProvider methods return the same cached engine instance for each URI, there is no need to statically cache an engine instance in your application.

An existing Esper engine instance can be reset via the initialize method on the EPServiceProvider instance. This operation stops and removes all statements and resets the engine to the configuration provided when the engine instance for that URI was obtained. If no configuration is provided, an empty (default) configuration applies.

After initialize your application must obtain new administrative and runtime services. Any administrative and runtime services obtained before the initialize are invalid and have undefined behavior.

The next code snippet outlines a typical sequence of use:

// Configure the engine, this is optional
Configuration config = new Configuration();
config.configure("configuration.xml");	// load a configuration from file
config.set....(...);    // make additional configuration settings

// Obtain an engine instance
EPServiceProvider epService = EPServiceProviderManager.getDefaultProvider(config);

// Optionally, use initialize if the same engine instance has been used before to start clean
epService.initialize();

// Optionally, make runtime configuration changes
epService.getEPAdministrator().getConfiguration().add...(...);

// Destroy the engine instance when no longer needed, frees up resources
epService.destroy();

An existing Esper engine instance can be destroyed via the destroy method on the EPServiceProvider instance. This stops and removes all statements as well as frees all resources held by the instance. After a destroy the engine can no longer be used.

The EPServiceStateListener interface may be implemented by your application to receive callbacks when an engine instance is about to be destroyed and after an engine instance has been initialized. Listeners are registered via the addServiceStateListener method. The EPStatementStateListener interface is used to receive callbacks when a new statement gets created and when a statement gets started, stopped or destroyed. Listeners are registered via the addStatementStateListener method.

When destroying an engine instance your application must make sure that threads that are sending events into the engine have completed their work. More generally, the engine should not be currently in use during or after the destroy operation.

As engine instances are completely independent, your application may not send EventBean instances obtained from one engine instance into a second engine instance since the event type space between two engine instances is not shared.

Create event pattern expression and EPL statements via the administrative interface EPAdministrator.

This code snippet gets an Esper engine then creates an event pattern and an EPL statement.

EPServiceProvider epService = EPServiceProviderManager.getDefaultProvider();
EPAdministrator admin = epService.getEPAdministrator();

EPStatement 10secRecurTrigger = admin.createPattern(
  "every timer:at(*, *, *, *, *, */10)");

EPStatement countStmt = admin.createEPL(
  "select count(*) from MarketDataBean.win:time(60 sec)");

Note that event pattern expressions can also occur within EPL statements. This is outlined in more detail in Section 5.4.2, “Pattern-based Event Streams”.

The create methods on EPAdministrator are overloaded and allow an optional statement name to be passed to the engine. A statement name can be useful for retrieving a statement by name from the engine at a later time. The engine assigns a statement name if no statement name is supplied on statement creation.

The createPattern and createEPL methods return EPStatement instances. Statements are automatically started and active when created. A statement can also be stopped and started again via the stop and start methods shown in the code snippet below.

countStmt.stop();
countStmt.start();

The create methods on EPAdministrator also accept a user object. The user object is associated with a statement at time of statement creation and is a single, unnamed field that is stored with every statement. Applications may put arbitrary objects in this field. Use the getUserObject method on EPStatement to obtain the user object of a statement and StatementAwareUpdateListener for listeners.

Your application may create new statements or stop and destroy existing statements using any thread and also within listener or subscriber code. If using POJO events, your application may not create or manage statements in the event object itself while the same event is currently being processed by a statement.

Esper provides three choices for your application to receive statement results. Your application can use all three mechanisms alone or in any combination for each statement. The choices are:

Table 14.1. Choices For Receiving Statement Results

NameMethods on EPStatementDescription
Listener CallbacksaddListener and removeListener

Your application provides implementations of the UpdateListener or the StatementAwareUpdateListener interface to the statement. Listeners receive EventBean instances containing statement results.

The engine continuously indicates results to all listeners as soon they occur, and following output rate limiting clauses if specified.

Subscriber ObjectsetSubscriber

Your application provides a POJO (plain Java object) that exposes methods to receive statement results.

The engine continuously indicates results to the single subscriber as soon they occur, and following output rate limiting clauses if specified.

This is the fastest method to receive statement results, as the engine delivers strongly-typed results directly to your application objects without the need for building an EventBean result set as in the Listener Callback choice.

There can be at most 1 Subscriber Object registered per statement. If you require more than one listener, use the Listener Callback instead (or in addition). The Subscriber Object is bound to the statement with a strongly typed support which ensure direct delivery of new events without type conversion. This optimization is made possible because there can only be 0 or 1 Subscriber Object per statement.

Pull APIsafeIterator and iterator

Your application asks the statement for results and receives a set of events via java.util.Iterator<EventBean>.

This is useful if your application does not need continuous indication of new results in real-time.


Your application may attach one or more listeners, zero or one single subscriber and in addition use the Pull API on the same statement. There are no limitations to the use of iterator, subscriber or listener alone or in combination to receive statement results.

The best delivery performance can generally be achieved by attaching a subscriber and by not attaching listeners. The engine is aware of the listeners and subscriber attached to a statement. The engine uses this information internally to reduce statement overhead. For example, if your statement does not have listeners or a subscriber attached, the engine does not need to continuously generate results for delivery.

If your application attaches both a subscriber and one or more listeners then the subscriber receives the result first before any of the listeners.

If your application attaches more then one listener then the UpdateListener listeners receive results first in the order they were added to the statement, and StatementAwareUpdateListener listeners receive results next in the order they were added to the statement. To change the order of delivery among listeners your application can add and remove listeners at runtime.

If you have configured outbound threading, it means a thread from the outbound thread pool delivers results to the subscriber and listeners instead of the processing or event-sending thread.

If outbound threading is turned on, we recommend turning off the engine setting preserving the order of events delivered to listeners as described in Section 15.4.10.1, “Preserving the order of events delivered to listeners”. If outbound threading is turned on statement execution is not blocked for the configured time in the case a subscriber or listener takes too much time.

A subscriber object is a direct binding of query results to a Java object. The object, a POJO, receives statement results via method invocation. The subscriber class does not need to implement an interface or extend a superclass. Only one subscriber object may be set for a statement.

Subscriber objects have several advantages over listeners. First, they offer a substantial performance benefit: Query results are delivered directly to your method(s) through Java virtual machine method calls, and there is no intermediate representation (EventBean). Second, as subscribers receive strongly-typed parameters, the subscriber code tends to be simpler.

This chapter describes the requirements towards the methods provided by your subscriber class.

The engine can deliver results to your subscriber in two ways:

  1. Each evert in the insert stream results in a method invocation, and each event in the remove stream results in further method invocations. This is termed row-by-row delivery.

  2. A single method invocation that delivers all rows of the insert and remove stream. This is termed multi-row delivery.

Your subscriber class must provide a method by name update to receive insert stream events row-by-row. The number and types of parameters declared by the update method must match the number and types of columns as specified in the select clause, in the same order as in the select clause.

For example, if your statement is:

select orderId, price, count(*) from OrderEvent

Then your subscriber update method looks as follows:

public class MySubscriber {
  ...
  public void update(String orderId, double price, long count) {...}
  ...
}

Each method parameter declared by the update method must be assignable from the respective column type as listed in the select-clause, in the order selected. The assignability rules are:

  • Widening of types follows Java standards. For example, if your select clause selects an integer value, the method parameter for the same column can be typed int, long, float or double (or any equivalent boxed type).

  • Auto-boxing and unboxing follows Java standards. For example, if your select clause selects an java.lang.Integer value, the method parameter for the same column can be typed int. Note that if your select clause column may generate null values, an exception may occur at runtime unboxing the null value.

  • Interfaces and super-classes are honored in the test for assignability. Therefore java.lang.Object can be used to accept any select clause column type

In the case that your subscriber class offers multiple update method footprints, the engine selects the closest-matching footprint by comparing the output types and method parameter types. The engine prefers the update method that is an exact match of types, followed by an update method that requires boxing or unboxing, followed by an update method that requires widening and finally any other allowable update method.

In place of row-by-row delivery, your subscriber can receive all events in the insert and remove stream via a single method invocation. This is applicable when an EPL delivers multiple output rows for a given input event or time advancing, for example when multiple pattern matches occur for the same incoming event, for a join producing multiple output rows or with output rate limiting, for example.

The event delivery follow the scheme as described earlier in Section 14.3.3.1.2, “Row Delivery as Map and Object Array ”. The subscriber class must provide one of the following methods:


Your application can subscribe to updates posted by a statement via the addListener and removeListener methods on EPStatement . Your application must to provide an implementation of the UpdateListener or the StatementAwareUpdateListener interface to the statement:

UpdateListener myListener = new MyUpdateListener();
countStmt.addListener(myListener);

EPL statements and event patterns publish old data and new data to registered UpdateListener listeners. New data published by statements is the events representing the new values of derived data held by the statement. Old data published by statements constists of the events representing the prior values of derived data held by the statement.

Important

UpdateListener listeners receive multiple result rows in one invocation by the engine: the new data and old data parameters to your listener are array parameters. For example, if your application uses one of the batch data windows, or your application creates a pattern that matches multiple times when a single event arrives, then the engine indicates such multiple result rows in one invocation and your new data array carries two or more rows.

A second listener interface is the StatementAwareUpdateListener interface. A StatementAwareUpdateListener is especially useful for registering the same listener object with multiple statements, as the listener receives the statement instance and engine instance in addition to new and old data when the engine indicates new results to a listener.

StatementAwareUpdateListener myListener = new MyStmtAwareUpdateListener();
statement.addListener(myListener);

To indicate results the engine invokes this method on StatementAwareUpdateListener listeners: update(EventBean[] newEvents, EventBean[] oldEvents, EPStatement statement, EPServiceProvider epServiceProvider)

Subscribing to events posted by a statement is following a push model. The engine pushes data to listeners when events are received that cause data to change or patterns to match. Alternatively, you need to know that statements serve up data that your application can obtain via the safeIterator and iterator methods on EPStatement. This is called the pull API and can come in handy if your application is not interested in all new updates, and only needs to perform a frequent or infrequent poll for the latest data.

The safeIterator method on EPStatement returns a concurrency-safe iterator returning current statement results, even while concurrent threads may send events into the engine for processing. The engine employs a read-write lock per context partition and obtains a read lock for iteration. Thus safe iterator guarantees correct results even as events are being processed by other threads and other context partitions. The cost is that the iterator obtains and holds zero, one or multiple context partition locks for that statement that must be released via the close method on the SafeIterator instance.

The iterator method on EPStatement returns a concurrency-unsafe iterator. This iterator is only useful for applications that are single-threaded, or applications that themselves perform coordination between the iterating thread and the threads that send events into the engine for processing. The advantage to this iterator is that it does not hold a lock.

When statements are used with contexts and context partitions, the APIs to identify, filter and select context partitions for statement iteration are described in Section 14.19, “Context Partition Selection”.

The next code snippet shows a short example of use of safe iterators:

EPStatement statement = epAdmin.createEPL("select avg(price) as avgPrice from MyTick");
// .. send events into the engine
// then use the pull API...
SafeIterator<EventBean> safeIter = statement.safeIterator();
try {
  for (;safeIter.hasNext();) {
     // .. process event ..
     EventBean event = safeIter.next();
     System.out.println("avg:" + event.get("avgPrice");
  }
}
finally {
  safeIter.close();	// Note: safe iterators must be closed
}

This is a short example of use of the regular iterator that is not safe for concurrent event processing:

double averagePrice = (Double) eplStatement.iterator().next().get("average");

The safeIterator and iterator methods can be used to pull results out of all statements, including statements that join streams, contain aggregation functions, pattern statements, and statements that contain a where clause, group by clause, having clause or order by clause.

For statements without an order by clause, the iterator method returns events in the order maintained by the data window. For statements that contain an order by clause, the iterator method returns events in the order indicated by the order by clause.

Consider using the on-select clause and a named window if your application requires iterating over a partial result set or requires indexed access for fast iteration; Note that on-select requires that you sent a trigger event, which may contain the key values for indexed access.

Esper places the following restrictions on the pull API and usage of the safeIterator and iterator methods:

  1. In multithreaded applications, use the safeIterator method. Note: make sure your application closes the iterator via the close method when done, otherwise the iterated statement context partitions stay locked and event processing for statement context partitions does not resume.

  2. In multithreaded applications, the iterator method does not hold any locks. The iterator returned by this method does not make any guarantees towards correctness of results and fail-behavior, if your application processes events into the engine instance by multiple threads. Use the safeIterator method for concurrency-safe iteration instead.

  3. Since the safeIterator and iterator methods return events to the application immediately, the iterator does not honor an output rate limiting clause, if present. That is, the iterator returns results as if there is no output-rate clause for the statement in statements without grouping or aggregation. For statements with grouping or aggregation, the iterator in combintion with an output clause returns last output group and aggregation results. Use a separate statement and the insert into clause to control the output rate for iteration, if so required.

  4. When iterating a statement that operates on an unbound stream (no data window declared), please note the following:

    • When iterating a statement that groups and aggregates values from an unbound stream and that specifies output snapshot, the engine retains groups and aggregations for output as iteration results or upon the output snapshot condition .

    • When iterating a statement that groups and aggregates values from an unbound stream and that does not specify output snapshot, the engine only retains the last aggregation values and the iterated result contains only the last updated group.

    • When iterating a statement that operates on an unbound stream and does not group and aggregate, the iterator returns the last event.

Certain configuration changes are available to perform on an engine instance while in operation. Such configuration operations are available via the getConfiguration method on EPAdministrator, which returns a ConfigurationOperations object.

Please consult the JavaDoc of ConfigurationOperations for further information. The section Section 15.6, “Runtime Configuration” provides a summary of available configurations.

In summary, the configuration operations available on a running engine instance are as follows:

  • Add new event types for all event representations, check if an event type exists, update an existing event type, remove an event type, query a list of types and obtain a type by name.

  • Add and remove variables (get and set variable values is done via the runtime API).

  • Add a variant stream.

  • Add a revision event type.

  • Add event types for all event classes in a given Java package, using the simple class name as the event name.

  • Add import for user-defined functions.

  • Add a plug-in aggregation function, plug-in single row function, plug-in event type, plug-in event type resolution URIs.

  • Control metrics reporting.

  • Additional items please see the ConfigurationOperations interface.

For examples of above runtime configuration API functions please consider the Configuration chapter, which applies to both static configuration and runtime configuration as the ConfigurationOperations interface is the same.

The EPRuntime interface is used to send events for processing into an Esper engine, set and get variable values and execute on-demand queries.

The below code snippet shows how to send a Java object event to the engine. Note that the sendEvent method is overloaded. As events can take on different representation classes in Java, the sendEvent takes parameters to reflect the different types of events that can be send into the engine. The Chapter 2, Event Representations section explains the types of events accepted.

EPServiceProvider epService = EPServiceProviderManager.getDefaultProvider();
EPRuntime runtime = epService.getEPRuntime();

// Send an example event containing stock market data
runtime.sendEvent(new MarketDataBean('IBM', 75.0));		

Tip

Events, in theoretical terms, are observations of a state change that occurred in the past. Since one cannot change an event that happened in the past, events are best modelled as immutable objects.

Caution

The engine relies on events that are sent into an engine to not change their state. Typically, applications create a new event object for every new event, to represent that new event. Application should not modify an existing event that was sent into the engine.

Important

Another important method in the runtime interface is the route method. This method is designed for use by UpdateListener and subscriber implementations as well as engine extensions that need to send events into an engine instance to avoid the possibility of a stack overflow due to nested calls to sendEvent and to ensure correct processing of the current and routed event.

The EventSender interface processes event objects that are of a known type. This facility can reduce the overhead of event object reflection and type lookup as an event sender is always associated to a single concrete event type.

Use the method getEventSender(String eventTypeName) to obtain an event sender for processing events of the named type:

EventSender sender = epService.getEPRuntime().getEventSender("MyEvent");
sender.sendEvent(myEvent);

For events backed by a Java class (JavaBean events), the event sender ensures that the event object equals the underlying class, or implements or extends the underlying class for the given event type name.

For events backed by a java.util.Map (Map events), the event sender does not perform any checking other then checking that the event object implements Map.

For events backed by a Object[] (Object-array events), the event sender does not perform any checking other then checking that the event object implements Object[]. The array elements must be in the exact same order of properties as declared and array length must always be at least the number of properties declared.

For events backed by a org.w3c.Node (XML DOM events), the event sender checks that the root element name equals the root element name for the event type.

A second method to obtain an event sender is the method getEventSender(URI[]), which takes an array of URIs. This method is for use with plug-in event representations. The event sender returned by this method processes event objects that are of one of the types of one or more plug-in event representations. Please consult Section 17.8, “Event Type And Event Object” for more information.

As your application may not require streaming results and may not know each query in advance, the on-demand query facility provides for ad-hoc execution of an EPL expression.

On-demand queries are not continuous in nature: The query engine executes the query once and returns all result rows to the application. On-demand query execution is very lightweight as the engine performs no statement creation and the query leaves no traces within the engine.

Esper provides the facility to explicitly index named windows to speed up on-demand and continuous queries. Please consult Section 5.15.13, “Explicitly Indexing Named Windows” for more information.

When named windows are used with contexts and context partitions, the APIs to identify, filter and select context partitions for on-demand queries can be found in Section 14.19, “Context Partition Selection”.

The EPRuntime interface provides three ways to run on-demand queries:

  1. Use the executeQuery method to executes a given on-demand query exactly once, see Section 14.5.1, “On-Demand Query Single Execution”.

  2. Use the prepareQuery method to prepare a given on-demand query such that the same query can be executed multiple times without repeated parsing, see Section 14.5.2, “On-Demand Query Prepared Unparameterized Execution”.

  3. Use the prepareQueryWithParameters method to prepare a given on-demand query that may have substitution parameters such that the same query can be parameterized and executed multiple times without repeated parsing, see Section 14.5.3, “On-Demand Query Prepared Parameterized Execution”

If your application must execute the same EPL on-demand query multiple times with different parameters use prepareQueryWithParameters.

If your application must execute the same EPL on-demand query multiple times without use either prepareQuery or prepareQueryWithParameters and specify no substitution parameters.

By using any of the prepare... methods the engine can compile an EPL query string or object model once and reuse the object and thereby speed up repeated execution.

The following limitations apply:

  • An on-demand EPL expression only evaluates against the named windows that your application creates. On-demand queries may not specify any other streams or application event types.

  • The following clauses are not allowed in on-demand EPL: insert into and output.

  • Views and patterns are not allowed to appear in on-demand queries.

  • On-demand EPL may not perform subqueries.

  • The previous and prior functions may not be used.

Prepared on-demand queries are designed for repeated execution and may perform better then the dynamic single-execution method if running the same query multiple times. For use with parameter placeholders please see Section 14.5.3, “On-Demand Query Prepared Parameterized Execution”.

The next code snippet demonstrates prepared on-demand queries without parameter placeholder:

String query = "select * from MyNamedWindow where orderId = '123'"
EPOnDemandPreparedQuery prepared = epRuntime.prepareQuery(query);
EPOnDemandQueryResult result = prepared.execute();

// ...later on execute once more ...
prepared.execute();	// execute a second time

Substitution parameters are inserted into an on-demand query as a single question mark character '?'. The engine assigns the first substitution parameter an index of 1 and subsequent parameters increment the index by one.

Substitution parameters can be inserted into any EPL construct that takes an expression.

All substitution parameters must be replaced by actual values before an on-demand query with substitution parameters can be executed. Substitution parameters can be replaced with an actual value using the setObject method for each index. Substitution parameters can be set to new values and the query executed more than once.

While the setObject method allows substitution parameters to assume any actual value including application Java objects or enumeration values, the application must provide the correct type of substitution parameter that matches the requirements of the expression the parameter resides in.

The next program listing runs a prepared and parameterized on-demand query against a named window MyNamedWindow:

String query = "select * from MyNamedWindow where orderId = ?";
EPOnDemandPreparedQueryParameterized prepared = epRuntime.prepareQueryWithParameters(query);

// Set the required parameter values before each execution
prepared.setObject(1, "123");
result = epRuntime.executeQuery(prepared);

// ...execute a second time with new parameter values...
prepared.setObject(1, "456");
result = epRuntime.executeQuery(prepared);

This second example uses the in operator and has multiple parameters:

String query = "select * from MyNamedWindow where orderId in (?) and price > ?";
EPOnDemandPreparedQueryParameterized prepared = epRuntime.prepareQueryWithParameters(query);
prepared.setObject(1, new String[] {"123", "456"});
prepared.setObject(2, 1000.0});

An EventBean object represents a row (event) in your continuous query's result set. Each EventBean object has an associated EventType object providing event metadata.

An UpdateListener implementation receives one or more EventBean events with each invocation. Via the iterator method on EPStatement your application can poll or read data out of statements. Statement iterators also return EventBean instances.

Each statement provides the event type of the events it produces, available via the getEventType method on EPStatement.

An EventType object encapulates all the metadata about a certain type of events. As Esper supports an inheritance hierarchy for event types, it also provides information about super-types to an event type.

An EventType object provides the following information:

For each property of an event type, there is an EventPropertyDescriptor object that describes the property. The EventPropertyDescriptor contains flags that indicate whether a property is an indexed (array) or a mapped property and whether access to property values require an integer index value (indexed properties only) or string key value (mapped properties only). The descriptor also contains a fragment flag that indicates whether a property value is available as a fragment.

The term fragment means an event property value that is itself an event, or a property value that can be represented as an event. The getFragmentType on EventType may be used to determine a fragment's event type in advance.

A fragment event type and thereby fragment events allow navigation over a statement's results even if the statement result contains nested events or a graph of events. There is no need to use the Java reflection API to navigate events, since fragments allow the querying of nested event properties or array values, including nested Java classes.

When using the Map or Object-array event representation, any named Map type or Object-array type nested within a Map or Object-array as a simple or array property is also available as a fragment. When using Java objects either directly or within Map or Object-array events, any object that is neither a primitive or boxed built-in type, and that is not an enumeration and does not implement the Map interface is also available as a fragment.

The nested, indexed and mapped property syntax can be combined to a property expression that may query an event property graph. Most of the methods on the EventType interface allow a property expression to be passed.

Your application may use an EventType object to obtain special getter-objects. A getter-object is a fast accessor to a property value of an event of a given type. All getter objects implement the EventPropertyGetter interface. Getter-objects work only for events of the same type or sub-types as the EventType that provides the EventPropertyGetter. The performance section provides additional information and samples on using getter-objects.

Consider a statement that returns the symbol, count of events per symbol and average price per symbol for tick events. Our sample statement may declare a fully-qualified Java class name as the event type: org.sample.StockTickEvent. Assume that this class exists and exposes a symbol property of type String, and a price property of type (Java primitive) double.

select symbol, avg(price) as avgprice, count(*) as mycount 
from org.sample.StockTickEvent 
group by symbol

The next table summarizes the property names and types as posted by the statement above:


A code snippet out of a possible UpdateListener implementation to this statement may look as below:

String symbol = (String) newEvents[0].get("symbol");
Double price= (Double) newEvents[0].get("avgprice");
Long count= (Long) newEvents[0].get("mycount");

The engine supplies the boxed java.lang.Double and java.lang.Long types as property values rather then primitive Java types. This is because aggregated values can return a null value to indicate that no data is available for aggregation. Also, in a select statement that computes expressions, the underlying event objects to EventBean instances are either of type Object[] (object-array) or of type java.util.Map.

Use statement.getEventType().getUnderlyingType() to inspect the underlying type for all events delivered to listeners. Whether the engine delivers Map or Object-array events to listeners can be specified as follows. If the statement provides the @EventRepresentation(array=true) annotation the engine delivers the output events as object array. If the statement provides the @EventRepresentation(array=false) annotation the engine delivers output events as a Map. If neither annotation is provided, the engine delivers the configured default event representation as discussed in Section 15.4.11.1, “Default Event Representation”.

Consider the next statement that specifies a wildcard selecting the same type of event:

select * from org.sample.StockTickEvent where price > 100

The property names and types provided by an EventBean query result row, as posted by the statement above are as follows:


As an alternative to querying individual event properties via the get methods, the getUnderlying method on EventBean returns the underlying object representing the query result. In the sample statement that features a wildcard-select, the underlying event object is of type org.sample.StockTickEvent:

StockTickEvent tick = (StockTickEvent) newEvents[0].getUnderlying();

Composite events are events that aggregate one or more other events. Composite events are typically created by the engine for statements that join two event streams, and for event patterns in which the causal events are retained and reported in a composite event. The example below shows such an event pattern.

// Look for a pattern where BEvent follows AEvent
String pattern = "a=AEvent -> b=BEvent";
EPStatement stmt = epService.getEPAdministrator().createPattern(pattern);
stmt.addListener(testListener);
// Example listener code
public class MyUpdateListener implements UpdateListener {
  public void update(EventBean[] newData, EventBean[] oldData) {
    System.out.println("a event=" + newData[0].get("a"));
    System.out.println("b event=" + newData[0].get("b"));
  }
}

Note that the update method can receive multiple events at once as it accepts an array of EventBean instances. For example, a time batch window may post multiple events to listeners representing a batch of events received during a given time period.

Pattern statements can also produce multiple events delivered to update listeners in one invocation. The pattern statement below, for instance, delivers an event for each A event that was not followed by a B event with the same id property within 60 seconds of the A event. The engine may deliver all matching A events as an array of events in a single invocation of the update method of each listener to the statement:

select * from pattern[
  every a=A -> (timer:interval(60 sec) and not B(id=a.id))]

A code snippet out of a possible UpdateListener implementation to this statement that retrives the events as fragments may look as below:

EventBean a = (EventBean) newEvents[0].getFragment("a");
// ... or using a nested property expression to get a value out of A event...
double value = (Double) newEvent[0].get("a.value");

Some pattern objects return an array of events. An example is the unbound repeat operator. Here is a sample pattern that collects all A events until a B event arrives:

select * from pattern [a=A until b=B]

A possible code to retrieve different fragments or property values:

EventBean[] a = (EventBean[]) newEvents[0].getFragment("a");
// ... or using a nested property expression to get a value out of A event...
double value = (Double) newEvent[0].get("a[0].value");

Esper is designed from the ground up to operate as a component to multi-threaded, highly-concurrent applications that require efficient use of Java VM resources. In addition, multi-threaded execution requires guarantees in predictability of results and deterministic processing. This section discusses these concerns in detail.

In Esper, an engine instance is a unit of separation. Applications can obtain and discard (initialize) one or more engine instances within the same Java VM and can provide the same or different engine configurations to each instance. An engine instance efficiently shares resources between statements. For example, consider two statements that declare the same data window. The engine matches up view declarations provided by each statement and can thus provide a single data window representation shared between the two statements.

Applications can use Esper APIs to concurrently, by multiple threads of execution, perform such functions as creating and managing statements, or sending events into an engine instance for processing. Applications can use application-managed threads or thread pools or any set of same or different threads of execution with any of the public Esper APIs. There are no restrictions towards threading other then those noted in specific sections of this document.

Esper does not prescribe a specific threading model. Applications using Esper retain full control over threading, allowing an engine to be easily embedded and used as a component or library in your favorite Java container or process.

In the default configuration it is up to the application code to use multiple threads for processing events by the engine, if so desired. All event processing takes places within your application thread call stack. The exception is timer-based processing if your engine instance relies on the internal timer (default). If your application relies on external timer events instead of the internal timer then there need not be any Esper-managed internal threads.

The fact that event processing can take place within your application thread's call stack makes developing applications with Esper easier: Any common Java integrated development environment (IDE) can host an Esper engine instance. This allows developers to easily set up test cases, debug through listener code and inspect input or output events, or trace their call stack.

In the default configuration, each engine instance maintains a single timer thread (internal timer) providing for time or schedule-based processing within the engine. The default resolution at which the internal timer operates is 100 milliseconds. The internal timer thread can be disabled and applications can instead send external time events to an engine instance to perform timer or scheduled processing at the resolution required by an application.

Each engine instance performs minimal locking to enable high levels of concurrency. An engine instance locks on a context partition level to protect context partition resources. For stateless EPL select-statements the engine does not use a context-partition lock and operates lock-free for the context partition. For stateful statements, the maximum (theoretical) degree of parallelism is 2^31-1 (2,147,483,647) parallel threads working to process a single EPL statement under a hash segmented context.

You may turn off context partition locking engine-wide (also read the caution notice) as described in Section 15.4.23.3, “Disable Locking”. You may disable context partition locking for a given statement by providing the @NoLock annotation as part of your EPL. Note, we provide the @NoLock annotation for the purpose of identifying locking overhead, or when your application is single-threaded, or when using an external mechanism for concurrency control or for example with virtual data windows or plug-in data windows to allow customizing concurrency for a given statement or named window. Using this annotation may have unpredictable results unless your application is taking concurrency under consideration.

For an engine instance to produce predictable results from the viewpoint of listeners to statements, an engine instance by default ensures that it dispatches statement result events to listeners in the order in which a statement produced result events. Applications that require the highest possible concurrency and do not require predictable order of delivery of events to listeners, this feature can be turned off via configuration, see Section 15.4.10.1, “Preserving the order of events delivered to listeners”. For example, assume thread T1 processes an event applied to statement S producing output event O1. Assume thread T2 processes another event applied to statement S and produces output event O2. The engine employs a configurable latch system to ensure that listeners to statement S receive and may complete processing of O1 before receiving O2. When using outbound threading (advanced threading options) or changing the configuration this guarantee is weakened or removed.

In multithreaded environments, when one or more statements make result events available via the insert into clause to further statements, the engine preserves the order of events inserted into the generated insert-into stream, allowing statements that consume other statement's events to behave deterministic. This feature can also be turned off via configuration, see , see Section 15.4.10.2, “Preserving the order of events for insert-into streams”. For example, assume thread T1 processes an event applied to statement S and thread T2 processes another event applied to statement S. Assume statement S inserts into into stream ST. T1 produces an output event O1 for processing by consumers of ST1 and T2 produces an output event O2 for processing by consumers of ST. The engine employs a configurable latch system such that O1 is processed before O2 by consumers of ST. When using route execution threading (advanced threading options) or changing the configuration this guarantee is weakened or removed.

We generally recommended that listener implementations block minimally or do not block at all. By implementing listener code as non-blocking code execution threads can often achieve higher levels of concurrency.

We recommended that, when using a single listener or subscriber instance to receive output from multiple statements, that the listener or subscriber code is multithread-safe. If your application has shared state between listener or subscriber instances then such shared state should be thread-safe.

In the default configuration the same application thread that invokes any of the sendEvent methods will process the event fully and also deliver output events to listeners and subscribers. By default the single internal timer thread based on system time performs time-based processing and delivery of time-based results.

This default configuration reduces the processing overhead associated with thread context switching, is lightweight and fast and works well in many environments such as J2EE, server or client. Latency and throughput requirements are largely use case dependant, and Esper provides engine-level facilities for controlling concurrency that are described next.

Inbound Threading queues all incoming events: A pool of engine-managed threads performs the event processing. The application thread that sends an event via any of the sendEvent methods returns without blocking.

Outbound Threading queues events for delivery to listeners and subscribers, such that slow or blocking listeners or subscribers do not block event processing.

Timer Execution Threading means time-based event processing is performed by a pool of engine-managed threads. With this option the internal timer thread (or external timer event) serves only as a metronome, providing units-of-work to the engine-managed threads in the timer execution pool, pushing threading to the level of each statement for time-based execution.

Route Execution Threading means that the thread sending in an event via any of the sendEvent methods (or the inbound threading pooled thread if inbound threading is enabled) only identifies and pre-processes an event, and a pool of engine-managed threads handles the actual processing of the event for each statement, pushing threading to the level of each statement for event-arrival-based execution.

The engine starts engine-managed threads as daemon threads when the engine instance is first obtained. The engine stops engine-managed threads when the engine instance is destroyed via the destroy method. When the engine is initialized via the initialize method the existing engine-managed threads are stopped and new threads are created. When shutting down your application, use the destroy method to stop engine-managed threads.

Note that the options discussed herein may introduce additional processing overhead into your system, as each option involves work queue management and thread context switching.

If your use cases require ordered processing of events or do not tolerate disorder, the threading options described herein may not be the right choice. For enforcing a processing order within a given criteria, your application must enforce such processing order using Java or .NET code. Esper will not enforce order of processing if you enable inbound or route threading. Your application code could, for example, utilize a thread per group of criteria keys, a latch per criteria key, or a queue per criteria key, or use Java's completion service, all depending on your ordering requirements.

If your use cases require loss-less processing of events, wherein the threading options mean that events are held in an in-memory queue, the threading options described herein may not be the right choice.

Care should be taken to consider arrival rates and queue depth. Threading options utilize unbound queues or capacity-bound queues with blocking-put, depending on your configuration, and may therefore introduce an overload or blocking situation to your application. You may use the service provider interface as outlined below to manage queue sizes, if required, and to help tune the engine to your application needs. Consider throttling down the event send rate when the API (see below) indicates that events are getting queued.

All threading options are on the level of an engine. If you require different threading behavior for certain statements then consider using multiple engine instances, consider using the route method or consider using application threads instead.

Please consult Section 15.4.10, “Engine Settings related to Concurrency and Threading” for instructions on how to configure threading options. Threading options take effect at engine initialization time.

There are two modes for an engine to keep track of time: The internal timer based on JVM system time (the default), and externally-controlled time giving your application full control over the concept of time within an engine or isolated service.

An isolated service is an execution environment separate from the main engine runtime, allowing full control over the concept of time for a group of statements, as further described in Section 14.10, “Service Isolation”.

By default the internal timer provides time and evaluates schedules. External clocking can be used to supply time ticks to the engine instead. The latter is useful for testing time-based event sequences or for synchronizing the engine with an external time source.

The internal timer relies on the java.util.concurrent.ScheduledThreadPoolExecutor class for time tick events. The next section describes timer resolution for the internal timer, by default set to 100 milliseconds but is configurable via the threading options. When using externally-controlled time the timer resolution is in your control.

To disable the internal timer and use externally-provided time instead, there are two options. The first option is to use the configuration API at engine initialization time. The second option toggles on and off the internal timer at runtime, via special timer control events that are sent into the engine like any other event.

If using a timer execution thread pool as discussed above, the internal timer or external time event provide the schedule evaluation however do not actually perform the time-based processing. The time-based processing is performed by the threads in the timer execution thread pool.

This code snippet shows the use of the configuration API to disable the internal timer and thereby turn on externally-provided time (see the Configuration section for configuration via XML file):

Configuration config = new Configuration();
config.getEngineDefaults().getThreading().setInternalTimerEnabled(false);
EPServiceProvider epService = EPServiceProviderManager.getDefaultProvider(config);

After disabling the internal timer, it is wise to set a defined time so that any statements created thereafter start relative to the time defined. Use the CurrentTimeEvent class to indicate current time to the engine and to move time forward for the engine (a.k.a application-time model).

This code snippet obtains the current time and sends a timer event in:

long timeInMillis = System.currentTimeMillis();
CurrentTimeEvent timeEvent = new CurrentTimeEvent(timeInMillis);
epService.getEPRuntime().sendEvent(timeEvent);

Alternatively, you can use special timer control events to enable or disable the internal timer. Use the TimerControlEvent class to control timer operation at runtime.

The next code snippet demonstrates toggling to external timer at runtime, by sending in a TimerControlEvent event:

EPServiceProvider epService = EPServiceProviderManager.getDefaultProvider();
EPRuntime runtime = epService.getEPRuntime();
// switch to external clocking
runtime.sendEvent(new TimerControlEvent(TimerControlEvent.ClockType.CLOCK_EXTERNAL));

Your application sends a CurrentTimeEvent event when it desires to move the time forward. All aspects of Esper engine time related to EPL statements and patterns are driven by the time provided by the CurrentTimeEvent that your application sends in.

The next example sequence of instructions sets time to zero, then creates a statement, then moves time forward to 1 seconds later and then 6 seconds later:

// Set start time at zero.
runtime.sendEvent(new CurrentTimeEvent(0));

// create a statement here
epAdministrator.createEPL("select * from MyEvent output every 5 seconds");

// move time forward 1 second
runtime.sendEvent(new CurrentTimeEvent(1000));

// move time forward 5 seconds
runtime.sendEvent(new CurrentTimeEvent(6000));

When sending external timer events, your application should make sure that long-type time values are ascending. That is, each long-type value should be either the same value or a larger value then the prior value provided by a CurrentTimeEvent. The engine outputs a warning if time events move back in time.

Your application may use the getNextScheduledTime method in EPRuntime to determine the earliest time a schedule for any statement requires evaluation.

The following code snippet sets the current time, creates a statement and prints the next scheduled time which is 1 minute later then the current time:

// Set start time to the current time.
runtime.sendEvent(new CurrentTimeEvent(System.currentTimeMillis()));

// Create a statement.
epService.getEPAdministrator().createEPL("select * from pattern[timer:interval(1 minute)]");

// Print next schedule time
System.out.println("Next schedule at " + new Date(runtime.getNextScheduledTime());

With CurrentTimeEvent, as described above, your application can advance engine time to a given point in time. In addition, the getNextScheduledTime method in EPRuntime returns the next scheduled time according to started statements. You would typically use CurrentTimeEvent to advance time at a relatively high resolution.

To advance time for a span of time without sending individual CurrentTimeEvent events to the engine, the API provides the class CurrentTimeSpanEvent. You may use CurrentTimeSpanEvent with or without a resolution.

If your application only provides the target end time of time span to CurrentTimeSpanEvent and no resolution, the engine advances time up to the target time by stepping through all relevant times according to started statements.

If your application provides the target end time of time span and in addition a long-typed resolution, the engine advances time up to the target time by incrementing time according to the resolution (regardless of next scheduled time according to started statements).

Consider the following example:

// Set start time to Jan.1, 2010, 00:00 am for this example
SimpleDateFormat format = new SimpleDateFormat("yyyy MM dd HH:mm:ss SSS");
Date startTime = format.parse("2010 01 01 00:00:00 000");
runtime.sendEvent(new CurrentTimeEvent(startTime.getTime()));

// Create a statement.
EPStatement stmt = epService.getEPAdministrator().createEPL("select current_timestamp() as ct " +
  "from pattern[every timer:interval(1 minute)]");
stmt.addListener(...);	// add a listener

// Advance time to 10 minutes after start time
runtime.sendEvent(new CurrentTimeSpanEvent(startTime.getTime() + 10*60*1000));

The above example advances time to 10 minutes after the time set using CurrentTimeSpanEvent. As the example does not pass a resolution, the engine advances time according to statement schedules. Upon sending the CurrentTimeSpanEvent the listener sees 10 invocations for minute 1 to minute 10.

To advance time according to a given resolution, you may provide the resolution as shown below:

// Advance time to 10 minutes after start time at 100 msec resolution
runtime.sendEvent(new CurrentTimeSpanEvent(startTime.getTime() + 10*60*1000, 100));

The minimum resolution that all data windows, patterns and output rate limiting operate at is the millisecond. Parameters to time window views, pattern operators or the output clause that are less then 1 millisecond are not allowed. As stated earlier, the default frequency at which the internal timer operates is 100 milliseconds (configurable).

The internal timer thread, by default, uses the call System.currentTimeMillis() to obtain system time. Please see the JIRA issue ESPER-191 Support nano/microsecond resolution for more information on Java system time-call performance, accuracy and drift.

The internal timer thread can be configured to use nano-second time as returned by System.nanoTime(). If configured for nano-second time, the engine computes an offset of the nano-second ticks to wall clock time upon startup to present back an accurate millisecond wall clock time. Please see section Section 15.4.18, “Engine Settings related to Time Source” to configure the internal timer thread to use System.nanoTime().

The internal timer is based on java.util.concurrent.ScheduledThreadPoolExecutor (java.util.Timer does not support high accuracy VM time).

Your application can achieve a higher tick rate then 1 tick per millisecond by sending external timer events that carry a long-value which is not based on milliseconds since January 1, 1970, 00:00:00 GMT. In this case, your time interval parameters need to take consideration of the changed use of engine time.

Thus, if your external timer events send long values that represents microseconds (1E-6 sec), then your time window interval must be 1000-times larger, i.e. "win:time(1000)" becomes a 1-second time window.

And therefore, if your external timer events send long values that represents nanoseconds (1E-9 sec), then your time window interval must be 1000000-times larger, i.e. "win:time(1000000)" becomes a 1-second time window.

An isolated service allows an application to control event visibility and the concept of time as desired on a statement level: Events sent into an isolated service are visible only to those statements that currently reside in the isolated service and are not visible to statements outside of that isolated service. Within an isolated service an application can control time independently, start time at a point in time and advance time at the resolution and pace suitable for the statements added to that isolated service.

As discussed before, a single Java Virtual Machine may hold multiple Esper engine instances unique by engine URI. Within an Esper engine instance the default execution environment for statements is the EPRuntime engine runtime, which coordinates all statement's reaction to incoming events and to time passing (via internal or external timer).

Subordinate to an Esper engine instance, your application can additionally allocate multiple isolated services (or execution environments), uniquely identified by a name and represented by the EPServiceProviderIsolated interface. In the isolated service, time passes only when you application sends timer events to the EPRuntimeIsolated instance. Only events explicitly sent to the isolated service are visible to statements added.

Your application can create new statements that start in an isolated service. You can also move existing statements back and forth between the engine and an isolated service.

Isolation does not apply to variables: Variables are global in nature. Also, as named windows are globally visibly data windows, consumers to named windows see changes in named windows even though a consumer or the named window (through the create statement) may be in an isolated service.

An isolated service allows an application to:

While a statement resides in an isolated runtime it receives only those events explicitly sent to the isolated runtime, and performs time-based processing based on the timer events provided to that isolated runtime.

Use the getEPServiceIsolated method on EPServiceProvider passing a name to obtain an isolated runtime:

EPServiceProviderIsolated isolatedService = epServiceManager.getEPServiceIsolated("name");

Set the start time for your isolated runtime via the CurrentTimeEvent timer event:

// In this example start the time at the system time
long startInMillis = System.currentTimeMillis();	
isolatedService.getEPRuntime().sendEvent(new CurrentTimeEvent(startInMillis));

Use the addStatement method on EPAdministratorIsolated to move an existing statement out of the engine runtime into the isolated runtime:

// look up the existing statement
EPStatement stmt = epServiceManager.getEPAdministrator().getStatement("MyStmt");

// move it to an isolated service
isolatedService.getEPAdministrator().addStatement(stmt);

To remove the statement from isolation and return the statement back to the engine runtime, use the removeStatement method on EPAdministratorIsolated:

isolatedService.getEPAdministrator().removeStatement(stmt);

To create a new statement in the isolated service, use the createEPL method on EPAdministratorIsolated:

isolatedService.getEPAdministrator().createEPL(
  "@Name('MyStmt') select * from Event", null, null); 
// the example is passing the statement name in an annotation and no user object

The destroy method on EPServiceProviderIsolated moves all currently-isolated statements for that isolated service provider back to engine runtime.

When moving a statement between engine runtime and isolated service or back, the algorithm ensures that events are aged according to the time that passed and time schedules stay intact.

To use isolated services, your configuration must have view sharing disabled as described in Section 15.4.12.1, “Sharing View Resources between Statements”.

This example creates a statement in the isolated service, replays some events and advances time, then merges back the statement to the engine to let it participate in incoming events and engine time processing.

First, allocate an isolated service and explicitly set it to a start time. Assuming that myStartTime is a long millisecond time value that marks the beginning of the data to replay, the sequence is as follows:

EPServiceProviderIsolated isolatedService = epServiceManager.getEPServiceIsolated("suspendedStmts");
isolatedService.getEPRuntime().sendEvent(new CurrentTimeEvent(myStartTime));

Next, create the statement. The sample statement is a pattern statement looking for temperature events following each other within 60 seconds:

EPStatement stmt = epAdmin.createEPL(
  "select * from pattern[every a=TemperatureEvent -> b=TemperatureEvent where timer:within(60)]");

For each historical event to be played, advance time and send an event. This code snippet assumes that currentTime is a time greater then myStartTime and reflects the time that the historical event should be processed at. It also assumes historyEvent is the historical event object.

isolatedService.getEPRuntime().sendEvent(new CurrentTimeEvent(currentTime));
isolatedService.getEPRuntime().send(historyEvent);
// repeat the above advancing time until no more events

Finally, when done replaying events, merge the statement back with the engine:

isolatedService.getEPAdministrator().removeStatement(stmt);

You may register one or more exception handlers for the engine to invoke in the case it encounters an exception processing a continuously-executing statement. By default and without exception handlers the engine cancels execution of the current EPL statement that encountered the exception, logs the exception and continues to the next statement, if any. The configuration is described in Section 15.4.24, “Engine Settings related to Exception Handling”.

If your application registers exception handlers as part of engine configuration, the engine invokes the exception handlers in the order they are registered passing relevant exception information such as EPL statement name, expression and the exception itself.

Exception handlers receive any EPL statement unchecked exception such as internal exceptions or exceptions thrown by plug-in aggregation functions or plug-in views. The engine does not provide to exception handlers any exceptions thrown by static method invocations for function calls, method invocations in joins, methods on event classes and listeners or subscriber exceptions.

An exception handler can itself throw a runtime exception to cancel execution of the current event against any further statements.

For on-demand queries the API indicates any exception directly back to the caller without the exception handlers being invoked, as exception handlers apply to continuous queries only. The same applies to any API calls other then sendEvent and the EventSender methods.

As the configuration section describes, your application registers one or more classes that implement the ExceptionHandlerFactory interface in the engine configuration. Upon engine initialization the engine obtains a factory instance from the class name that then provides the exception handler instance. The exception handler class must implement the ExceptionHandler interface.

You may register one or more condition handlers for the engine to invoke in the case it encounters certain conditions, as outlined below, when executing a statement. By default and without condition handlers the engine logs the condition at informational level and continues processing. The configuration is described in Section 15.4.25, “Engine Settings related to Condition Handling”.

If your application registers condition handlers as part of engine configuration, the engine invokes the condition handlers in the order they are registered passing relevant condition information such as EPL statement name, expression and the condition information itself.

Currently the only conditions indicated by this facility are raised by the pattern followed-by operator, see Section 6.5.8.1, “Limiting Sub-Expression Count” and see Section 6.5.8.2, “Limiting Engine-wide Sub-Expression Count”.

A condition handler may not itself throw a runtime exception or return any value.

As the configuration section describes, your application registers one or more classes that implement the ConditionHandlerFactory interface in the engine configuration. Upon engine initialization the engine obtains a factory instance from the class name that then provides the condition handler instance. The condition handler class must implement the ConditionHandler interface.

The statement object model is a set of classes that provide an object-oriented representation of an EPL or pattern statement. The object model classes are found in package com.espertech.esper.client.soda. An instance of EPStatementObjectModel represents a statement's object model.

The statement object model classes are a full and complete specification of a statement. All EPL and pattern constructs including expressions and sub-queries are available via the statement object model.

In conjunction with the administrative API, the statement object model provides the means to build, change or interrogate statements beyond the EPL or pattern syntax string representation. The object graph of the statement object model is fully navigable for easy querying by code, and is also serializable allowing applications to persist or transport statements in object form, when required.

The statement object model supports full round-trip from object model to EPL statement string and back to object model: A statement object model can be rendered into an EPL string representation via the toEPL method on EPStatementObjectModel. Further, the administrative API allows to compile a statement string into an object model representation via the compileEPL method on EPAdministrator.

The statement object model is fully mutable. Mutating a any list such as returned by getChildren(), for example, is acceptable and supported.

The create method on EPAdministrator creates and starts a statement as represented by an object model. In order to obtain an object model from an existing statement, obtain the statement expression text of the statement via the getText method on EPStatement and use the compileEPL method to obtain the object model.

The following limitations apply:

  • Statement object model classes are not safe for sharing between threads other then for read access.

  • Between versions of Esper, the serialized form of the object model is subject to change. Esper makes no guarantees that the serialized object model of one version will be fully compatible with the serialized object model generated by another version of Esper. Please consider this issue when storing Esper object models in persistent store.

A EPStatementObjectModel consists of an object graph representing all possible clauses that can be part of an EPL statement.

Among all clauses, the SelectClause and FromClause objects are required clauses that must be present, in order to define what to select and where to select from.


Part of the statement object model package are convenient builder classes that make it easy to build a new object model or change an existing object model. The SelectClause and FromClause are such builder classes and provide convenient create methods.

Within the from-clause we have a choice of different streams to select on. The FilterStream class represents a stream that is filled by events of a certain type and that pass an optional filter expression.

We can use the classes introduced above to create a simple statement object model:

EPStatementObjectModel model = new EPStatementObjectModel();
model.setSelectClause(SelectClause.createWildcard());
model.setFromClause(FromClause.create(FilterStream.create("com.chipmaker.ReadyEvent")));

The model as above is equivalent to the EPL :

select * from com.chipmaker.ReadyEvent

Last, the code snippet below creates a statement from the object model:

EPStatement stmt = epService.getEPAdministrator().create(model);

Notes on usage:

  • Variable names can simply be treated as property names.

  • When selecting from named windows, the name of the named window is the event type name for use in FilterStream instances or patterns.

  • To compile an arbitrary sub-expression text into an Expression object representation, simply add the expression text to a where clause, compile the EPL string into an object model via the compileEPL on EPAdministrator, and obtain the compiled where from the EPStatementObjectModel via the getWhereClause method.

This sample statement creates a named window:

create window OrdersTimeWindow.win:time(30 sec) as select symbol as sym, volume as vol, price from OrderEvent

The is the code that builds the create-window statement as above:

EPStatementObjectModel model = new EPStatementObjectModel();
model.setCreateWindow(CreateWindowClause.create("OrdersTimeWindow").addView("win", "time", 30));
model.setSelectClause(SelectClause.create()
		.addWithName("symbol", "sym")
		.addWithName("volume", "vol")
		.add("price"));
model.setFromClause(FromClause.create(FilterStream.create("OrderEvent)));

A second statement deletes from the named window:

on NewOrderEvent as myNewOrders
delete from AllOrdersNamedWindow as myNamedWindow
where myNamedWindow.symbol = myNewOrders.symbol

The object model is built by:

EPStatementObjectModel model = new EPStatementObjectModel();
model.setOnExpr(OnClause.createOnDelete("AllOrdersNamedWindow", "myNamedWindow"));
model.setFromClause(FromClause.create(FilterStream.create("NewOrderEvent", "myNewOrders")));
model.setWhereClause(Expressions.eqProperty("myNamedWindow.symbol", "myNewOrders.symbol"));
EPStatement stmtOnDelete = epService.getEPAdministrator().create(model);

A third statement selects from the named window using the non-continuous on-demand selection via on-select:

on QueryEvent(volume>0) as query
select count(*) from OrdersNamedWindow as win
where win.symbol = query.symbol

The on-select statement is built from scratch via the object model as follows:

EPStatementObjectModel model = new EPStatementObjectModel();
model.setOnExpr(OnClause.createOnSelect("OrdersNamedWindow", "win"));
model.setWhereClause(Expressions.eqProperty("win.symbol", "query.symbol"));
model.setFromClause(FromClause.create(FilterStream.create("QueryEvent", "query", 
  Expressions.gt("volume", 0))));
model.setSelectClause(SelectClause.create().add(Expressions.countStar()));
EPStatement stmtOnSelect = epService.getEPAdministrator().create(model);

The prepare method that is part of the administrative API pre-compiles an EPL statement and stores the precompiled statement in an EPPreparedStatement object. This object can then be used to efficiently start the parameterized statement multiple times.

Substitution parameters are inserted into an EPL statement as a single question mark character '?'. The engine assigns the first substitution parameter an index of 1 and subsequent parameters increment the index by one.

Substitution parameters can be inserted into any EPL construct that takes an expression. They are therefore valid in any clauses such as the select-clause, from-clause filters, where-clause, group-by-clause, having-clause or order-by-clause, including view parameters and pattern observers and guards. Substitution parameters cannot be used where a numeric constant is required rather then an expression and in SQL statements.

All substitution parameters must be replaced by actual values before a statement with substitution parameters can be started. Substitution parameters can be replaced with an actual value using the setObject method for each index. Substitution parameters can be set to new values and new statements can be created from the same EPPreparedStatement object more then once.

While the setObject method allows substitution parameters to assume any actual value including application Java objects or enumeration values, the application must provide the correct type of substitution parameter that matches the requirements of the expression the parameter resides in.

In the following example of setting parameters on a prepared statement and starting the prepared statement, epService represents an engine instance:

String stmt = "select * from com.chipmaker.ReadyEvent(line=?)";
EPPreparedStatement prepared = epService.getEPAdministrator().prepareEPL(stmt);
prepared.setObject(1, 8);
EPStatement statement = epService.getEPAdministrator().create(prepared);

The engine can report key processing metrics through the JMX platform mbean server by setting a single configuration flag described in Section 15.4.19, “Engine Settings related to JMX Metrics”. For additional detailed reporting and metrics events, please read on.

Metrics reporting is a feature that allows an application to receive ongoing reports about key engine-level and statement-level metrics. Examples are the number of incoming events, the CPU time and wall time taken by statement executions or the number of output events per statement.

Metrics reporting is, by default, disabled. To enable reporting, please follow the steps as outlined in Section 15.4.20, “Engine Settings related to Metrics Reporting”. Metrics reporting must be enabled at engine initialization time. Reporting intervals can be controlled at runtime via the ConfigurationOperations interface available from the administrative API.

Your application can receive metrics at configurable intervals via EPL statement. A metric datapoint is simply a well-defined event. The events are EngineMetric and StatementMetric and the Java class representing the events can be found in the client API in package com.espertech.esper.client.metric.

Since metric events are processed by the engine the same as application events, your EPL may use any construct on such events. For example, your application may select, filter, aggregate properties, sort or insert into a stream or named window all metric events the same as application events.

This example statement selects all engine metric events:

select * from com.espertech.esper.client.metric.EngineMetric

The next statement selects all statement metric events:

select * from com.espertech.esper.client.metric.StatementMetric

Make sure to have metrics reporting enabled since only then do listeners or subscribers to a statement such as above receive metric events.

The engine provides metric events after the configured interval of time has passed. By default, only started statements that have activity within an interval (in the form of event or timer processing) are reported upon.

The default configuration performs the publishing of metric events in an Esper daemon thread under the control of the engine instance. Metrics reporting honors externally-supplied time, if using external timer events.

Via runtime configuration options provided by ConfigurationOperations, your application may enable and disable metrics reporting globally, provided that metrics reporting was enabled at initialization time. Your application may also enable and disable metrics reporting for individual statements by statement name.

Statement groups is a configuration feature that allows to assign reporting intervals to statements. Statement groups are described further in the Section 15.4.20, “Engine Settings related to Metrics Reporting” section. Statement groups cannot be added or removed at runtime.

The following limitations apply:

  • If your Java VM version does not report current thread CPU time (most JVM do), then CPU time is reported as zero (use ManagementFactory.getThreadMXBean().isCurrentThreadCpuTimeSupported() to determine if your JVM supports this feature).

    Note: In some JVM the accuracy of CPU time returned is very low (in the order of 10 milliseconds off) which can impact the usefulness of CPU metrics returned. Consider measuring CPU time in your application thread after sending a number of events in the same thread, external to the engine as an alternative.

  • Your Java VM may not provide high resolution time via System.nanoTime. In such case wall time may be inaccurate and inprecise.

  • CPU time and wall time have nanosecond precision but not necessarily nanosecond accuracy, please check with your Java VM provider.

  • There is a performance cost to collecting and reporting metrics.

  • Not all statements may report metrics: The engine performs certain runtime optimizations sharing resources between similar statements, thereby not reporting on certain statements unless resource sharing is disabled through configuration.

Your application may use the built-in XML and JSON formatters to render output events into a readable textual format, such as for integration or debugging purposes. This section introduces the utility classes in the client util package for rendering events to strings. Further API information can be found in the JavaDocs.

The EventRenderer interface accessible from the runtime interface via the getEventRenderer method provides access to JSON and XML rendering. For repeated rendering of events of the same event type or subtypes, it is recommended to obtain a JSONEventRenderer or XMLEventRenderer instance and use the render method provided by the interface. This allows the renderer implementations to cache event type metadata for fast rendering.

In this example we show how one may obtain a renderer for repeated rendering of events of the same type, assuming that statement is an instance of EPStatement:

JSONEventRenderer jsonRenderer = epService.getEPRuntime().
    getEventRenderer().getJSONRenderer(statement.getEventType());

Assuming that event is an instance of EventBean, this code snippet renders an event into the JSON format:

String jsonEventText = jsonRenderer.render("MyEvent", event);

The XML renderer works the same:

XMLEventRenderer xmlRenderer = epService.getEPRuntime().
    getEventRenderer().getXMLRenderer(statement.getEventType());

...and...

String xmlEventText = xmlRenderer.render("MyEvent", event);

If the event type is not known in advance or if you application does not want to obtain a renderer instance per event type for fast rendering, your application can use one of the following methods to render an event to a XML or JSON textual format:

String json = epService.getEPRuntime().getEventRenderer().renderJSON(event);
String xml = epService.getEPRuntime().getEventRenderer().renderXML(event);

Use the JSONRenderingOptions or XMLRenderingOptions classes to control how events are rendered. To render specific event properties using a custom event property renderer, specify an EventPropertyRenderer as part of the options that renders event property values to strings. Please see the JavaDoc documentation for more information.

A plug-in loader is for general use with input adapters, output adapters or EPL code deployment or any other task that can benefits from being part of an Esper configuration file and that follows engine lifecycle.

A plug-in loader implements the com.espertech.esper.plugin.PluginLoader interface and can be listed in the configuration.

Each configured plug-in loader follows the engine instance lifecycle: When an engine instance initializes, it instantiates each PluginLoader implementation class listed in the configuration. The engine then invokes the lifecycle methods of the PluginLoader implementation class before and after the engine is fully initialized and before an engine instance is destroyed.

Declare a plug-in loader in your configuration XML as follows:

...
  <plugin-loader name="MyLoader" class-name="org.mypackage.MyLoader">
    <init-arg name="property1" value="val1"/>
  </plugin-loader>
...

Alternatively, add the plug-in loader via the configuration API:

Configuration config = new Configuration();
Properties props = new Properties();
props.put("property1", "value1");
config.addPluginLoader("MyLoader", "org.mypackage.MyLoader", props);

Implement the init method of your PluginLoader implementation to receive initialization parameters. The engine invokes this method before the engine is fully initialized, therefore your implementation should not yet rely on the engine instance within the method body:

public class MyPluginLoader implements PluginLoader {
  public void init(String loaderName, Properties properties, EPServiceProviderSPI epService) {
     // save the configuration for later, perform checking
  }
  ...

The engine calls the postInitialize method once the engine completed initialization and to indicate the engine is ready for traffic.

public void postInitialize() {
  // Start the actual interaction with external feeds or the engine here
}
...

The engine calls the destroy method once the engine is destroyed or initialized for a second time.

public void destroy() {
  // Destroy resources allocated as the engine instance is being destroyed
}

As discussed in Section 5.2.7, “Annotation” an EPL annotation is an addition made to information in an EPL statement. The API and examples to interrogate annotations are described here.

You may use the getAnnotations method of EPStatement to obtain annotations specified for an EPL statement. Or when compiling an EPL expression to a EPStatementObjectModel statement object model you may also query, change or add annotations.

The following example code demonstrates iterating over an EPStatement statement's annotations and retrieving values:

String exampleEPL = "@Tag(name='direct-output', value='sink 1') select * from RootEvent";
EPStatement stmt = epService.getEPAdministrator().createEPL(exampleEPL);
for (Annotation annotation : stmt.getAnnotations()) {
  if (annotation instanceof Tag) {
    Tag tag = (Tag) annotation;
    System.out.println("Tag name " + tag.name() + " value " + tag.value());
  }
}

The output of the sample code shown above is Tag name direct-output value sink 1.

This chapter discusses how to select context partitions. Contexts are discussed in Chapter 4, Context and Context Partitions and the reasons for context partition selection are introduced in Section 4.7, “Operations on Specific Context Partitions”.

The section is only relevant when you declare a context. It applies to all different types of hash, partitioned, category, overlapping or other temporal contexts. The section uses a category context for the purpose of illustration. The API discussed herein is general and handles all different types of contexts including nested contexts.

Consider a category context that separates bank transactions into small, medium and large:

// declare category context
create context TxnCategoryContext 
  group by amount < 100 as small, 
  group by amount between 100 and 1000 as medium, 
  group by amount > 1000 as large from BankTxn
// retain 1 minute of events of each category separately
context TxnCategoryContext select * from BankTxn.win:time(1 minute)

In order for your application to iterate one or more specific categories it is necessary to identify which category, i.e. which context partition, to iterate. Similarly for on-demand queries, to execute on-demand queries against one or more specific categories, it is necessary to identify which context partition to execute the on-demand query against.

Your application may iterate one or more specific context partitions using either the iterate or safeIterate method of EPStatement by providing an implementation of the ContextPartitionSelector interface.

For example, assume your application must obtain all bank transactions for small amounts. It may use the API to identify the category and iterate the associated context partition:

ContextPartitionSelectorCategory categorySmall = new ContextPartitionSelectorCategory() {
    public Set<String> getLabels() {
      return Collections.singleton("small");
    }
  };
Iterator<EventBean> it = stmt.iterator(categorySmall);

Your application may execute on-demand queries against one or more specific context partitions by using the executeQuery method on EPRuntime or the execute method on EPOnDemandPreparedQuery and by providing an implementation of ContextPartitionSelector.

On-demand queries execute against named windows, therefore below EPL statement creates a named window which the engine manages separately for small, medium and large transactions according to the context declared earlier:

// Named window per category
context TxnCategoryContext create window BankTxnWindow.win:time(1 min) as BankTxn

The following code demonstrates how to fire an on-demand query against the small and the medium category:

ContextPartitionSelectorCategory categorySmallMed = new ContextPartitionSelectorCategory() {
    public Set<String> getLabels() {
      return new HashSet<String>(Arrays.asList("small", "medium"));
    }
  };
epService.getEPRuntime().executeQuery(
   "select count(*) from BankTxnWindow", 
   new ContextPartitionSelector[] {categorySmallMed});

The following limitations apply:

  • On-demand queries may not join named windows that declare a context.

This section summarizes the selector interfaces that are available for use to identify and interrogate context partitions. Please refer to the JavaDoc documentation for package com.espertech.esper.client.context and classes therein for additional information.

Use an implementation of ContextPartitionSelectorAll or the ContextPartitionSelectorAll.INSTANCE object to instruct the engine to consider all context partitions.

Use an implementation of ContextPartitionSelectorById if your application knows the context partition ids to query. This selector instructs the engine to consider only those provided context partitions based on their integer id value. The engine outputs the context partition id in the built-in property context.id.

Use an implementation of ContextPartitionSelectorFiltered to receive and interrogate context partitions. Use the filter method that receives a ContextPartitionIdentifier to return a boolean indicator whether to include the context partition or not. The ContextPartitionIdentifier provides information about each context partition. Your application may not retain ContextPartitionIdentifier instances between filter method invocations as the engine reuses the same instance. This selector is not supported with nested contexts.

Use an implementation of ContextPartitionSelectorCategory with category contexts.

Use an implementation of ContextPartitionSelectorSegmented with keyed segmented contexts.

Use an implementation of ContextPartitionSelectorHash with hash segmented contexts.

Use an implementation of ContextPartitionSelectorNested in combination with the selectors described above with nested contexts.

This chapter briefly discusses the API to manage context partitions. Contexts are discussed in Chapter 4, Context and Context Partitions.

The section is only relevant when you declare a context. It applies to all different types of hash, partitioned, category, overlapping or other temporal contexts.

The administrative API for context partitions is EPContextPartitionAdmin. Use the getContextPartitionAdmin method of the EPAdministrator interface to obtain said service.

The context partition admin API allows an application to:

  • Start, stop and destroy individual context partitions.

  • Interrogate the state and identifiers for existing context partitions.

  • Determine statements associated to a context and context nesting level.

Stopping individual context partitions is useful to drop state, free memory and suspend a given context partition without stopping or destroying any associated statements. For example, assume a keyed segmented context per user id. To suspend and free the memory for a given user id your application can stop the user id's context partition. The engine does not allocate a context partition for this user id again, until your application destroys or starts the context partition.

Destroying individual context partitions is useful to drop state, free memory and deregister the given context partition without stopping or destroying any associated statements. For example, assume a keyed segmented context per user id. To deregister and free the memory for a given user id your application can destroy the user id's context partition. The engine can allocate a fresh context partition for this user id when events for this user id arrive.

Please see the JavaDoc documentation for more information.

Esper offers a listener and an assertions class to facilitate automated testing of EPL rules, for example when using a test framework such as JUnit or TestNG.

Esper does not require any specific test framework. If your application has the JUnit test framework in classpath Esper uses junit.framework.AssertionFailedError to indicate assertion errors, so as to integrate with continuous integration tools.

For detailed method-level information, please consult the JavaDoc of the package com.espertech.esper.client.scopetest.

The class com.espertech.esper.client.scopetest.EPAssertionUtil provides methods to assert or compare event property values as well as perform various array arthithmatic, sort events and convert events or iterators to arrays.

The class com.espertech.esper.client.scopetest.SupportUpdateListener provides an UpdateListener implementation that collects events and returns event data for assertion.

The class com.espertech.esper.client.scopetest.SupportSubscriber provides a subscriber implementation that collects events and returns event data for assertion. The SupportSubscriberMRD is a subscriber that accepts events multi-row delivery. The SupportSubscriber and SupportSubscriberMRD work similar to SupportUpdateListener that is introduced in more detail below.

Esper engine configuration is entirely optional. Esper has a very small number of configuration parameters that can be used to simplify event pattern and EPL statements, and to tune the engine behavior to specific requirements. The Esper engine works out-of-the-box without configuration.

An application can supply configuration at the time of engine allocation using the Configuration class, and can also use XML files to hold configuration. Configuration can be changed at runtime via the ConfigurationOperations interface available from EPAdministrator via the getConfiguration method.

The difference between using a Configuration object and the ConfigurationOperations interface is that for the latter, all configuration including event types added through that interface are considered runtime configurations. This means they will be discarded when calling the initialize method on an EPServiceProvider instance.

Esper can process Java classes that provide event properties through other means then through JavaBean-style getter methods. It is not necessary that the method and member variable names in your Java class adhere to the JavaBean convention - any public methods and public member variables can be exposed as event properties via the below configuration.

A Java class can optionally be configured with an accessor style attribute. This attribute instructs the engine how it should expose methods and fields for use as event properties in statements.


Using the public setting for the accessor-style attribute instructs the engine to expose an event property for each public method and public member variable of a Java class. The engine assigns event property names of the same name as the name of the method or member variable in the Java class.

For example, assuming the class MyLegacyEvent exposes a method named readValue and a member variable named myField, we can then use properties as shown.

select readValue, myField from MyLegacyEvent

Using the explicit setting for the accessor-style attribute requires that event properties are declared via configuration. This is outlined in the next chapter.

When configuring an engine instance from a XML configuration file, the XML snippet below demonstrates the use of the legacy-type element and the accessor-style attribute.

<event-type name="MyLegacyEvent" class="com.mycompany.mypackage.MyLegacyEventClass">
  <legacy-type accessor-style="public"/>
</event-type>

When configuring an engine instance via Configuration API, the sample code below shows how to set the accessor style.

Configuration configuration = new Configuration();
ConfigurationEventTypeLegacy legacyDef = new ConfigurationEventTypeLegacy();
legacyDef.setAccessorStyle(ConfigurationEventTypeLegacy.AccessorStyle.PUBLIC);
config.addEventType("MyLegacyEvent", MyLegacyEventClass.class.getName(), legacyDef);

EPServiceProvider epService = EPServiceProviderManager.getProvider("sample", configuration);

The insert into clause and directly instantiate and populate your event object. By default the engine invokes the default constructor to instantiate an event object. To change this behavior, you may configure a factory method. The factory method is a method name or a class name plus a method name (in the format class.method) that returns an instance of the class.

The update clause can change event properties on an event object. For the purpose of maintaining consistency, the engine may have to copy your event object via serialization (implement the java.io.Serializable interface). If instead you do not want any copy operations to occur, or your application needs to control the copy operation, you may configure a copy method. The copy method is the name of a method on the event object that copies the event object.

The sample below shows this option in XML configuration, however the setting can also be changed via ConfigurationEventTypeLegacy:

<event-type name="MyLegacyEvent" class="com.mycompany.package.MyLegacyEventClass">
  <legacy-type factory-method="com.mycompany.myapp.MySampleEventFactory.createMyLegacyTypeEvent" copy-method="myCopyMethod"/>
</event-type>

The copy method should be a public method that takes no parameters and returns a new event object (it may not return this). The copy method may not be a static method and may not take parameters.

The Beacon data flow operator in connection with the Sun JVM can use sun.reflect.ReflectionFactory if the class has no default no-argument constructor.

The engine can process java.util.Map events via the sendEvent(Map map, String eventTypeName) method on the EPRuntime interface. Entries in the Map represent event properties. Keys must be of type java.util.String for the engine to be able to look up event property names in pattern or EPL statements. Values can be of any type. JavaBean-style objects as values in a Map can be processed by the engine, and strongly-typed nested maps are also supported. Please see the Chapter 2, Event Representations section for details on how to use Map events with the engine.

Via configuration you can provide an event type name for Map events for use in statements, and the event property names and types enabling the engine to validate properties in statements.

The below snippet of XML configuration configures an event named MyMapEvent.

<event-type name="MyMapEvent">
  <java-util-map>
    <map-property name="carId" class="int"/>
    <map-property name="carType" class="string"/>
    <map-property name="assembly" class="com.mycompany.Assembly"/>    
  </java-util-map>
</event-type>

This configuration defines the carId property of MyMapEvent events to be of type int, and the carType property to be of type java.util.String. The assembly property of the Map event will contain instances of com.mycompany.Assembly for the engine to query.

The valid types for the class attribute are listed in Section 15.5, “Type Names”. In addition, any fully-qualified Java class name that can be resolved via Class.forName is allowed.

You can also use the configuration API to configure Map event types, as the short code snippet below demonstrates:

Map<String, Object> properties = new Map<String, Object>();
properties.put("carId", "int");
properties.put("carType", "string");
properties.put("assembly", Assembly.class.getName());

Configuration configuration = new Configuration();
configuration.addEventType("MyMapEvent", properties);

For strongly-typed nested maps (maps-within-maps), the configuration API method addEventType can also used to define the nested types. The XML configuration does not provide the capability to configure nested maps.

Finally, here is a sample EPL statement that uses the configured MyMapEvent map event. This statement uses the chassisTag and numParts properties of Assembly objects in each map.

select carType, assembly.chassisTag, count(assembly.numParts) from MyMapEvent.win:time(60 sec)

A Map event type may also become a subtype of one or more supertypes that must also be Map event types. The java-util-map element provides the optional attribute supertype-names that accepts a comma-separated list of names of Map event types that are supertypes to the type:

<event-type name="AccountUpdate">
<java-util-map supertype-names="BaseUpdate, AccountEvent">
...

For initialization time configuration, the addMapSuperType method can be used to add Map hierarchy information. For runtime configuration, pass the supertype names to the addEventType method in ConfigurationOperations.

A Map event type may declare a start and end timestamp property name. The XML shown next instructs the engine that the startts property carries the event start timestamp and the endts property carries the event end timestamp:

<event-type name="AccountUpdate">
<java-util-map start-timestamp-property-name="startts" end-timestamp-property-name="endts">
...

The engine can process Object-array (Object[]) events via the sendEvent(Object[] array, String eventTypeName) method on the EPRuntime interface. Elements in the Object array represent event properties. Values can be of any type. JavaBean-style objects as values in an Object[] can be processed by the engine, and strongly-typed nested map or object-array event types are also supported. Please see the Chapter 2, Event Representations section for details on how to use Object[] events with the engine.

Via configuration you can provide an event type name for Object[] events for use in statements, and the event property names and types enabling the engine to validate properties in statements.

The below snippet of XML configuration configures an event named MyObjectArrayEvent.

<event-type name="MyObjectArrayEvent">
  <objectarray>
    <objectarray-property name="carId" class="int"/>
    <objectarray-property name="carType" class="string"/>
    <objectarray-property name="assembly" class="com.mycompany.Assembly"/>    
  </objectarray>
</event-type>

This configuration defines the carId property of MyObjectArrayEvent events to be of type int and in the object array first element ([0]). The carType property to be of type java.util.String is expected in the second array element ([1]) . The assembly property of the object array event will contain instances of com.mycompany.Assembly for the engine to query in element two ([2]).

Note that the engine does not verify the length and property values of object array events when your application sends object-array events into the engine. For the example above, the proper object array would look as follows: new Object[] {carId, carType, assembly}.

The valid types for the class attribute are listed in Section 15.5, “Type Names”. In addition, any fully-qualified Java class name that can be resolved via Class.forName is allowed.

You can also use the configuration API to configure Object[] event types, as the short code snippet below demonstrates:

String[] propertyNames = {"carId", "carType", "assembly"};
Object[] propertyTypes = {int.class, String.class, Assembly.class};

Configuration configuration = new Configuration();
configuration.addEventType("MyObjectArrayEvent", propertyNames, propertyTypes);

Finally, here is a sample EPL statement that uses the configured MyObjectArrayEvent object-array event. This statement uses the chassisTag and numParts properties of Assembly objects.

select carType, assembly.chassisTag, count(assembly.numParts) from MyObjectArrayEvent.win:time(60 sec)

An Object-array event type may also become a subtype of one supertype that must also be an Object-array event type. The objectarray element provides the optional attribute supertype-names that accepts a single name of an Object-array event type that is the supertype to the type:

<event-type name="AccountUpdate">
<objectarray supertype-names="BaseUpdate">
...

An Object-array event type may declare a start and end timestamp property name. The XML shown next instructs the engine that the startts property carries the event start timestamp and the endts property carries the event end timestamp:

<event-type name="AccountUpdate">
<objectarray start-timestamp-property-name="startts" end-timestamp-property-name="endts">
...

Via this configuration item the Esper engine can natively process org.w3c.dom.Node instances, i.e. XML document object model (DOM) nodes. Please see the Chapter 2, Event Representations section for details on how to use Node events with the engine.

Esper allows configuring XPath expressions as event properties. You can specify arbitrary XPath functions or expressions and provide a property name by which their result values will be available for use in expressions.

For XML documents that follow a XML schema, Esper can load and interrogate your schema and validate event property names and types against the schema information.

Nested, mapped and indexed event properties are also supported in expressions against org.w3c.dom.Node events. Thus XML trees can conveniently be interrogated using the existing event property syntax for querying JavaBean objects, JavaBean object graphs or java.util.Map events.

In the simplest form, the Esper engine only requires a configuration entry containing the root element name and the event type name in order to process org.w3c.dom.Node events:

<event-type name="MyXMLNodeEvent">
  <xml-dom root-element-name="myevent" />
</event-type>

You can also use the configuration API to configure XML event types, as the short example below demonstrates. In fact, all configuration options available through XML configuration can also be provided via setter methods on the ConfigurationEventTypeXMLDOM class.

Configuration configuration = new Configuration();
ConfigurationEventTypeXMLDOM desc = new ConfigurationEventTypeXMLDOM();
desc.setRootElementName("myevent");
desc.addXPathProperty("name1", "/element/@attribute", XPathConstants.STRING);
desc.addXPathProperty("name2", "/element/subelement", XPathConstants.NUMBER);
configuration.addEventType("MyXMLNodeEvent", desc);

The next example presents configuration options in a sample configuration entry.

<event-type name="AutoIdRFIDEvent">
  <xml-dom root-element-name="Sensor" schema-resource="data/AutoIdPmlCore.xsd" 
       default-namespace="urn:autoid:specification:interchange:PMLCore:xml:schema:1">
    <namespace-prefix prefix="pmlcore" 
       namespace="urn:autoid:specification:interchange:PMLCore:xml:schema:1"/>
    <xpath-property property-name="countTags" 
       xpath="count(/pmlcore:Sensor/pmlcore:Observation/pmlcore:Tag)" type="number"/>
  </xml-dom>
</event-type>

This example configures an event property named countTags whose value is computed by an XPath expression. The namespace prefixes and default namespace are for use with XPath expressions and must also be made known to the engine in order for the engine to compile XPath expressions. Via the schema-resource attribute we instruct the engine to load a schema file. You may also use schema-text instead to provide the actual text of the schema.

Here is an example EPL statement using the configured event type named AutoIdRFIDEvent.

select ID, countTags from AutoIdRFIDEvent.win:time(30 sec)

This setting indicates that when properties are compiled to XPath expressions that the compilation should generate an absolute XPath expression or a deep (find element) XPath expression.

For example, consider the following statement against an event type that is represented by a XML DOM document, assuming the event type GetQuote has been configured with the engine as a XML DOM event type:

select request, request.symbol from GetQuote

By default, the engine compiles the "request" property name to an XPath expression "/GetQuote/request". It compiles the nested property named "request.symbol" to an XPath expression "/GetQuote/request/symbol", wherein the root element node is "GetQuote".

By setting absolute property resolution to false, the engine compiles the "request" property name to an XPath expression "//request". It compiles the nested property named "request.symbol" to an XPath expression "//request/symbol". This enables these elements to be located anywhere in the XML document.

The setting is available in XML via the attribute resolve-properties-absolute.

The configuration API provides the above settings as shown here in a sample code:

ConfigurationEventTypeXMLDOM desc = new ConfigurationEventTypeXMLDOM();
desc.setRootElementName("GetQuote");
desc.setDefaultNamespace("http://services.samples/xsd");
desc.setRootElementNamespace("http://services.samples/xsd");
desc.addNamespacePrefix("m0", "http://services.samples/xsd");
desc.setResolvePropertiesAbsolute(false);
configuration.addEventType("GetQuote", desc);

As part of the extension API plug-in event representations allows an application to create new event types and event instances based on information available elsewhere. Please see Section 17.8, “Event Type And Event Object” for details.

The configuration examples shown next use the configuration API to select settings. All options are also configurable via XML, please refer to the sample configuration XML in file esper.sample.cfg.xml.

Esper allows invocations of static Java library functions in expressions, as outlined in Section 9.1, “Single-row Function Reference”. This configuration item can be set to allow a partial rather than a fully qualified class name in such invocations. The imports work in the same way as in Java files, so both packages and classes can be imported.

select Math.max(priceOne, PriceTwo)
// via configuration equivalent to
select java.lang.Math.max(priceOne, priceTwo)

Esper auto-imports the following Java library packages. Any additional imports that are specified in configuration files or through the API are added to the configuration in addition to the imports below.

  • java.lang.*

  • java.math.*

  • java.text.*

  • java.util.*

In a XML configuration file the auto-import configuration may look as below:

<auto-import import-name="com.mycompany.mypackage.*"/>
<auto-import import-name="com.mycompany.myapp.MyUtilityClass"/>

Here is an example of providing imports via the API:

Configuration config = new Configuration();
config.addImport("com.mycompany.mypackage.*");	// package import
config.addImport("com.mycompany.mypackage.MyLib");   // class import

Method invocations are allowed in the from clause in EPL, such that your application may join event streams to the data returned by a web service, or to data read from a distributed cache or object-oriented database, or obtain data by other means. A local cache may be placed in front of such method invocations through the configuration settings described herein.

The LRU cache is described in detail in Section 15.4.9.6.1, “LRU Cache”. The expiry-time cache documentation can be found in Section 15.4.9.6.2, “Expiry-time Cache”

The next XML snippet is a sample cache configuration that applies to methods provided by the classes 'MyFromClauseLookupLib' and 'MyFromClauseWebServiceLib'. The XML and API configuration understand both the fully-qualified Java class name, as well as the simple class name:

<method-reference class-name="com.mycompany.MyFromClauseLookupLib">
  <expiry-time-cache max-age-seconds="10" purge-interval-seconds="10" ref-type="weak"/>
</method-reference> 	
<method-reference class-name="MyFromClauseWebServiceLib">
  <lru-cache size="1000"/>
</method-reference> 

Esper has the capability to join event streams against historical data sources, such as a relational database. This section describes the configuration entries that the engine requires to access data stored in your database. Please see Section 5.13, “Accessing Relational Data via SQL” for information on the use of EPL queries that include historical data sources.

EPL queries that poll data from a relational database specify the name of the database as part of the EPL statement. The engine uses the configuration information described here to resolve the database name in the statement to database settings. The required and optional database settings are summarized below.

  • Database connections can be obtained via JDBC javax.xml.DataSource, via java.sql.DriverManager and via data source factory. Either one of these methods to obtain database connections is a required configuration.

  • Optionally, JDBC connection-level settings such as auto-commit, transaction isolation level, read-only and the catalog name can be defined.

  • Optionally, a connection lifecycle can be set to indicate to the engine whether the engine must retain connections or must obtain a new connection for each lookup and close the connection when the lookup is done (pooled).

  • Optionally, define a cache policy to allow the engine to retrieve data from a query cache, reducing the number of query executions.

Some of the settings can have important performance implications that need to be carefully considered in relationship to your database software, JDBC driver and runtime environment. This section attempts to outline such implications where appropriate.

The sample XML configuration file in the "etc" folder can be used as a template for configuring database settings. All settings are also available by means of the configuration API through the classes Configuration and ConfigurationDBRef.

This configuration causes Esper to obtain a database connection from a javax.sql.DataSource available from your JNDI provider.

The setting is most useful when running within an application server or when a JNDI directory is otherwise present in your Java VM. If your application environment does not provide an available DataSource, the next section outlines how to use Apache DBCP as a DataSource implementation with connection pooling options and outlines how to use a custom factory for DataSource implementations.

If your DataSource provides connections out of a connection pool, your configuration should set the collection lifecycle setting to pooled.

The snippet of XML below configures a database named mydb1 to obtain connections via a javax.sql.DataSource. The datasource-connection element instructs the engine to obtain new connections to the database mydb1 by performing a lookup via javax.naming.InitialContext for the given object lookup name. Optional environment properties for the InitialContext are also shown in the example.

<database-reference name="mydb1">
  <datasource-connection context-lookup-name="java:comp/env/jdbc/mydb">
    <env-property name="java.naming.factory.initial" value ="com.myclass.CtxFactory"/>
    <env-property name="java.naming.provider.url" value ="iiop://localhost:1050"/>
  </datasource-connection>
</database-reference>

To help you better understand how the engine uses this information to obtain connections, we have included the logic below.

if (envProperties.size() > 0) {
  initialContext = new InitialContext(envProperties);
}
else {
  initialContext = new InitialContext();
}
DataSource dataSource = (DataSource) initialContext.lookup(lookupName);
Connection connection = dataSource.getConnection();

In order to plug-in your own implementation of the DataSource interface, your application may use an existing JNDI provider as provided by an application server if running in a J2EE environment.

In case your application does not have an existing JNDI implementation to register a DataSource to provide connections, you may set the java.naming.factory.initial property in the configuration to point to your application's own implementation of the javax.naming.spi.InitialContextFactory interface that can return your application DataSource though the javax.naming.Context provided by the factory implementation. Please see Java Naming and Directory Interface (JNDI) API documentation for further information.

This section describes how to use Apache Commons Database Connection Pooling (Apache DBCP) with Esper. We also explain how to provide a custom application-specific DataSource factory if not using Apache DBCP.

If your DataSource provides connections out of a connection pool, your configuration should set the collection lifecycle setting to pooled.

Apache DBCP provides comprehensive means to test for dead connections or grow and shrik a connection pool. Configuration properties for Apache DBCP can be found at Apache DBCP configuration. The listed properties are passed to Apache DBCP via the properties list provided as part of the Esper configuration.

The snippet of XML below is an example that configures a database named mydb3 to obtain connections via the pooling DataSource provided by Apache DBCP BasicDataSourceFactory.

The listed properties are passed to DBCP to instruct DBCP how to manage the connection pool. The settings below initialize the connection pool to 2 connections and provide the validation query select 1 from dual for DBCP to validate a connection before providing a connection from the pool to Esper:

<database-reference name="mydb3">
  <!-- For a complete list of properties see Apache DBCP. -->
  <datasourcefactory-connection class-name="org.apache.commons.dbcp.BasicDataSourceFactory">	
    <env-property name="username" value ="myusername"/>
    <env-property name="password" value ="mypassword"/>
    <env-property name="driverClassName" value ="com.mysql.jdbc.Driver"/>
    <env-property name="url" value ="jdbc:mysql://localhost/test"/>
    <env-property name="initialSize" value ="2"/>
    <env-property name="validationQuery" value ="select 1 from dual"/>
  </datasourcefactory-connection>
  <connection-lifecycle value="pooled"/>
</database-reference>

The same configuration options provided through the API:

Properties props = new Properties();
props.put("username", "myusername");
props.put("password", "mypassword");
props.put("driverClassName", "com.mysql.jdbc.Driver");
props.put("url", "jdbc:mysql://localhost/test");
props.put("initialSize", 2);
props.put("validationQuery", "select 1 from dual");

ConfigurationDBRef configDB = new ConfigurationDBRef();
// BasicDataSourceFactory is an Apache DBCP import
configDB.setDataSourceFactory(props, BasicDataSourceFactory.class.getName());
configDB.setConnectionLifecycleEnum(ConfigurationDBRef.ConnectionLifecycleEnum.POOLED);

Configuration configuration = new Configuration();;
configuration.addDatabaseReference("mydb3", configDB);

Apache Commons DBCP is a separate download and not provided as part of the Esper distribution. The Apache Commons DBCP jar file requires the Apache Commons Pool jar file.

Your application can provide its own factory implementation for DataSource instances: Set the class name to the name of the application class that provides a public static method named createDataSource which takes a single Properties object as parameter and returns a DataSource implementation. For example:

configDB.setDataSourceFactory(props, MyOwnDataSourceFactory.class.getName());
...
class MyOwnDataSourceFactory {
  public static DataSource createDataSource(Properties properties) {
    return new MyDataSourceImpl(properties);
  }
}

Cache settings can dramatically reduce the number of database queries that the engine executes for EPL statements. If no cache setting is specified, the engine does not cache query results and executes a separate database query for every event.

Caches store the results of database queries and make these results available to subsequent queries using the exact same query parameters as the query for which the result was stored. If your query returns one or more rows, the cache keep the result rows of the query keyed to the parameters of the query. If your query returns no rows, the cache also keeps the empty result. Query results are held by a cache until the cache entry is evicted. The strategies available for evicting cached query results are listed next.

The expiry time cache is configured by a maximum age in seconds, a purge interval and an optional reference type. The cache discards (on the get operation) any query results that are older then the maximum age so that stale data is not used. If the cache is not empty, then every purge interval number of seconds the engine purges any expired entries from the cache.

The XML configuration entry for an expiry-time cache is as follows. The example configures an expiry time cache in which prior query results are valid for 60 seconds and which the engine inspects every 2 minutes to remove query results older then 60 seconds.

<database-reference name="mydb">
... configure data source or driver manager settings...
    <expiry-time-cache max-age-seconds="60" purge-interval-seconds="120" />
</database-reference>

By default, the expiry-time cache is backed by a java.util.WeakHashMap and thus relies on weak references. That means that cached SQL results can be freed during garbage collection.

Via XML or using the configuration API the type of reference can be configured to not allow entries to be garbage collected, by setting the ref-type property to hard:

<database-reference name="mydb">
... configure data source or driver manager settings...
    <expiry-time-cache max-age-seconds="60" purge-interval-seconds="120" ref-type="hard"/>
</database-reference>

The last setting for the cache reference type is soft: This strategy allows the garbage collection of cache entries only when all other weak references have been collected.

In multithreaded environments, this setting controls whether dispatches of statement result events to listeners preserve the ordering in which a statement processes events. By default the engine guarantees that it delivers a statement's result events to statement listeners in the order in which the result is generated. This behavior can be turned off via configuration as below.

The next code snippet shows how to control this feature:

Configuration config = new Configuration();
config.getEngineDefaults().getThreading().setListenerDispatchPreserveOrder(false);
engine = EPServiceProviderManager.getDefaultProvider(config);

And the XML configuration file can also control this feature by adding the following elements:

<engine-settings>
  <defaults>
    <threading>
      <listener-dispatch preserve-order="true" timeout-msec="1000" locking="spin"/>
    </threading>
  </defaults>
</engine-settings>

As discussed, by default the engine can temporarily block another processing thread when delivering result events to listeners in order to preserve the order in which results are delivered to a given statement. The maximum time the engine blocks a thread can also be configured, and by default is set to 1 second.

As such delivery locks are typically held for a very short amount of time, the default blocking technique employs a spin lock (There are two techniques for implementing blocking; having the operating system suspend the thread until it is awakened later or using spin locks). While spin locks are CPU-intensive and appear inefficient, a spin lock can be more efficient than suspending the thread and subsequently waking it up, especially if the lock in question is held for a very short time. That is because there is significant overhead to suspending and rescheduling a thread.

The locking technique can be changed to use a blocking strategy that suspends the thread, by means of setting the locking property to 'suspend'.

In multithreaded environments, this setting controls whether statements producing events for other statements via insert-into preserve the order of delivery within the producing and consuming statements, allowing statements that consume other statement's events to behave deterministic in multithreaded applications, if the consuming statement requires such determinism. By default, the engine makes this guarantee (the setting is on).

Take, for example, an application where a single statement (S1) inserts events into a stream that another statement (S2) further evaluates. A multithreaded application may have multiple threads processing events into statement S1. As statement S1 produces events for consumption by statement S2, such results may need to be delivered in the exact order produced as the consuming statement may rely on the order received. For example, if the first statement counts the number of events, the second statement may employ a pattern that inspects counts and thus expect the counts posted by statement S1 to continuously increase by 1 even though multiple threads process events.

The engine may need to block a thread such that order of delivery is maintained, and statements that require order (such as pattern detection, previous and prior functions) receive a deterministic order of events. The settings available control the blocking technique and parameters. As described in the section immediately prior, the default blocking technique employs spin locks per statement inserting events for consumption, as the locks in questions are typically held a very short time. The 'suspend' blocking technique can be configured and a timeout value can also defined.

The XML configuration file may change settings via the following elements:

<engine-settings>
  <defaults>
    <threading>
      <insert-into-dispatch preserve-order="true" timeout-msec="100" locking="spin"/>
    </threading>
  </defaults>
</engine-settings>

The engine-wide settings discussed here are used when you want to control case sensitivity or accessor style for all event classes as a default. The two settings are found under class-property-resolution under event-meta in the XML configuration.

To control the case sensitivity as discussed in Section 15.4.1.6, “Case Sensitivity and Property Names”, add the style attribute in the XML configuration to set a default case sensitivity applicable to all event classes unless specifically overridden by class-specific configuration. The default case sensitivity is case_sensitive (case sensitivity turned on).

To control the accessor style as discussed in Section 15.4.1.3, “Non-JavaBean and Legacy Java Event Classes”, add the accessor-style attribute in the XML configuration to set a default accessor style applicable to all event classes unless specifically overridden by class-specific configuration. The default accessor style is javabean JavaBean accessor style.

The next code snippet shows how to control this feature via the API:

Configuration config = new Configuration();
config.getEngineDefaults().getEventMeta().setClassPropertyResolutionStyle(
    Configuration.PropertyResolutionStyle.CASE_INSENSITIVE);
config.getEngineDefaults().getEventMeta().setDefaultAccessorStyle(
    ConfigurationEventTypeLegacy.AccessorStyle.PUBLIC);

By default, when combining multiple data window views, Esper applies an intersection of the data windows unless the retain-union keyword is provided which instructs to apply an union. The setting described herein may be used primarily for backward compatibility to instruct that intersection should not be the default.

Here is a sample statement that specifies multiple expiry policies:

select * from MyEvent.std:unique(price).std:unique(quantity)

By default Esper applies intersection as described in Section 5.4.4, “Multiple Data Window Views”.

Here is the setting to allow multiple data windows without the intersection default:

Configuration config = new Configuration();
config.getEngineDefaults().getViewResources().setAllowMultipleExpiryPolicies(true);

When setting this option to true, and when using multiple data window views for a given stream, the behavior is as follows: The top-most data window receives an insert stream of events. It passes each insert stream event to each further data window view in the chain. Each data window view may remove events according to its expiry policy. Such remove stream events are only passed to data window views further in the chain, and are not made available to data window views earlier in the chain.

It is recommended to leave the default setting at false.

By default, the engine does not produce debug output for the event processing execution paths even when Log4j or Logger configurations have been set to output debug level logs. To enable debug level logging, set this option in the configuration as well as in your Log4j configuration file.

Statement-level processing information can be output via the @Audit annotation, please see Section 16.3.1, “@Audit Annotation”.

When debug-level logging is enabled by setting the flag as below and by setting DEBUG in the Log4j configuration file, then the timer processing may produce extensive debug output that you may not want to have in the log file. The timer-debug setting in the XML or via API as below disables timer debug output which is enabled by default.

The API to use to enable debug logging and disable timer event output is shown here:

Configuration config = new Configuration();
config.getEngineDefaults().getLogging().setEnableExecutionDebug(true);
config.getEngineDefaults().getLogging().setEnableTimerDebug(false);

Note: this is a configuration option that applies to all engine instances of a given Java module or VM.

The XML snippet is:

<esper-configuration>
  <engine-settings>
    <defaults>
      <logging>
        <execution-path enabled="true"/>
        <timer-debug enabled="false"/>
    </logging>
    </defaults>
  </engine-settings>
</esper-configuration>

This setting only applies if internal timer events control engine time (default). If external timer events provide engine clocking, the setting does not apply.

By default, the internal timer uses the call System.currentTimeMillis() to determine engine time in milliseconds. Via this setting the internal timer can be instructed to use System.nanoTime() instead. Please see Section 14.9, “Time Resolution” for more information.

Note: This is a Java VM global setting. If running multiple engine instances in a Java VM, the timer setting is global and applies to all engine instances in the same Java VM, for performance reasons.

A sample XML configuration for this setting is shown below, whereas the sample setting sets the time source to the nanosecond time provider:

<engine-settings>
  <defaults>
    <time-source>
      <time-source-type value="nano" />
    </time-source>
  </defaults>
</engine-settings>

The equivalent code snippet using the configuration API is here:

Configuration config = new Configuration();
config.getEngineDefaults().getTimeSource().
      setTimeSourceType(ConfigurationEngineDefaults.TimeSourceType.NANO);

This section explains how to enable and configure metrics reporting, which is by default disabled. Please see Section 14.15, “Engine and Statement Metrics Reporting” for more information on the metrics data reported to your application.

The flag that enables metrics reporting is global to a Java virtual machine. If metrics reporting is enabled, the overhead incurred for reporting metrics is carried by all engine instances per Java VM.

Metrics reporting occurs by an engine-controlled separate daemon thread that each engine instance starts at engine initialization time, if metrics reporting and threading is enabled (threading enabled is the default).

Engine and statement metric intervals are in milliseconds. A negative or zero millisecond interval value may be provided to disable reporting.

To control statement metric reporting for individual statements or groups of statements, the engine provides a facility that groups statements by statement name. Each such statement group may have different reporting intervals configured, and intervals can be changed at runtime through runtime configuration. A statement group is assigned a group name at configuration time to identify the group.

Metrics reporting configuration is part of the engine default settings. All configuration options are also available via the Configuration API.

A sample XML configuration is shown below:

<engine-settings>
  <defaults>
    <metrics-reporting enabled="true" engine-interval="1000" statement-interval="1000" 
        threading="true"/>
  </defaults>
</engine-settings>

The engine-interval setting (defaults to 10 seconds) determines the frequency in milliseconds at which the engine reports engine metrics, in this example every 1 second. The statement-interval is for statement metrics. The threading flag is true by default since reporting takes place by a dedicated engine thread and can be set to false to use the external or internal timer thread instead.

The next example XML declares a statement group: The statements that have statement names that fall within the group follow a different reporting frequency:

<metrics-reporting enabled="true" statement-interval="0">
  <stmtgroup name="MyStmtGroup" interval="2000" default-include="true" num-stmts="100" 
        report-inactive="true">
    <exclude-regex>.*test.*</exclude-regex>
  </stmtgroup>
</metrics-reporting>

The above example configuration sets the statement-interval to zero to disable reporting for all statements. It defines a statement group by name MyStmtGroup and specifies a 2-second interval. The example sets the default-include flag to true (by default false) to include all statements in the statement group. The example also sets report-inactive to true (by default false) to report inactive statements.

The exclude-regex element may be used to specify a regular expression that serves to exclude statements from the group. Any statement whose statement name matches the exclude regular expression is not included in the group. In the above example, all statements with the characters 'test' inside their statement name are excluded from the group.

Any statement not belonging to any of the statement groups follow the configured statement interval.

There are additional elements available to include and exclude statements: include-regex, include-like and exclude-like. The latter two apply SQL-like matching. All patterns are case-sensitive.

Here is a further example of a possible statement group definition, which includes statements whose statement name have the characters @REPORT or @STREAM, and excludes statements whose statement name have the characters @IGNORE or @METRICS inside.

<metrics-reporting enabled="true">
  <stmtgroup name="MyStmtGroup" interval="1000">
    <include-like>%@REPORT%</include-like>
    <include-regex>.*@STREAM.*</include-like>
    <exclude-like>%@IGNORE%</exclude-like>
    <exclude-regex>.*@METRICS.*</exclude-regex>
  </stmtgroup>
</metrics-reporting>

By default Esper returns double-typed values for divisions regardless of operand types. Division by zero returns positive or negative double infinity.

To have Esper use Java-standard integer division instead, use this setting as described here. In Java integer division, when dividing integer types, the result is an integer type. This means that if you divide an integer unevenly by another integer, it returns the whole number part of the result, does not perform any rounding and the fraction part is dropped. If Java-standard integer division is enabled, when dividing an integer numerator by an integer denominator, the result is an integer number. Thus the expression 1 / 4 results in an integer zero. Your EPL must then convert at least one of the numbers to a double value before the division, for example by specifying 1.0 / 4 or by using cast(myint, double).

When using Java integer division, division by zero for integer-typed operands always returns null. However division by zero for double-type operands still returns positive or negative double infinity. To also return null upon division by zero for double-type operands, set the flag to true as below (default is false).

The XML configuration is as follows:

<engine-settings>
  <defaults>
    <expression integer-division="false" division-by-zero-is-null="false"/>
  </defaults>
</engine-settings>

The API to change the setting:

Configuration config = new Configuration();
config.getEngineDefaults().getExpression().setIntegerDivision(true);
config.getEngineDefaults().getExpression().setDivisionByZeroReturnsNull(true);

Use the settings as described here to register an exception handler factory class that provides an exception handler. The engine invokes exception handlers in the order they are listed to handle a continues-query unchecked exception, as further described in Section 14.11, “Exception Handling”.

Please provide the full-qualified class name of each class that implements the com.espertech.esper.client.hook.ExceptionHandlerFactory interface in the engine defaults configuration as below.

The XML configuration is as follows:

<engine-settings>
  <defaults>
    <exceptionHandling>
      <handlerFactory class="my.company.cep.MyCEPEngineExceptionHandlerFactory"/>
    </exceptionHandling>
  </defaults>
</engine-settings>

The API calls to register an exception handler factory are as follows:

Configuration config = new Configuration();
config.getEngineDefaults().getExceptionHandling().addClass(MyCEPEngineExceptionHandlerFactory.class);

Use the settings as described here to register a condition handler factory class that provides a condition handler. The engine invokes condition handlers in the order they are listed to indicate conditions, which is the term used for notification when certain predefined limits are reached, as further described in Section 14.12, “Condition Handling”.

Please provide the full-qualified class name of each class that implements the com.espertech.esper.client.hook.ConditionHandlerFactory interface in the engine defaults configuration as below.

The XML configuration is as follows:

<engine-settings>
  <defaults>
    <conditionHandling>
      <handlerFactory class="my.company.cep.MyCEPEngineConditionHandlerFactory"/>
    </conditionHandling>
  </defaults>
</engine-settings>

The API calls to register a condition handler factory are as follows:

Configuration config = new Configuration();
config.getEngineDefaults().getConditionHandling().addClass(MyCEPEngineConditionHandlerFactory.class);

Revision event types reflect a versioning relationship between events of same or different event types. Please refer to Section 2.10, “Updating, Merging and Versioning Events” and Section 5.15.14, “Versioning and Revision Event Type Use with Named Windows”.

The configuration consists of the following:

  • An name of an event type whose events are base events.

  • Zero, one or more names of event types whose events are delta events.

  • One or more property names that supply the key values that tie base and delta events to existing revision events. Properties must exist on the event type as simple properties. Nested, indexed or mapped properties are not allowed.

  • Optionally, a strategy for overlaying or merging properties. The default strategy is Overlay Declared as described below.

The XML configuration for this setting is shown below:

<revision-event-type name="UserProfileRevisions">
  <base-event-type name="ProfileCreation"/>
  <delta-event-type name="ProfileUpdate"/>
  <key-property name="userid"/>
</revision-event-type>

If configuring via runtime or initialization-time API, this code snippet explains how:

Configuration config = new Configuration();
ConfigurationRevisionEventType configRev = new ConfigurationRevisionEventType();
configRev.setNameBaseEventType("ProfileCreation");
configRev.addNameDeltaEventType("ProfileUpdate");
configRev.setKeyPropertyNames(new String[] {"userid"});
config.addRevisionEventType("UserProfileRevisions", configRev);

As the configuration provides names of base and delta event types, such names must be configured for JavaBean, Map or XML events as the previous sections outline.

The next table outlines the available strategies:

Table 15.5. Property Revision Strategies

NameDescription
Overlay Declared (default)

A fast strategy for revising events that groups properties provided by base and delta events and overlays contributed properties to compute a revision.

For use when there is a limited number of combinations of properties that change on an event, and such combinations are known in advance.

The properties available on the output revision events are all properties of the base event type. Delta event types do not add any additional properties that are not present on the base event type.

Any null values or non-existing property on a delta (or base) event results in a null values for the same property on the output revision event.

Merge Declared

A strategy for revising events by merging properties provided by base and delta events, considering null values and non-existing (dynamic) properties as well.

For use when there is a limited number of combinations of properties that change on an event, and such combinations are known in advance.

The properties available on the output revision events are all properties of the base event type plus all additional properties that any of the delta event types provide.

Any null values or non-existing property on a delta (or base) event results in a null values for the same property on the output revision event.

Merge Non-null

A strategy for revising events by merging properties provided by base and delta events, considering only non-null values.

For use when there is an unlimited number of combinations of properties that change on an event, or combinations are not known in advance.

The properties available on the output revision events are all properties of the base event type plus all additional properties that any of the delta event types provide.

Null values returned by delta (or base) event properties provide no value to output revision events, i.e. null values are not merged.

Merge Exists

A strategy for revising events by merging properties provided by base and delta events, considering only values supplied by event properties that exist.

For use when there is an unlimited number of combinations of properties that change on an event, or combinations are not known in advance.

The properties available on the output revision events are all properties of the base event type plus all additional properties that any of the delta event types provide.

All properties are treated as dynamic properties: If an event property does not exist on a delta event (or base) event the property provides no value to output revision events, i.e. non-existing property values are not merged.


Esper logs all messages to Apache commons logging under an appropriate log level. To output log messages you can add Log4j to classpath and configure Log4j as below.

Esper's only direct dependency for logging is the Apache Commons Logging interfaces. You may use Log4j as described here, or you may use SLF4J instead (for example) as described in http://www.slf4j.org/legacy.html. You can also redirect SLF4J to any backend you want - nop, logback, jul as needed.

Statement-level processing information can be output, please see Section 16.3.1, “@Audit Annotation”.

For performance reasons, Esper does not log any debug-level or informational-level messages for event execution unless explicitly configured via Section 15.4.13.1, “Execution Path Debug Logging”.

A callback API for receiving certain critical engine reports is available as described in Section 14.11, “Exception Handling”.

More information on configuring engine-level settings for logging are at Section 15.4.13, “Engine Settings related to Logging”.

Apache Commons Logging uses an automatic discovery mechanism to find a log framework that it will delegate to. If your application has another component using logback, please inspect you logback configuration and add com.espertech.

The next table explains the log levels:


Log4j is the default logging component. Please find additional information for Log4j configuration and extension in http://logging.apache.org/log4j.

The easiest way to configure Log4j is by providing a Log4J configuration file, similar to the log4j.xml file shipped in the etc folder of the distribution.

Add the log4j.configuration system property to the java command line and provide the file name of the Log4j configuration file, making sure your classpath also includes the directory of the file:

java -Dlog4j.configuration=log4j.xml ...

This chapter presents information related to the development lifecycle for developing an event processing application with EPL. It includes information on authoring, testing, debugging, packaging and deploying.

One important tool for debugging is the parameterized @Audit annotation. This annotation allows to output, on statement-level, detailed information about many aspects of statement processing.

Another tool for logging engine-level detail is Section 15.4.13.1, “Execution Path Debug Logging”.

Please see Section 15.7, “Logging Configuration” for information on configuring logging in general.

Use the @Audit annotation to have the engine output detailed information about statement processing. The engine reports, at INFO level, the information under log name com.espertech.esper.audit. You may define an output format for audit information via configuration.

You may provide a comma-separated list of category names to @Audit to output information related to specific categories only. The table below lists all available categories. If no parameter is provided, the engine outputs information for all categories. Category names are not case-sensitive.

For the next statement the engine produces detailed processing information (all categories) for the statement:

@Name('All Order Events') @Audit select * from OrderEvent

For the next statement the engine provides information about new events and also about event property values (2 categories are listed):

@Name('All Order Events') @Audit('stream,property') select price from OrderEvent

Here is a more complete example that uses the API to create the schema, create above statement and send an event:

epService.getEPAdministrator().createEPL("create schema OrderEvent(price double)");
String epl = "@Name('All-Order-Events') @Audit('stream,property') select price from OrderEvent";
epService.getEPAdministrator().createEPL(epl).addListener(listener);
epService.getEPRuntime().sendEvent(Collections.singletonMap("price", 100d), "OrderEvent");

The output is similar to the following:

INFO  [audit] Statement All-Order-Events stream OrderEvent inserted {price=100.0}
INFO  [audit] Statement All-Order-Events property price value 100.0

Note that the engine only evaluates select-clause expressions if either a listener or subscriber is attached to the statement or if used with insert-into.

An EPL module file is a plain text file in which EPL statements appear separated by the semicolon (;) character. It bundles EPL statements with optional deployment instructions. A service provider instance keeps track of the known and/or deployed EPL modules and makes it easy to add, remove, deploy and undeploy EPL modules.

The synopsis of an EPL module file is:

[module module_name;]
[uses module_name; | import import_name;] [uses module_name; | import import_name;] [...]
[epl_statement;] [epl_statement;] [...]

Use the module keyword followed a module_name identifier or a package (identifiers separated by dots) to declare the name of the module. The module name declaration must be at the beginning of the file, comments and whitespace excluded. The module name serves to check uses-dependences of other modules.

If a module file requires certain constructs that may be shared by other module files, such as named windows, variables, event types, variant streams or inserted-into streams required by statements, a module file may specify zero to many dependent modules with the uses keyword. At deployment time the engine checks the uses-dependencies and ensures that a module of that name is already deployed or will be deployed as part of the deployments. The deployment API supports ordering modules according to their uses-relationship.

If the EPL statements in the module require Java classes such as for underlying events or user-defined functions, use the import keyword followed by the fully-qualified class name or package name in the format package.*. The uses and import keywords are optional and must occur after the module declaration.

Following the optional deployment instructions are any number of epl_statement EPL statements that are separated by semicolon (;).

The following is a sample EPL module file explained in detail thereafter:

// Declare the name for the module
module org.myorganization.switchmonitor;

// Declare other module(s) that this module depends on
uses org.myorganization.common;

// Import any Java/.NET classes in an application package 
import org.myorganization.events.*;

// Declare an event type based on a Java class in the package that was imported as above
create schema MySwitchEvent as MySwitchEventPOJO;

// Sample statement
@Name('Off-On-Detector')
insert into MyOffOnStream
select * from pattern[every-distinct(id) a=MySwitchEvent(status='off') 
  -> b=MySwitchEvent(id=a.id, status='on')];

// Sample statement
@Name('Count-Switched-On')
@Description('Count per switch id of the number of Off-to-On switches in the last 1 hour')
select id, count(*) from MyOffOnStream.win:time(1 hour) group by id;

The example above declares a module name of org.myorganization.switchmonitor. As defined by the uses keyword, it ensures that the org.myorganization.common module is already deployed. The example demonstrates the import keyword to make a package name known to the engine for resolving POJO class names, as the example assumes that MySwitchEventPOJO is a POJO event class. In addition the example module contains two statements separated by semicolon characters.

Your application code may, after deployment, look up a statement and attach listeners as shown here:

epService.getEPAdministrator().getStatement("Count-Switched-On").addListener(...);

The com.espertech.esper.client.deploy.EPDeploymentAdmin service available from the EPAdministrator interface by method getDeploymentAdmin provides the functionality available to manage packaging and deployment. Please consult the JavaDoc documentation for more information.

The deployment API allows to read resources and parse text strings to obtain an object representation of the EPL module, the Module. A Module object can also be simply constructed.

After your application obtains a Module instance it may either use deploy to deploy the module directly, starting all statements of the module. Alternatively your application may add a module, making it known without starting statements for later deployment. In each case the module is assigned a deployment id, which acts as a unique primary key for all known modules. Your application may assign its own deployment id or may have the engine generate a deployment id (two footprints for add and deploy methods).

A module may be in two states: undeployed or deployed. When calling add to add a module, it starts life in the undeployed state. When calling deploy to deploy a module, it starts life in the deployed state. A module may be transitioned by providing the deployment id and by calling the deploy or undeploy methods.

Your code can remove a module in undeployed state using the remove method or the undeployRemove method. If the module is in deployed state, use undeployRemove to undeploy and remove the module.

The DeploymentOptions instance that can be passed to the deploy method when validating or deploying modules controls validation, fail-fast, rollback and the isolated service provider, if any, for the deployment. Also use DeploymentOptions to set a user object per statement or to set a statement name per statement.

We also provide additional sample code to read and deploy modules as part of the J2EE considerations below.

Esper can well be deployed as part of a J2EE web or enterprise application archive to a web application server. When designing for deployment into a J2EE web application server, please consider the items discussed here.

We provide a sample servlet context listener in this section that uses the deployment API to deploy and undeploy modules as part of the servlet lifecycle.

The distribution provides a message-driven bean (MDB) example that you may find useful.

Esper does not have a dependency on any J2EE or Servlet APIs to allow the engine to run in any environment or container.

As multiple web applications deployed to a J2EE web application server typically have a separate classloader per application, you should consider whether engine instances need to be shared between applications or can remain separate engine instances. Consider the EPServiceProviderManager a Singleton. When deploying multiple web applications, your J2EE container classloader may provide a separate instance of the Singleton EPServiceProviderManager to each web application resulting in multiple independent engine instances.

To share EPServiceProvider instances between web applications, one approach is to add the Esper jar files to the system classpath. A second approach can be to have multiple web applications share the same servet context and have your application place the EPServiceProvider instance into a servlet context attribute for sharing. Architecturally you may also consider a single archived application (such as an message-driven bean) that all your web applications communicate to via the JMS broker provided by your application server or an external JMS broker.

As per J2EE standards there are restrictions in regards to starting new threads in J2EE application code. Esper adheres to these restrictions: It allows to be driven entirely by external events. To remove all Esper threads, set the internal timer off and leave the advanced threading options turned off. To provide timer events when the internal timer is turned off, you should check with your J2EE application container for support of the Java system timer or for support of batch or work loading to send timer events to an engine instance.

As per J2EE standards there are restrictions in regards to input and output by J2EE application code. Esper adheres to these restrictions: By itself it does not start socket listeners or performs any file IO.

When deploying a J2EE archive that contains EPL modules files we provide sample code to read and deploy EPL modules files packaged with the enterprise or web application archive when the servlet initializes. The sample undeploys EPL modules when the servlet context gets destroyed.

A sample web.xml configuration extract is:

<?xml version="1.0" encoding="UTF-8"?>
<web-app>
  <listener>
    <listener-class>SampleServletListener</listener-class>
  </listener>
  <context-param>
    <param-name>eplmodules</param-name>
    <param-value>switchmonitor.epl</param-value>
</context-param>
</web-app>

A servet listener that deploys EPL module files packaged into the archive on context initialization and that undeploys when the application server destroys the context is shown here:

public class SampleServletListener implements ServletContextListener {

  private List<String> deploymentIds = new ArrayList<String>();
  
  public void contextInitialized(ServletContextEvent servletContextEvent) {
    try {
      EPServiceProvider epServiceProvider = EPServiceProviderManager.getDefaultProvider();
      String modulesList = servletContextEvent.getServletContext().getInitParameter("eplmodules");
      List<Module> modules = new ArrayList<Module>();
      if (modulesList != null) {
        String[] split = modulesList.split(",");
        for (int i = 0; i < split.length; i++) {
          String resourceName = split[i].trim();
          if (resourceName.length() == 0) {
            continue;
          }
          String realPath = servletContextEvent.getServletContext().getRealPath(resourceName);
  		Module module = epServiceProvider.getEPAdministrator()
  		        .getDeploymentAdmin().read(new File(realPath));
          modules.add(module);
        }
      }
    
      // Determine deployment order
      DeploymentOrder order = epServiceProvider.getEPAdministrator()
                  .getDeploymentAdmin().getDeploymentOrder(modules, null);
  
      // Deploy
      for (Module module : order.getOrdered()) {
        DeploymentResult result = epServiceProvider.getEPAdministrator()
                  .getDeploymentAdmin().deploy(module, new DeploymentOptions());
        deploymentIds.add(result.getDeploymentId());
      }
    }
    catch (Exception ex) {
      ex.printStackTrace();
    }
  }
  
  public void contextDestroyed(ServletContextEvent servletContextEvent) {
    EPServiceProvider epServiceProvider = EPServiceProviderManager.getDefaultProvider();
    for (String deploymentId : deploymentIds) {
       epServiceProvider.getEPAdministrator().getDeploymentAdmin().undeployRemove(deploymentId);
    }
  }
}

This chapter summarizes integration and describes in detail each of the extension APIs that allow integrating external data and/or extend engine functionality.

For information on input and output adapters that connect to an event transport and perform event transformation for incoming and outgoing on-the-wire event data, for use with streaming data, please see the EsperIO reference documentation. The data flow instances as described in Chapter 13, EPL Reference: Data Flow are an easy way to plug in operators that perform input and output. Data flows allow providing parameters and managing individual flows independent of engine lifecycle. Also consider using the Plug-in Loader API for creating a new adapter that starts or stops as part of the CEP engine initialization and destroy lifecycle, see Section 14.17, “Plug-in Loader”.

To join data that resides in a relational database and that is accessible via JDBC driver and SQL statement the engine offers a syntax for using SQL within EPL, see Section 5.13, “Accessing Relational Data via SQL”. A relational database input and output adapter for streaming input from and output to a relational database also exists (EsperIO).

To join data that resides in a non-relational store the engine offers a two means: First, the virtual data window, as described below, for transparently integrating the external store as a named window. The second mechanism is a special join syntax based on static method invocation, see Section 5.14, “Accessing Non-Relational Data via Method Invocation”.

Tip

The best way to test that your extension code works correctly is to write unit tests against an EPL statement that utilizes the extension code. Samples can be obtained from Esper regression test code base.

Note

For all extension code and similar to listeners and subscribers, to send events into the engine from extension code the route method should be used (and not sendEvent) to avoid the possibility of stack overflow due to event-callback looping and ensure correct processing of the current and routed event .

Note

For all extension code it is not safe to administrate the engine within the extension code. For example, it is not safe to implement a data window view that creates a new statement or destroys an existing statement.

Use a virtual data window if you have a (large) external data store that you want to access as a named window. The access is transparent: There is no need to use special syntax or join syntax. All regular queries including subqueries, joins, on-merge, on-select, on-insert, on-delete, on-update and fire-and-forget are supported with virtual data windows.

There is no need to keep any data or events in memory with virtual data windows. The only requirement for virtual data windows is that all data rows returned are EventBean instances.

When implementing a virtual data window it is not necessary to send any events into the engine or to use insert-into. The event content is simply assumed to exist and accessible to the engine via the API implementation you provide.

The distribution ships with a sample virtual data window in the examples folder under the name virtualdw. The code snippets below are extracts from the example.

We use the term store here to mean a source set of data that is managed by the virtual data window. We use the term store row or just row to mean a single data item provided by the store. We use the term lookup to mean a read operation against the store returning zero, one or many rows.

Virtual data windows allow high-performance low-latency lookup by exposing all relevant EPL query access path information. This makes it possible for the virtual data window to choose the desired access method into its store.

The following steps are required to develop and use a virtual data window:

Once you have completed above steps, the virtual data window is ready to use in EPL statements.

From a threading perspective, virtual data window implementation classes must be thread-safe if objects are shared between multiple named windows. If no objects are shared between multiple different named windows, thereby each object is only used for the same named window and other named windows receive a separate instance, it is no necessary that the implementation classes are thread-safe.

Your application must first register the virtual data window factory as part of engine configuration:

Configuration config = new Configuration();
config.addPlugInVirtualDataWindow("sample", "samplevdw", 
    SampleVirtualDataWindowFactory.class.getName());

Your application may then create a named window backed by a virtual data window.

For example, assume that the SampleEvent event type is declared as follows:

create schema SampleEvent as (key1 string, key2 string, value1 int, value2 double)

The next EPL statement creates a named window MySampleWindow that provides SampleEvent events and is backed by a virtual data window provided by SampleVirtualDataWindowFactory as configured above:

create window MySampleWindow.sample:samplevdw() as SampleEvent

You may then access the named window, same as any other named window, for example by subquery, join, on-action, fire-and-forget query or by consuming its insert and remove stream. While this example uses Map-type events, the example code is the same for POJO or other events.

Your application may obtain a reference to the virtual data window from the engine context.

This code snippet looks up the virtual data window by the named window name:

try {
  return (VirtualDataWindow) epService.getContext().lookup("/virtualdw/MySampleWindow");
}
catch (NamingException e) {
  throw new RuntimeException("Failed to look up virtual data window, is it created yet?");
}

When you application registers a subquery, join or on-action query or executes a fire-and-forget query against a virtual data window the engine interacts with the virtual data window. The interaction is a two-step process.

At time of EPL statement creation (once), the engine analyzes the EPL where-clause, if present. It then compiles a list of hash-index and binary tree (btree, i.e. sorted) index properties. It passes the property names that are queried as well as the operators (i.e. =, >, range etc.) to the virtual data window. The virtual data window returns a lookup strategy object to the engine.

At time of EPL statement execution (repeatedly as triggered) , the engine uses that lookup strategy object to execute a lookup. It passes to the lookup all actual key values (hash, btree including ranges) to make fast and efficient lookup achievable.

To explain in detail, assume that your application creates an EPL statement with a subquery as follows:

select (select * from MySampleWindow where key1 = 'A1') from OtherEvent

At the time of creation of the EPL query above the engine analyzes the EPL query. It determines that the subquery queries a virtual data window. It determines from the where-clause that the lookup uses property key1 and hash-equals semantics. The engine then provides this information as part of VirtualDataWindowLookupContext passed to the getLookup method. Your application may inspect hash and btree properties and may determine the appropriate store access method to use.

The hash and btree property lookup information is for informational purposes, to enable fast and performant queries that returns the smallest number of rows possible. Your implementation classes may use some or none of the information provided and may also instead return some or perhaps even all rows, as is practical to your implementation. The where-clause still remains in effect and gets evaluated on all rows that are returned by the lookup strategy.

Following the above example, the sub-query executes once when a OtherEvent event arrives. At time of execution the engine delivers the string value A1 to the VirtualDataWindowLookup lookup implementation provided by your application. The lookup object queries the store and returns store rows as EventBean instances.

As a second example, consider an EPL join statement as follows:

select * from MySampleWindow, MyTriggerEvent where key1 = trigger1 and key2 = trigger2

The engine analyzes the query and passes to the virtual data window the information that the lookup occurs on properties key1 and key2 under hash-equals semantics. When a MyTriggerEvent arrives, it passes the actual value of the trigger1 and trigger2 properties of the current MyTriggerEvent to the lookup.

As a last example, consider an EPL fire-and-forget statement as follows:

select * from MySampleWindow key1 = 'A2' and value1 between 0 and 1000

The engine analyzes the query and passes to the virtual data window the lookup information. The lookup occurs on property key1 under hash-equals semantics and on property value1 under btree-open-range semantics. When you application executes the fire-and-forget query the engine passes A2 and the range endpoints 0 and 1000 to the lookup.

For more information, please consult the JavaDoc API documentation for class com.espertech.esper.client.hook.VirtualDataWindow, VirtualDataWindowLookupContext or VirtualDataWindowLookupFieldDesc.

For each named window that refers to the virtual data window, the engine instantiates one instance of the factory.

A virtual data window factory class is responsible for the following functions:

The engine instantiates a VirtualDataWindowFactory instance for each named window created via create window. The engine invokes the initialize method once in respect to the named window being created passing a VirtualDataWindowFactoryContext context object.

If not using contexts, the engine calls the create method once after calling the initialize method. If using contexts, the engine calls the create method every time it allocates a context partition. If using contexts and your virtual data window implementation operates thread-safe, you may return the same virtual data window implementation object for each context partition. If using contexts and your implementation object is not thread safe, return a separate thread-safe implementation object for each context partition.

The engine invokes the destroyAllContextPartitions once when the named window is stopped or destroyed. If not using contexts, the engine calls the destroy method of the virtual data window implementation object before calling the destroyAllContextPartitions method on the factory object. If using contexts, the engine calls the destroy method on each instance associates to a context partition at the time when the associated context partition terminates.

The sample code shown here can be found among the examples in the distribution under virtualdw:

public class SampleVirtualDataWindowFactory implements VirtualDataWindowFactory {

    public void initialize(VirtualDataWindowFactoryContext factoryContext) {
		// Can add initialization logic here.
    }

  public VirtualDataWindow create(VirtualDataWindowContext context) {
    // This example allocates a new virtual data window (one per context partitions if using contexts).
    // For sharing the virtual data window instance between context partitions, return the same reference.
    return new SampleVirtualDataWindow(context);
  }
  
  public void destroyAllContextPartitions() {
    // Release shared resources here
  }
}

Your factory class must implement the create method which receives a VirtualDataWindowContext object. This method is called once for each EPL that creates a virtual data window (see example create window above).

The VirtualDataWindowContext provides to your application:

String namedWindowName;	// Name of named window being created.
Object[] parameters;  // Any optional parameters provided as part of create-window.
EventType eventType;  // The event type of events.
EventBeanFactory eventFactory;  // A factory for creating EventBean instances from store rows.
VirtualDataWindowOutStream outputStream;  // For stream output to consuming statements.
AgentInstanceContext agentInstanceContext;  // Other EPL statement information in statement context.

When using contexts you can decide whether your factory returns a new virtual data window for each context partition or returns the same virtual data window instance for all context partitions. Your extension code may refer to the named window name to identify the named window and may refer to the agent instance context that holds the agent instance id which is the id of the context partition.

A virtual data window implementation is responsible for the following functions:

The sample code shown here can be found among the examples in the distribution under virtualdw.

The implementation class must implement the VirtualDataWindow interface like so:

public class SampleVirtualDataWindow implements VirtualDataWindow {

  private final VirtualDataWindowContext context;
  
  public SampleVirtualDataWindow(VirtualDataWindowContext context) {
    this.context = context;
  } ...

When the engine compiles an EPL statement and detects a virtual data window, the engine invokes the getLookup method indicating hash and btree access path information by passing a VirtualDataWindowLookupContext context. The lookup method must return a VirtualDataWindowLookup implementation that the EPL statement uses for all lookups until the EPL statement is stopped or destroyed.

The sample implementation does not use the hash and btree access path information and simply returns a lookup object:

public VirtualDataWindowLookup getLookup(VirtualDataWindowLookupContext desc) {

  // Place any code that interrogates the hash-index and btree-index fields here.

  // Return the lookup strategy.
  return new SampleVirtualDataWindowLookup(context);
}

If your virtual data window returns null instead of a lookup object, the EPL query creation fails and throws an EPStatementException.

The engine calls the update method when data changes because of on-merge, on-delete, on-update or insert-into. For example, if you have an on-merge statement that is triggered and that updates the virtual data window, the newData parameter receives the new (updated) event and the oldData parameters receives the event prior to the update. Your code may use these events to update the store or delete from the store, if needed.

If your application plans to consume data from the virtual data window, for example via select * from MySampleWindow, then the code must implement the update method to forward insert and remove stream events, as shown below, to receive the events in consuming statements. To post insert and remove stream data, use the VirtualDataWindowOutStream provided by the context object as follows.

public void update(EventBean[] newData, EventBean[] oldData) {
  // This sample simply posts into the insert and remove stream what is received.
  context.getOutputStream().update(newData, oldData);
}

Your application should not use VirtualDataWindowOutStream to post new events that originate from the store. The object is intended for use with on-action EPL statements. Use insert-into instead for any new events that originate from the store.

A lookup implementation is responsible for the following functions:

The sample code shown here can be found among the examples in the distribution under virtualdw.

The implementation class must implement the VirtualDataWindowLookup interface:

public class SampleVirtualDataWindowLookup implements VirtualDataWindowLookup {

  private final VirtualDataWindowContext context;
  
  public SampleVirtualDataWindowLookup(VirtualDataWindowContext context) {
    this.context = context;
  } ...

When an EPL query fires, the engine invokes the lookup and provides the actual lookup values. The lookup values are provided in the same exact order as the access path information that the engine provided when obtaining the lookup.

Each store row must be wrapped as an EventBean instance. The context object provides an EventBeanFactory implementation returned by getEventFactory() that can be used to wrap rows.

The sample implementation does not use the lookup values and simply returns a hardcoded sample event:

public Set<EventBean> lookup(Object[] lookupValues) {
  // Add code to interogate lookup values here.

  // Create sample event.
  // This example uses Map events; Other underlying events such as POJO are exactly the same code.
  Map<String, Object> eventData = new HashMap<String, Object>();
  eventData.put("key1", "sample1");
  eventData.put("key2", "sample2");
  eventData.put("value1", 100);
  eventData.put("value2", 1.5d);
  EventBean event = context.getEventFactory().wrap(eventData);
  return Collections.singleton(event);
}

The lookupValues object array represents all actual joined property values or expression results if you where-clause criteria are expressions. The code may use these keys to for efficient store access.

When a key value is a range, the key value is an instance of VirtualDataWindowKeyRange.

Single-row functions return a single value. They are not expected to aggregate rows but instead should be stateless functions. These functions can appear in any expressions and can be passed any number of parameters.

The following steps are required to develop and use a custom single-row function with Esper.

  1. Implement a class providing one or more public static methods accepting the number and type of parameters as required.

  2. Register the single-row function class and method name with the engine by supplying a function name, via the engine configuration file or the configuration API.

You may not override a built-in function with a single-row function provided by you. The single-row function you register must have a different name then any of the built-in functions.

An example single-row function can also be found in the examples under the runtime configuration example.

Views in Esper are used to derive information from an event stream, and to represent data windows onto an event stream. This chapter describes how to plug-in a new, custom view.

The following steps are required to develop and use a custom view with Esper.

  1. Implement a view factory class. View factories are classes that accept and check view parameters and instantiate the appropriate view class.

  2. Implement a view class. A view class commonly represents a data window or derives new information from a stream.

  3. Configure the view factory class supplying a view namespace and name in the engine configuration file.

The example view factory and view class that are used in this chapter can be found in the examples source folder in the OHLC (open-high-low-close) example. The class names are OHLCBarPlugInViewFactory and OHLCBarPlugInView.

Views can make use of the following engine services available via StatementServiceContext:

  • The SchedulingService interface allows views to schedule timer callbacks to a view

  • The EventAdapterService interface allows views to create new event types and event instances of a given type.

  • The StatementStopService interface allows view to register a callback that the engine invokes to indicate that the view's statement has been stopped

Section 17.4.3, “View Contract” outlines the requirements for correct behavior of a your custom view within the engine.

Note that custom views may use engine services and APIs that can be subject to change between major releases. The engine services discussed above and view APIs are considered part of the engine internal public API and are stable. Any changes to such APIs are disclosed through the release change logs and history. Please also consider contributing your custom view to the Esper project team by submitting the view code through the mailing list or via a JIRA issue.

A view factory class is responsible for the following functions:

View factory classes simply subclass com.espertech.esper.view.ViewFactorySupport:

public class OHLCBarPlugInViewFactory extends ViewFactorySupport { ...

Your view factory class must implement the setViewParameters method to accept and parse view parameters. The next code snippet shows an implementation of this method. The code checks the number of parameters and retains the parameters passed to the method:

public class OHLCBarPlugInViewFactory extends ViewFactorySupport {
    private ViewFactoryContext viewFactoryContext;
    private List<ExprNode> viewParameters;
    private ExprNode timestampExpression;
    private ExprNode valueExpression;

    public void setViewParameters(ViewFactoryContext viewFactoryContext, 
            List<ExprNode> viewParameters) throws ViewParameterException {
        this.viewFactoryContext = viewFactoryContext;
        if (viewParameters.size() != 2) {
            throw new ViewParameterException(
                "View requires a two parameters: " +
                "the expression returning timestamps and the expression supplying OHLC data points");
        }
        this.viewParameters = viewParameters;
    }
  ...

After the engine supplied view parameters to the factory, the engine will ask the view to attach to its parent view and validate any parameter expressions against the parent view's event type. If the view will be generating events of a different type then the events generated by the parent view, then the view factory can create the new event type in this method:

public void attach(EventType parentEventType, 
		StatementContext statementContext, 
		ViewFactory optionalParentFactory, 
		List<ViewFactory> parentViewFactories) throws ViewParameterException {
		
    ExprNode[] validatedNodes = ViewFactorySupport.validate("OHLC view", 
	      parentEventType, statementContext, viewParameters, false);

    timestampExpression = validatedNodes[0];
    valueExpression = validatedNodes[1];

    if ((timestampExpression.getExprEvaluator().getType() != long.class) && 
        (timestampExpression.getExprEvaluator().getType() != Long.class)) {
        throw new ViewParameterException(
            "View requires long-typed timestamp values in parameter 1");
    }
    if ((valueExpression.getExprEvaluator().getType() != double.class) && 
        (valueExpression.getExprEvaluator().getType() != Double.class)) {
        throw new ViewParameterException(
            "View requires double-typed values for in parameter 2");
    }
}

Finally, the engine asks the view factory to create a view instance, and asks for the type of event generated by the view:

public View makeView(AgentInstanceViewFactoryChainContext agentInstanceViewFactoryContext) {
    return new OHLCBarPlugInView(agentInstanceViewFactoryContext, timestampExpression, valueExpression);
}

public EventType getEventType() {
    return OHLCBarPlugInView.getEventType(viewFactoryContext.getEventAdapterService());
}

The update method must adhere to the following conventions, to prevent memory leaks and to enable correct behavior within the engine:

Your view implementation can register a callback indicating when a statement using the view, or a context partition using the view, is stopped or terminated. Your view code must implement, or provide an implementation, of the com.espertech.esper.util.StopCallback interface. Register the stop callback in order for the engine to invoke the callback:

agentInstanceContext.getTerminationCallbacks().add(this);

Please refer to the sample views for a code sample on how to implement iterator and cloneView methods.

In terms of multiple threads accessing view state, there is no need for your custom view factory or view implementation to perform any synchronization to protect internal state. The iterator of the custom view implementation does also not need to be thread-safe. The engine ensures the custom view executes in the context of a single thread at a time. If your view uses shared external state, such external state must be still considered for synchronization when using multiple threads.

Aggregation functions are stateful functions that aggregate events, event property values or expression results. Examples for built-in aggregation functions are count(*), sum(price * volume), window(*) or maxby(volume).

Esper allows two different ways for your application to provide aggregation functions. We use the name aggregation single-function and aggregation multi-function for the two independent extension APIs for aggregation functions.

The aggregation single-function API is simple to use however it imposes certain restrictions on how expressions that contain aggregation functions share state and are evaluated.

The aggregation multi-function API is more powerful and provides control over how expressions that contain aggregation functions share state and are evaluated.

The next table compares the two aggregation function extension API's:


The following sections discuss developing an aggregation single-function first, followed by the subject of developing an aggregation multi-function.

Note

The aggregation multi-function API is a powerful and lower-level API to extend the engine. Any classes that are not part of the client, plugin or agg.access package are subject to change between minor and major releases of the engine.

This section describes the aggregation single-function extension API for providing aggregation functions.

The following steps are required to develop and use a custom aggregation single-function with Esper.

The optional keyword distinct ensures that only distinct (unique) values are aggregated and duplicate values are ignored by the aggregation function. Custom plug-in aggregation single-functions do not need to implement the logic to handle distinct values. This is because when the engine encounters the distinct keyword, it eliminates any non-distinct values before passing the value for aggregation to the custom aggregation single-function.

Custom aggregation functions can also be passed multiple parameters, as further described in Section 17.5.1.4, “Aggregation Single-Function: Accepting Multiple Parameters”. In the example below the aggregation function accepts a single parameter.

The code for the example aggregation function as shown in this chapter can be found in the runtime configuration example in the package com.espertech.esper.example.runtimeconfig by the name MyConcatAggregationFunction. The sample function simply concatenates string-type values.

An aggregation function factory class is responsible for the following functions:

Aggregation function classes implement the interface com.espertech.esper.client.hook.AggregationFunctionFactory:

public class MyConcatAggregationFunctionFactory implements AggregationFunctionFactory { ...

The engine generally constructs one instance of the aggregation function factory class for each time the function is listed in an EPL statement, however the engine may decide to reduce the number of aggregation class instances if it finds equivalent aggregations.

The aggregation function factory instance receives the aggregation function name via set setFunctionName method.

The sample concatenation function factory provides an empty setFunctionName method:

public void setFunctionName(String functionName) {
  // no action taken
}

An aggregation function factory must provide an implementation of the validate method that is passed a AggregationValidationContext validation context object. Within the validation context you find the result type of each of the parameters expressions to the aggregation function as well as information about constant values and data window use. Please see the JavaDoc API documentation for a comprehensive list of validation context information.

Since the example concatenation function requires string types, it implements a type check:

public void validate(AggregationValidationContext validationContext) {
  if ((validationContext.getParameterTypes().length != 1) ||
    (validationContext.getParameterTypes()[0] != String.class)) {
    throw new IllegalArgumentException("Concat aggregation requires a single parameter of type String");
  }
}

In order for the engine to validate the type returned by the aggregation function against the types expected by enclosing expressions, the getValueType must return the result type of any values produced by the aggregation function:

public Class getValueType() {
  return String.class;
}

Finally the factory implementation must provide a newAggregator method that returns instances of AggregationMethod. The engine invokes this method for each new aggregation state to be allocated.

public AggregationMethod newAggregator() {
  return new MyConcatAggregationFunction();
}

An aggregation function class is responsible for the following functions:

Aggregation function classes implement the interface AggregationMethod:

public class MyConcatAggregationFunction implements AggregationMethod { ...

The class that provides the aggregation and implements AggregationMethod does not have to be threadsafe.

The constructor initializes the aggregation function:

public class MyConcatAggregationFunction implements AggregationMethod {
  private final static char DELIMITER = ' ';
  private StringBuilder builder;
  private String delimiter;

  public MyConcatAggregationFunction() {
    builder = new StringBuilder();
    delimiter = "";
  }
  ...

In order for the engine to validate the type returned by the aggregation function against the types expected by enclosing expressions, the getValueType must return the result type of any values produced by the aggregation function:

public Class getValueType() {
  return String.class;
}

The enter method adds a datapoint to the current aggregation value. The example enter method shown below adds a delimiter and the string value to a string buffer:

public void enter(Object value) {
  if (value != null) {
    builder.append(delimiter);
    builder.append(value.toString());
    delimiter = String.valueOf(DELIMITER);
  }
}

Conversly, the leave method removes a datapoint from the current aggregation value. The example leave method removes from the string buffer:

public void leave(Object value) {
  if (value != null) {
    builder.delete(0, value.toString().length() + 1);
  }
}

Finally, the engine obtains the current aggregation value by means of the getValue method:

public Object getValue() {
  return builder.toString();
}

For on-demand queries the aggregation function must support resetting its value to empty or start values. Implement the clear function to reset the value as shown below:

public void clear() {
  builder = new StringBuilder();
  delimiter = "";
}

This section introduces the aggregation multi-function API. Please refer to the JavaDoc for more complete class and method-level documentation.

Among the Esper examples we provide an example use of the aggregation multi-function API in the example by name Cycle-Detect. Cycle-Detect takes incoming transaction events that have from-account and to-account fields. The example detects a cycle in the transactions between accounts in order to detect a possible transaction fraud. Please note that the graph and cycle detection logic of the example is not part of Esper: The example utilizes the jgrapht library.

In the Cycle-Detect example, the vertices of a graph are the account numbers. For example the account numbers Acct-1, Acct-2 and Acct-3. In the graph the edges are transaction events that identify a from-account and a to-account. An example edge is {from:Acct-1, to:Acct-2}. An example cycle is therefore in the three transactions {from:Acct-1, to:Acct-2}, {from:Acct-2, to:Acct-3} and {from:Acct-3, to:Acct-1}.

The code for the example aggregation multi-function as shown in this chapter can be found in the Cycle-Detect example in the package com.espertech.esper.example.cycledetect. The example provides two aggregation functions named cycledetected and cycleoutput:

In the Cycle-Detect example, the following statement utilizes the two functions cycledetected and cycleoutput that share the same graph state to detect a cycle among the last 1000 events:

@Name('CycleDetector') select cycleoutput() as cyclevertices
from TransactionEvent.win:length(1000)
having cycledetected(fromAcct, toAcct)

If instead the goal is to run graph cycle detection every 1 second (and not upon arrival of a new event), this sample EPL statement uses a pattern to trigger cycle detection:

@Name('CycleDetector')
select (select cycleoutput(fromAcct, toAcct) from TransactionEvent.win:length(1000)) as cyclevertices
from pattern [every timer:interval(1)]

The following steps are required to develop and use a custom aggregation multi-function with Esper.

An aggregation multi-function factory class is responsible for the following functions:

Aggregation multi-function factory classes implement the interface com.espertech.esper.plugin.PlugInAggregationMultiFunctionFactory:

public class CycleDetectorAggregationFactory implements PlugInAggregationMultiFunctionFactory { ...

The engine constructs a single instance of the aggregation multi-function factory class that is shared for all aggregation function expressions in a statement that have one of the function names provided in the configuration object.

The engine invokes the addAggregationFunction method at the time it parses an EPL statement or compiles a statement object model (SODA API). The method receives a declaration-time context object that provides the function name as well as additional information.

The sample Cycle-Detect factory class provides an empty addAggregationFunction method:

public void addAggregationFunction(PlugInAggregationMultiFunctionDeclarationContext declarationContext) {
  // no action taken
}

The engine invokes the validateGetHandler method at the time of expression validation. It passes a PlugInAggregationMultiFunctionValidationContext validation context object that contains actual parameters expressions. Please see the JavaDoc API documentation for a comprehensive list of validation context information.

The validateGetHandler method must return a handler object the implements the PlugInAggregationMultiFunctionHandler interface. Return a handler object for each aggregation function expression according to the aggregation function name and its parameters that are provided in the validation context.

The example cycledetect function takes two parameters that provide the cycle edge (from-account and to-account):

public PlugInAggregationMultiFunctionHandler validateGetHandler(PlugInAggregationMultiFunctionValidationContext validationContext) {
  if (validationContext.getParameterExpressions().length == 2) {
    fromExpression = validationContext.getParameterExpressions()[0].getExprEvaluator();
    toExpression = validationContext.getParameterExpressions()[1].getExprEvaluator();
  }
  return new CycleDetectorAggregationHandler(this, validationContext);
}

An aggregation multi-function handler class must implement the PlugInAggregationMultiFunctionHandler interface and is responsible for the following functions:

Typically your API returns a handler instance for each aggregation function in an EPL statement expression.

In the Cycle-Detect example, the class CycleDetectorAggregationHandler is the handler for all aggregation functions.

public class CycleDetectorAggregationHandler implements PlugInAggregationMultiFunctionHandler { ...

The getAccessor methods return a reader object according to whether the aggregation function name is cycledetected or cycleoutput:

public AggregationAccessor getAccessor() {
  if (validationContext.getFunctionName().toLowerCase().equals(CycleDetectorConstant.CYCLEOUTPUT_NAME)) {
    return new CycleDetectorAggregationAccessorOutput();
  }
  return new CycleDetectorAggregationAccessorDetect();
}

The getReturnType method provided by the handler instructs the engine about the return type of each aggregation accessor. The class com.espertech.esper.client.util.ExpressionReturnType holds return type information.

In the Cycle-Detect example the cycledetected function returns a single boolean value. The cycleoutput returns a collection of vertices:

public ExpressionReturnType getReturnType() {
  if (validationContext.getFunctionName().toLowerCase().equals(CycleDetectorConstant.CYCLEOUTPUT_NAME)) {
    return ExpressionReturnType.collectionOfSingleValue(factory.getFromExpression().getType());
  }
  return ExpressionReturnType.singleValue(Boolean.class) ;
}

The engine invokes the getAggregationStateUniqueKey method to determine whether multiple aggregation function expressions in the same statement can share the same aggregation state or should receive different aggregation state instances.

The getAggregationStateUniqueKey method must return an instance of AggregationStateKey. The engine uses equals-semantics (the hashCode and equals methods) to determine whether multiple aggregation function share the state object. If the key object returned for each aggregation function by the handler is an equal key object then the engine shares aggregation state between such aggregation functions for the same statement and context partition.

In the Cycle-Detect example the state is shared, which it achieves by simply returning the same key instance:

private static final AggregationStateKey CYCLE_KEY = new AggregationStateKey() {};

public AggregationStateKey getAggregationStateUniqueKey() {
  return CYCLE_KEY;   // Share the same aggregation state instance
}

The engine invokes the getStateFactory method to obtain an instance of PlugInAggregationMultiFunctionStateFactory. The state factory is responsible to instantiating separate aggregation state instances. If you statement does not have a group by clause, the engine obtains a single aggregation state from the state factory. If your statement has a group by clause, the engine obtains an aggregation state instance for each group when it encounters a new group.

In the Cycle-Detect example the method passes the expression evaluators providing the from-account and to-account expressions to the state factory:

public PlugInAggregationMultiFunctionStateFactory getStateFactory() {
  return new CycleDetectorAggregationStateFactory(factory.getFromExpression(), factory.getToExpression());
}

Pattern guards are pattern objects that control the lifecycle of the guarded sub-expression, and can filter the events fired by the subexpression.

The following steps are required to develop and use a custom guard object with Esper.

  1. Implement a guard factory class, responsible for creating guard object instances.

  2. Implement a guard class.

  3. Register the guard factory class with the engine by supplying a namespace and name, via the engine configuration file or the configuration API.

The code for the example guard object as shown in this chapter can be found in the test source folder in the package com.espertech.esper.regression.client by the name MyCountToPatternGuardFactory. The sample guard discussed here counts the number of events occurring up to a maximum number of events, and end the sub-expression when that maximum is reached.

A guard factory class is responsible for the following functions:

Guard factory classes subclass com.espertech.esper.pattern.guard.GuardFactorySupport:

public class MyCountToPatternGuardFactory extends GuardFactorySupport { ...

The engine constructs one instance of the guard factory class for each time the guard is listed in a statement.

The guard factory class implements the setGuardParameters method that is passed the parameters to the guard as supplied by the statement. It verifies the guard parameters, similar to the code snippet shown next. Our example counter guard takes a single numeric parameter:

public void setGuardParameters(List<ExprNode> guardParameters, 
			MatchedEventConvertor convertor) throws GuardParameterException {
    String message = "Count-to guard takes a single integer-value expression as parameter";
    if (guardParameters.size() != 1) {
        throw new GuardParameterException(message);
    }

    if (guardParameters.get(0).getExprEvaluator().getType() != Integer.class) {
        throw new GuardParameterException(message);
    }

    this.numCountToExpr = guardParameters.get(0);
    this.convertor = convertor;
}

The makeGuard method is called by the engine to create a new guard instance. The example makeGuard method shown below passes the maximum count of events to the guard instance. It also passes a Quitable implementation to the guard instance. The guard uses Quitable to indicate that the sub-expression contained within must stop (quit) listening for events.

public Guard makeGuard(PatternAgentInstanceContext context,
      MatchedEventMap beginState, 
      Quitable quitable, 
      Object stateNodeId, 
      Object guardState) {
      
    Object parameter = PatternExpressionUtil.evaluate("Count-to guard", 
        beginState, numCountToExpr, convertor);
    if (parameter == null) {
        throw new EPException("Count-to guard parameter evaluated to a null value");
    }

    Integer numCountTo = (Integer) parameter;
    return new MyCountToPatternGuard(numCountTo, quitable);
}

Pattern observers are pattern objects that are executed as part of a pattern expression and can observe events or test conditions. Examples for built-in observers are timer:at and timer:interval. Some suggested uses of observer objects are:

  • Implement custom scheduling logic using the engine's own scheduling and timer services

  • Test conditions related to prior events matching an expression

The following steps are required to develop and use a custom observer object within pattern statements:

  1. Implement an observer factory class, responsible for creating observer object instances.

  2. Implement an observer class.

  3. Register an observer factory class with the engine by supplying a namespace and name, via the engine configuration file or the configuration API.

The code for the example observer object as shown in this chapter can be found in the test source folder in package com.espertech.esper.regression.client by the name MyFileExistsObserver. The sample observer discussed here very simply checks if a file exists, using the filename supplied by the pattern statement, and via the java.io.File class.

An observer factory class is responsible for the following functions:

Observer factory classes subclass com.espertech.esper.pattern.observer.ObserverFactorySupport:

public class MyFileExistsObserverFactory extends ObserverFactorySupport { ...

The engine constructs one instance of the observer factory class for each time the observer is listed in a statement.

The observer factory class implements the setObserverParameters method that is passed the parameters to the observer as supplied by the statement. It verifies the observer parameters, similar to the code snippet shown next. Our example file-exists observer takes a single string parameter:

public void setObserverParameters(List<ExprNode> expressionParameters, 
			MatchedEventConvertor convertor) throws ObserverParameterException {
    String message = "File exists observer takes a single string filename parameter";
    if (expressionParameters.size() != 1) {
	    throw new ObserverParameterException(message);
    }
    if (!(expressionParameters.get(0).getExprEvaluator().getType() == String.class)) {
	    throw new ObserverParameterException(message);
    }

    this.filenameExpression = expressionParameters.get(0);
    this.convertor = convertor;
}

The pattern engine calls the makeObserver method to create a new observer instance. The example makeObserver method shown below passes parameters to the observer instance:

public EventObserver makeObserver(PatternAgentInstanceContext context,
			MatchedEventMap beginState, 
			ObserverEventEvaluator observerEventEvaluator, 
			Object stateNodeId, 
			Object observerState) {
    Object filename = PatternExpressionUtil.evaluate("File-exists observer ", beginState, filenameExpression, convertor);
    if (filename == null) {
	    throw new EPException("Filename evaluated to null");
    }

    return new MyFileExistsObserver(beginState, observerEventEvaluator, filename.toString());
}

The ObserverEventEvaluator parameter allows an observer to indicate events, and to indicate change of truth value to permanently false. Use this interface to indicate when your observer has received or witnessed an event, or changed it's truth value to true or permanently false.

The MatchedEventMap parameter provides a Map of all matching events for the expression prior to the observer's start. For example, consider a pattern as below:

a=MyEvent -> myplugin:my_observer(...)

The above pattern tagged the MyEvent instance with the tag "a". The pattern engine starts an instance of my_observer when it receives the first MyEvent. The observer can query the MatchedEventMap using "a" as a key and obtain the tagged event.

Creating a plug-in event representation can be useful under any of these conditions:

  • Your application has existing Java classes that carry event metadata and event property values and your application does not want to (or cannot) extract or transform such event metadata and event data into one of the built-in event representations (POJO Java objects, Map or XML DOM).

  • Your application wants to provide a faster or short-cut access path to event data, for example to access XML event data through a Streaming API for XML (StAX).

  • Your application must perform a network lookup or other dynamic resolution of event type and events.

Note that the classes to plug-in custom event representations are held stable between minor releases, but can be subject to change between major releases.

Currently, EsperIO provides the following additional event representations:

  • Apache Axiom provides access to XML event data on top of the fast Streaming API for XML (StAX).

The source code is available for these and they are therefore excellent examples for how to implement a plug-in event representation. Please see the EsperIO documentation for usage details.

Your application provides a plug-in event representation as an implementation of the com.espertech.esper.plugin.PlugInEventRepresentation interface. It registers the implementation class in the Configuration and at the same time provides a unique URI. This URI is called the root event representation URI. An example value for a root URI is type://xml/apacheaxiom/OMNode.

One can register multiple plug-in event representations. Each representation has a root URI. The root URI serves to divide the overall space of different event representations and plays a role in resolving event types and event objects.

There are two situations in an Esper engine instance asks an event representation for an event type:

The implementation of the PlugInEventRepresentation interface must provide implementations for two key interfaces: com.espertech.esper.client.EventType and EventBean. It must also implement several other related interfaces as described below.

The EventType methods provide event metadata including property names and property types. They also provide instances of EventPropertyGetter for retrieving event property values. Each instance of EventType represents a distinct type of event.

The EventBean implementation is the event itself and encapsulates the underlying event object.

Assume you have registered event representations using the following URIs:

When providing an array of child URIs for resolution, the engine compares each child URI to each of the event representation root URIs, in the order provided. Any event representation root URIs that spans the child URI space becomes a candidate event representation. If multiple root URIs match, the order is defined by the more specific root URI first, to the least specific root URI last.

During event type resolution and event sender resolution you provide a child URI. Assuming the child URI provided is type://myFormat/myProject/myName/myEvent?param1=abc&param2=true. In this example both root URIs #1 (the more specific) and #1 (the less specific) match, while root URI #3 is not a match. Thus at the time of type resolution the engine invokes the acceptType method on event presentation for URI #1 first (the more specific), before asking #2 (the less specific) to resolve the type.

The EventSender returned by the getEventSender(URI[]) method follows the same scheme. The event sender instance asks each matching event representation for each URI to resolve the event object in the order of most specific to least specific root URI, and the first event representation to return an instance of EventBean ends the resolution process for event objects.

The type:// part of the URI is an optional convention for the scheme part of an URI that your application may follow. URIs can also be simple names and can include parameters, as the Java software JavaDoc documents for class java.net.URI.

This section implements a minimal sample plug-in event representation. For the sake of keeping the example easy to understand, the event representation is rather straightforward: an event is a java.util.Properties object that consists of key-values pairs of type string.

The code shown next does not document method footprints. Please consult the JavaDoc API documentation for method details.

First, the sample shows how to implement the EventType interface. The event type provides information about property names and types, as well as supertypes of the event type.

Our EventType takes a set of valid property names:

public class MyPlugInPropertiesEventType implements EventType {
  private final Set<String> properties;

  public MyPlugInPropertiesEventType(Set<String> properties) {
    this.properties = properties;
  }

  public Class getPropertyType(String property) {
    if (!isProperty(property)) {
      return null;
    }
    return String.class;
  }

  public Class getUnderlyingType() {
    return Properties.class;
  }
  
  //... further methods below
}

An EventType is responsible for providing implementations of EventPropertyGetter to query actual events. The getter simply queries the Properties object underlying each event:

  public EventPropertyGetter getGetter(String property) {
    final String propertyName = property;
    
    return new EventPropertyGetter() {
      public Object get(EventBean eventBean) throws PropertyAccessException {
        MyPlugInPropertiesEventBean propBean = (MyPlugInPropertiesEventBean) eventBean;
        return propBean.getProperties().getProperty(propertyName);
      }
      
      public boolean isExistsProperty(EventBean eventBean) {
        MyPlugInPropertiesEventBean propBean = (MyPlugInPropertiesEventBean) eventBean;
        return propBean.getProperties().getProperty(propertyName) != null;
      }
      
      public Object getFragment(EventBean eventBean) {
	    return null;	// The property is not a fragment
      }
    };
  }

Our sample EventType does not have supertypes. Supertypes represent an extends-relationship between event types, and subtypes are expected to exhibit the same event property names and types as each of their supertypes combined:

  public EventType[] getSuperTypes() {
    return null;	// no supertype for this example
  }

  public Iterator<EventType> getDeepSuperTypes() {
    return null;
  }
  
  public String getName() {
    return name;
  }

  public EventPropertyDescriptor[] getPropertyDescriptors() {
    Collection<EventPropertyDescriptor> descriptorColl = descriptors.values();
    return descriptorColl.toArray(new EventPropertyDescriptor[descriptors.size()]);
  }

  public EventPropertyDescriptor getPropertyDescriptor(String propertyName) {
    return descriptors.get(propertyName);
  }

  public FragmentEventType getFragmentType(String property) {
    return null;  // sample does not provide any fragments
  }

The example event type as above does not provide fragments, which are properties of the event that can themselves be represented as an event, to keep the example simple.

A PlugInEventRepresentation serves to create EventType and EventBean instances through its related interfaces.

The sample event representation creates MyPlugInPropertiesEventType and MyPlugInPropertiesEventBean instances. The PlugInEventTypeHandler returns the EventType instance and an EventSender instance.

Our sample event representation accepts all requests for event types by returning boolean true on the acceptType method. When asked for the PlugInEventTypeHandler, it constructs a new EventType. The list of property names for the new type is passed as an initialization value provided through the configuration API or XML, as a comma-separated list of property names:

public class MyPlugInEventRepresentation implements PlugInEventRepresentation {

  private List<MyPlugInPropertiesEventType> types;

  public void init(PlugInEventRepresentationContext context) {
    types = new ArrayList<MyPlugInPropertiesEventType>();
  }

  public boolean acceptsType(PlugInEventTypeHandlerContext context) {
    return true;
  }

  public PlugInEventTypeHandler getTypeHandler(PlugInEventTypeHandlerContext eventTypeContext) {
    String proplist = (String) eventTypeContext.getTypeInitializer();
    String[] propertyList = proplist.split(",");

    Set<String> typeProps = new HashSet<String>(Arrays.asList(propertyList));

    MyPlugInPropertiesEventType eventType = new MyPlugInPropertiesEventType(typeProps);
    types.add(eventType);

    return new MyPlugInPropertiesEventTypeHandler(eventType);
  }
  // ... more methods below
}

The PlugInEventTypeHandler simply returns the EventType as well as an implementation of EventSender for processing same-type events:

public class MyPlugInPropertiesEventTypeHandler implements PlugInEventTypeHandler {
  private final MyPlugInPropertiesEventType eventType;

  public MyPlugInPropertiesEventTypeHandler(MyPlugInPropertiesEventType eventType) {
    this.eventType = eventType;
  }

  public EventSender getSender(EPRuntimeEventSender runtimeEventSender) {
    return new MyPlugInPropertiesEventSender(eventType, runtimeEventSender);
  }

  public EventType getType() {
    return eventType;
  }
}

The EventSender returned by PlugInEventTypeHandler is expected process events of the same type or any subtype:

public class MyPlugInPropertiesEventSender implements EventSender {
  private final MyPlugInPropertiesEventType type;
  private final EPRuntimeEventSender runtimeSender;

  public MyPlugInPropertiesEventSender(MyPlugInPropertiesEventType type, 
        EPRuntimeEventSender runtimeSender) {
    this.type = type;
    this.runtimeSender = runtimeSender;
  }

  public void sendEvent(Object event) {
    if (!(event instanceof Properties)) {
       throw new EPException("Sender expects a properties event");
    }
    EventBean eventBean = new MyPlugInPropertiesEventBean(type, (Properties) event);
    runtimeSender.processWrappedEvent(eventBean);
  }
}

The plug-in event representation may optionally provide an implementation of PlugInEventBeanFactory. A PlugInEventBeanFactory may inspect event objects and assign an event type dynamically based on resolution URIs and event properties.

Our sample event representation accepts all URIs and returns a MyPlugInPropertiesBeanFactory:

public class MyPlugInEventRepresentation implements PlugInEventRepresentation {

  // ... methods as seen earlier
  public boolean acceptsEventBeanResolution(
        PlugInEventBeanReflectorContext eventBeanContext) {
    return true;
  }

  public PlugInEventBeanFactory getEventBeanFactory(
        PlugInEventBeanReflectorContext eventBeanContext) {
    return new MyPlugInPropertiesBeanFactory(types);
   }
}

Last, the sample MyPlugInPropertiesBeanFactory implements the PlugInEventBeanFactory interface. It inspects incoming events and determines an event type based on whether all properties for that event type are present:

public class MyPlugInPropertiesBeanFactory implements PlugInEventBeanFactory {
  private final List<MyPlugInPropertiesEventType> knownTypes;

  public MyPlugInPropertiesBeanFactory(List<MyPlugInPropertiesEventType> types) {
    knownTypes = types;
  }

  public EventBean create(Object event, URI resolutionURI) {
    Properties properties = (Properties) event;

    // use the known types to determine the type of the object
    for (MyPlugInPropertiesEventType type : knownTypes) {
      // if there is one property the event does not contain, then its not the right type
      boolean hasAllProperties = true;
      for (String prop : type.getPropertyNames()) {
        if (!properties.containsKey(prop)) {
          hasAllProperties = false;
          break;
        }
      }

      if (hasAllProperties) {
        return new MyPlugInPropertiesEventBean(type, properties);
      }
    }
    return null; // none match, unknown event
  }
}

Esper allows the use scripting languages within EPL. You may use scripts for imperative programming to execute certain code as part of EPL processing by the engine.

The syntax and examples outlined below discuss how to declare a script that is visible to the same EPL statement that listed the script.

For declaring scripts that are visible across multiple EPL statements i.e. globally visible scripts please consult Section 5.19.2, “Declaring a Global Script” that explains the create expression clause.

Any scripting language that supports JSR 223 and also the MVEL scripting language can be specified in EPL. This section provides MVEL and JavaScript examples.

For more information on the MVEL scripting language and its syntax, please refer to MVEL At Codehaus. MVEL is an expression language that has a natural syntax for Java-based applications and compiles to provide fast execution times. To use MVEL with Esper, please make sure to add the MVEL jar file to the application classpath.

For more information on JSR 223 scripting languages, please refer to external resources. As JSR 223 defines a standard API for script engines, your application may use any script engine that implements the API. Current JVM versions ship with a JavaScript script engine. Other script engines such as Groovy, Ruby and Python scripts can be used as implementations of JSR 223.

As an alternative to a script consider providing a custom single row function as described in Section 17.3, “Single-Row Function”

The engine provides a built-in script object under the variable name epl to all scripts. Your scripts may use this script object to share and retain state by setting and reading script attributes.

The epl script object implements the interface com.espertech.esper.client.hook.EPLScriptContext. The EPLScriptContext interface has two methods: The void setScriptAttribute(String attribute, Object value) method to set an attribute value and the Object getScriptAttribute(String attribute) method to read an attribute value.

The engine maintains a separate script object per context partition, or per statement if not declaring a context. Therefore script attributes are not shared between statements.

The next example demonstrates the use of the epl script object. It outputs a flag value true when an RFID event matched because the location is A, and outputs a flag value false when an RFID event matched because the location is B. The example works the same for either MVEL or JavaScript dialects: You may simple replace the js dialect with mvel.

expression boolean js:setFlag(name, value, returnValue) [
  if (returnValue) epl.setScriptAttribute(name, value);
  returnValue;
]
expression js:getFlag(name) [
  epl.getScriptAttribute(name);
]
select getFlag('locA') as flag from RFIDEvent(zone = 'Z1' and
  (setFlag('locA', true, location = 'A') or setFlag('locA', false, location = 'B')) )

The example above utilizes two scripts: The setFlag script receives an attribute name, attribute value and a return value. The script sets the script attribute only when the return value is true. The getFlag script simply returns the script attribute value.

This chapter outlines the examples that come with Esper in the examples folder of the distribution. Each sample is in a separate folder that contains all files needed by the example, excluding jar files.

Here is an overview over the examples in alphabetical order:

Table 19.1. Examples

NameDescription
Section 19.3, “AutoID RFID Reader”

An array of RFID readers sense RFID tags as pallets are coming within the range of one of the readers.

Shows the use of an XSD schema and XML event representation. A single statement shows a rolling time window, a where-clause filter on a nested property and a group-by.

Section 19.6, “Market Data Feed Monitor”

Processes a raw market data feed and reports throughput statistics and detects when the data rate of a feed falls off unexpectedly.

Demonstrates a batch time window and a rolling time window with a having-clause. Multi-threaded example with a configurable number of threads and a simulator for generating feed data.

Section 19.12, “MatchMaker”

In the MatchMaker example every mobile user has an X and Y location and the task of the event patterns created by this example is to detect mobile users that are within proximity given a certain range, and for which certain properties match preferences.

Dynamically creates and removes event patterns that use range matching based on mobile user events received.

Section 19.13, “Named Window Query”

A mini-benchmark that handles temperature sensor events. The sample creates a named window and fills it with a large number of events. It then executes a large number of pre-defined queries as well as fire-and-forget queries and reports times.

Study this example if you are interested in named windows, Map event type representation, fire-and-forget queries as well as pre-defined queries via on-select, and the performance aspects.

Section 19.14, “Sample Virtual Data Window”

This example demonstrates the use of virtual data window to expose a (large) external data store, without any need to keep events in memory, and without sacrificing query performance.

Section 19.15, “Sample Cycle Detection”

This example showcases the aggregation multi-function extension API for use with a cycle-detection problem detecting cycles in transactions between accounts.

Section 19.7, “OHLC Plug-in View”

A plug-in custom view addressing a problem in the financial space: Computes open-high-low-close bars for minute-intervals of events that may arrive late, based on each event's timestamp.

A custom plug-in view based on the extension API can be a convenient and reusable way to express a domain-specific analysis problem as a unit, and this example includes the code for the OHLC view factory and view as well as simulator to test the view.

Section 20.3, “Using the performance kit”

A benchmark that is further described in the performance section of this document under performance kit.

Section 19.16, “Quality of Service”

This example develops some code for measuring quality-of-service levels such as for a service-level agreement (SLA).

This example combines patterns with select-statements, shows the use of the timer 'at' operator and followed-by operator ->, and uses the iterator API to poll for current results.

Section 19.10, “Assets Moving Across Zones - An RFID Example”

An example out of the RFID domain processes location report events. The example includes a simple Swing-based GUI for visualization allows moving tags from zone to zone visually. It also a contains comprehensive simulator to generate data for a large number of asset groups and their tracking.

The example hooks up statements that aggregate and detect patterns in the aggregated data to determine when an asset group constraint is violated.

Section 19.4, “Runtime Configuration”

Example code to demonstrate various key runtime configuration options such as adding event types on-the-fly, adding new variables, adding plug-in single-row and aggregation functions and adding variant streams and revision type definition.

Section 19.5, “JMS Server Shell and Client”

The server shell is a Java Messaging Service (JMS) -based server and client that send and listens to messages on a JMS destination. It also demonstrates a simple Java Management Extension (JMX) MBean for remote statement management.

A single EPL statement computes an average duration for each IP address on a rolling time window and outputs a snapshot every 2 seconds.

Section 19.11, “StockTicker”

An example from the financial domain that features event patterns to filter stock tick events based on price and symbol. The example is designed to provide a high volume of events and includes multithreaded unit test code as well as a simulting data generator.

Perhaps this is a good example to learn the API and get started with event patterns. The example dynamically creates and removes event patterns based on price limit events received.

Section 19.9, “Self-Service Terminal”

A J2EE-based self-service terminal managing system in an airport that gets a lot of events from connected terminals.

Contains a message-driven bean (EJB-MDB) for use in a J2EE container, a client and a simulator, as well as EPL statements for detecting various conditions. A version that runs outside of a J2EE container is also available.

Section 19.17, “Trivia Geeks Club”

Trivia Geeks Club demonstrates EPL for a scoring system computing scores in a trivia game.


In this example an array of RFID readers sense RFID tags as pallets are coming within the range of one of the readers. A reader generates XML documents with observation information such as reader sensor ID, observation time and tags observed. A statement computes the total number of tags per reader sensor ID within the last 60 seconds.

This example demonstrates how XML documents unmarshalled to org.w3c.dom.Node DOM document nodes can natively be processed by the engine without requiring Java object event representations. The example uses an XPath expression for an event property counting the number of tags observed by a sensor. The XML documents follow the AutoID (http://www.autoid.org/) organization standard.

The classes for this example can be found in package com.espertech.esper.example.autoid. As events are XML documents with no Java object representation, the example does not have event classes.

A simulator that can be run from the command line is also available for this example. The simulator generates a number of XML documents as specified by a command line argument and prints out the totals per sensor. Run "run_autoid.bat" (Windows) or "run_autoid.sh" (Unix) to start the AutoID simulator. Please see the readme file in the same folder for build instructions and command line parameters.

The code snippet below shows the simple statement to compute the total number of tags per sensor. The statement is created by class com.espertech.esper.example.autoid.RFIDTagsPerSensorStmt.

select ID as sensorId, sum(countTags) as numTagsPerSensor
from AutoIdRFIDExample.win:time(60 seconds)
where Observation[0].Command = 'READ_PALLET_TAGS_ONLY'
group by ID

The server shell is a Java Messaging Service (JMS) -based server that listens to messages on a JMS destination, and sends the received events into Esper. The example also demonstrates a Java Management Extension (JMX) MBean that allows remote dynamic statement management. This server has been designed to run with either Tibco (TM) Enterprise Messaging System (Tibco EMS), or with Apache ActiveMQ, controlled by a properties file.

The server shell has been created as an alternative to the EsperIO Spring JMSTemplate adapter. The server shell is a low-latency processor for byte messages. It employs JMS listeners to process message in multiple threads, this model reduces thread context switching for many JMS providers. The server is configurable and has been tested with two JMS providers. It consists of only 10 classes and is thus easy to understand.

The server shell sample comes with a client (server shell client) that sends events into the JMS-based server, and that also creates a statement on the server remotely through a JMX MBean proxy class.

The server shell classes for this example live in package com.espertech.esper.example.servershell. Configure the server to point to your JMS provider by changing the properties in the file servershell_config.properties in the etc folder. Make sure your JMS provider (ActiveMQ or Tibco EMS) is running, then run "run_servershell.bat" (Windows) or "run_servershell.sh" (Unix) to start the JMS server.

Start the server shell process first before starting the client, since the client also demonstrates remote statement management through JMX by attaching to the server process.

The client classes to the server shell can be found in package com.espertech.esper.example.servershellclient. The client shares the same configuration file as the server shell. Run "run_servershellclient.bat" (Windows) or "run_servershellclient.sh" (Unix) to start the JMS producer client that includes a JMX client as well.

This example processes a raw market data feed. It reports throughput statistics and detects when the data rate of a feed falls off unexpectedly. A rate fall-off may mean that the data is stale and we want to alert when there is a possible problem with the feed.

The classes for this example live in package com.espertech.esper.example.marketdatafeed. Run "run_mktdatafeed.bat" (Windows) or "run_mktdatafeed.sh" (Unix) in the examples/etc folder to start the market data feed simulator.

The classes for this example live in package com.espertech.esper.example.transaction. Run "run_txnsim.bat" (Windows) or "run_txnsim.sh" (Unix) to start the transaction simulator. Please see the readme file in the same folder for build instructions and command line parameters.

To make testing easier, standard and to demonstrate how the example works, the example is including an event generator. The generator generates events for a given number of transactions, using the following rules:

To make things harder, we don‘t want transaction events coming in order. This code ensures that they come completely out of order. To do this, we fill in a bucket with events and, when the bucket is full, we shuffle it. The buckets are sized so that some transactions‘ events will be split between buckets. So, you have a fairly randomized flow of events, representing the worst case from a big, distributed infrastructure.

The generator lets you change the size of the bucket (small, medium, large, larger, largerer). The larger the bucket size, the more events potentially come in between two events in a given transaction and so, the more the performance characteristics like buffers, hashes/indexes and other structures are put to the test as the bucket size increases.

The example is about a J2EE-based self-service terminal managing system in an airport that gets a lot of events from connected terminals. The event rate is around 500 events per second. Some events indicate abnormal situations such as 'paper low' or 'terminal out of order'. Other events observe activity as customers use a terminal to check in and print boarding tickets.

The example code in the distribution package implements a message-driven enterprise java bean (MDB EJB). We used an MDB as a convenient place for processing incoming events via a JMS message queue or topic. The example uses 2 JMS queues: One queue to receive events published by terminals, and a second queue to indicate situations detected via EPL statement and listener back to a receiving process.

This example has been packaged for deployment into a JBoss Java application server (see http://www.jboss.org) with default deployment configuration. JBoss is an open-source application server available under LGPL license. Of course the choice of application server does not indicate a requirement or preference for the use of Esper in a J2EE container. Other quality J2EE application servers are available and perhaps more suitable to run this example or a similar application.

The complete example code can be found in the "examples/terminalsvc" folder of the distribution. The standalone version that does not require a J2EE container is in "examples/terminalsvc-jse".

The example also contains an event simulator that generates meaningful events. The simulator can be run from the directory "examples/terminalsvc/etc" via the command "run_terminalsvc_sender.bat" (Windows) and "run_terminalsvc_sender.sh" (Linux). The event simulator generates a batch of at least 200 events every 1 second. Randomly, with a chance of 1 in 10 for each batch of events, the simulator generates either an OutOfOrder or a LowPaper event for a random terminal. Each batch the simulator generates 100 random terminal ids and generates a Checkin event for each. It then generates either a Cancelled or a Completed event for each. With a chance of 1 in 1000, it generates an OutOfOrder event instead of the Cancelled or Completed event for a terminal.

The event receiver listens to the MDB-outcoming queue for alerts and prints these out to console. The receiver can be run from the directory "examples/terminalsvc/etc" via the command "run_terminalsvc_receiver.bat" (Windows) and "run_terminalsvc_receiver.sh" (Linux). Before running please copy the jboss-client.jar file from your JBoss AS installation bin directory to the "terminalsvc/lib" folder.

The receiver and sender code use "guest" as user and "pass" as password. Add the "guest" user using the Jboss "add-user" script and assign the role "guest". Your JBoss server may need to start with "-c standalone-full.xml" to have the messaging subsystem available.

Add queue configurations to the messaging subsystem configuration as follows:

<jms-queue name="queue_a">
  <entry name="queue_a"/>
  <entry name="java:jboss/exported/jms/queue/queue_a"/>
</jms-queue>
<jms-queue name="queue_b">
  <entry name="queue_b"/>
  <entry name="java:jboss/exported/jms/queue/queue_b"/>
</jms-queue>

Disable persistence in the messaging subsystem for this example so we are not running out of disk space:

<persistence-enabled>false</persistence-enabled>

This example out of the RFID domain processes location report events. Each location report event indicates an asset id and the current zone of the asset. The example solves the problem that when a given set of assets is not moving together from zone to zone, then an alert must be fired.

Each asset group is tracked by 2 statements. The two statements to track a single asset group consisting of assets identified by asset ids {1, 2, 3} are as follows:

insert into CountZone_G1
select 1 as groupId, zone, count(*) as cnt
from LocationReport(assetId in 1, 2, 3).std:unique(assetId)
group by zone

select Part.zone from pattern [
  every Part=CountZone_G1(cnt in (1,2)) ->
    (timer:interval(10 sec)  and not CountZone_G1(zone=Part.zone, cnt in (0,3)))]

The classes for this example can be found in package com.espertech.esper.example.rfid.

This example provides a Swing-based GUI that can be run from the command line. The GUI allows drag-and-drop of three RFID tags that form one asset group from zone to zone. Each time you move an asset across the screen the example sends an event into the engine indicating the asset id and current zone. The example detects if within 10 seconds the three assets do not join each other within the same zone, but stay split across zones. Run "run_rfid_swing.bat" (Windows) or "run_rfid_swing.sh" (Unix) to start the example's Swing GUI.

The example also provides a simulator that can be run from the command line. The simulator generates a number of asset groups as specified by a command line argument and starts a number of threads as specified by a command line argument to send location report events into the engine. Run "run_rfid_sim.bat" (Windows) or "run_rfid_sim.sh" (Unix) to start the RFID location report event simulator. Please see the readme file in the same folder for build instructions and command line parameters.

This example develops some code for measuring quality-of-service levels such as for a service-level agreement (SLA). A SLA is a contract between 2 parties that defines service constraints such as maximum latency for service operations or error rates.

The example measures and monitors operation latency and error counts per customer and operation. When one of our operations oversteps these constraints, we want to be alerted right away. Additionally, we would like to have some monitoring in place that checks the health of our service and provides some information on how the operations are used.

Some of the constraints we need to check are:

The com.espertech.esper.example.qos_sla.events.OperationMeasurement event class with its latency and status properties is the main event used for the SLA analysis. The other event LatencyLimit serves to set latency limits on the fly.

The com.espertech.esper.example.qos_sla.monitor.AverageLatencyMonitor creates an EPL statement that computes latency statistics per customer and operation for the last 100 events. The DynaLatencySpikeMonitor uses an event pattern to listen to spikes in latency with dynamically set limits. The ErrorRateMonitor uses the timer 'at' operator in an event pattern that wakes up periodically and polls the error rate within the last 10 minutes. The ServiceHealthMonitor simply alerts when 3 errors occur, and the SpikeAndErrorMonitor alerts when a fixed latency is overstepped or an error status is reported.

Summary:

20.1. Performance Results
20.2. Performance Tips
20.2.1. Understand how to tune your Java virtual machine
20.2.2. Input and Output Bottlenecks
20.2.3. Theading
20.2.4. Select the underlying event rather than individual fields
20.2.5. Prefer stream-level filtering over where-clause filtering
20.2.6. Reduce the use of arithmetic in expressions
20.2.7. Remove Unneccessary Constructs
20.2.8. End Pattern Sub-Expressions
20.2.9. Consider using EventPropertyGetter for fast access to event properties
20.2.10. Consider casting the underlying event
20.2.11. Turn off logging and audit
20.2.12. Disable view sharing
20.2.13. Tune or disable delivery order guarantees
20.2.14. Use a Subscriber Object to Receive Events
20.2.15. Consider Data Flows
20.2.16. High-Arrival-Rate Streams and Single Statements
20.2.17. Subqueries versus Joins And Where-clause And Data Windows
20.2.18. Patterns and Pattern Sub-Expression Instances
20.2.19. Pattern Sub-Expression Instance Versus Data Window Use
20.2.20. The Keep-All Data Window
20.2.21. Statement Design for Reduced Memory Consumption - Diagnosing OutOfMemoryError
20.2.22. Performance, JVM, OS and hardware
20.2.23. Consider using Hints
20.2.24. Optimizing Stream Filter Expressions
20.2.25. Statement and Engine Metric Reporting
20.2.26. Expression Evaluation Order and Early Exit
20.2.27. Large Number of Threads
20.2.28. Context Partition Related Information
20.2.29. Prefer Constant Variables over Non-Constant Variables
20.2.30. Prefer Object-array Events
20.2.31. Composite or Compound Keys
20.2.32. Notes on Query Planning
20.2.33. Query Planning Index Hints
20.2.34. Measuring Throughput
20.2.35. Do not create the same EPL Statement X times
20.2.36. Comparing Single-Threaded and Multi-Threaded Performance
20.2.37. Incremental Versus Recomputed Aggregation for Named Window Events
20.2.38. When Does Memory Get Released
20.3. Using the performance kit
20.3.1. How to use the performance kit
20.3.2. How we use the performance kit

Esper has been highly optimized to handle very high throughput streams with very little latency between event receipt and output result posting. It is also possible to use Esper on a soft-real-time or hard-real-time JVM to maximize predictability even further.

This section describes performance best practices and explains how to assess Esper performance by using our provided performance kit.

We recommend using multiple threads to send events into Esper. We provide a test class below. Our test class does not use a blocking queue and thread pool so as to avoid a point of contention.

A sample code for testing performance with multiple threads is provided:

public class SampleClassThreading {

    public static void main(String[] args) throws InterruptedException {

        int numEvents = 1000000;
        int numThreads = 3;

        Configuration config = new Configuration();
        config.getEngineDefaults().getThreading()
          .setListenerDispatchPreserveOrder(false);
        config.getEngineDefaults().getThreading()
          .setInternalTimerEnabled(false);   // remove thread that handles time advancing
        EPServiceProvider engine = EPServiceProviderManager
          .getDefaultProvider(config);
        engine.getEPAdministrator().getConfiguration().addEventType(MyEvent.class);

        engine.getEPAdministrator().createEPL(
          "create context MyContext coalesce by consistent_hash_crc32(id) " +
          "from MyEvent granularity 64 preallocate");
        String epl = "context MyContext select count(*) from MyEvent group by id";
        EPStatement stmt = engine.getEPAdministrator().createEPL(epl);
        stmt.setSubscriber(new MySubscriber());

        Thread[] threads = new Thread[numThreads];
        CountDownLatch latch = new CountDownLatch(numThreads);

        int eventsPerThreads = numEvents / numThreads;
        for (int i = 0; i < numThreads; i++) {
            threads[i] = new Thread(
              new MyRunnable(latch, eventsPerThreads, engine.getEPRuntime()));
        }
        long startTime = System.currentTimeMillis();
        for (int i = 0; i < numThreads; i++) {
            threads[i].start();
        }

        latch.await(10, TimeUnit.MINUTES);
        if (latch.getCount() > 0) {
            throw new RuntimeException("Failed to complete in 10 minute");
        }
        long delta = System.currentTimeMillis() - startTime;
        System.out.println("Took " + delta + " millis");
    }

    public static class MySubscriber {
        public void update(Object[] args) {
        }
    }

    public static class MyRunnable implements Runnable {
        private final CountDownLatch latch;
        private final int numEvents;
        private final EPRuntime runtime;

        public MyRunnable(CountDownLatch latch, int numEvents, EPRuntime runtime) {
            this.latch = latch;
            this.numEvents = numEvents;
            this.runtime = runtime;
        }

        public void run() {
            Random r = new Random();
            for (int i = 0; i < numEvents; i++) {
                runtime.sendEvent(new MyEvent(r.nextInt(512)));
            }
            latch.countDown();
        }
    }

    public static class MyEvent {
        private final int id;

        public MyEvent(int id) {
            this.id = id;
        }

        public int getId() {
            return id;
        }
    }
}

We recommend using Java threads as above, or a blocking queue and thread pool with sendEvent() or alternatively we recommend configuring inbound threading if your application does not already employ threading. Esper provides the configuration option to use engine-level queues and threadpools for inbound, outbound and internal executions. See Section 14.7.1, “Advanced Threading” for more information.

We recommend the outbound threading if your listeners are blocking. For outbound threading also see the section below on tuning and disabling listener delivery guarantees.

If enabling advanced threading options keep in mind that the engine will maintain a queue and thread pool. There is additional overhead associated with entering work units into the queue, maintaining the queue and the hand-off between threads. The Java blocking queues are not necessarily fast on all JVM. It is not necessarily true that your application will perform better with any of the advanced threading options.

We found scalability better on Linux systems and running Java with -server and pinning threads to exclusive CPUs and after making sure CPUs are available on your system.

We recommend looking at LMAX Disruptor, an inter-thread messaging library.

Esper stream-level filtering is very well optimized, while filtering via the where-clause post any data windows is not optimized.

The same is true for named windows. If your application is only interested in a subset of named window data and such filters are not correlated to arriving events, place the filters into parenthesis after the named window name.

If your statement uses order by to order output events, consider removing order by unless your application does indeed require the events it receives to be ordered.

If your statement specifies group by but does not use aggregation functions, consider removing group by.

If your statement specifies group by but the filter criteria only allows one group, consider removing group by:

// Prefer:
select * from MarketData(symbol = 'GE') having sum(price) > 1000

// Don't use this since the filter specifies a single symbol:
select * from MarketData(symbol = 'GE') group by symbol having sum(price) > 1000

If your statement specifies the grouped data window std:groupwin but the window being grouped retains the same set of events regardless of grouping, remove std:groupwin:

// Prefer:
create window MarketDataWindow.win:keepall() as MarketDataEventType

// Don't use this, since keeping all events 
// or keeping all events per symbol is the same thing:
create window MarketDataWindow.std:groupwin(symbol).win:keepall() as MarketDataEventType

// Don't use this, since keeping the last 1-minute of events 
// or keeping 1-minute of events per symbol is the same thing:
create window MarketDataWindow.std:groupwin(symbol).win:time(1 min) as MarketDataEventType

It is not necessary to specify a data window for each stream.

// Prefer:
select * from MarketDataWindow

// Don't have a data window if just listening to events, prefer the above
select * from MarketDataWindow.std:lastevent()

If your statement specifies unique data window but the filter criteria only allows one unique criteria, consider removing the unique data window:

// Prefer:
select * from MarketDataWindow(symbol = 'GE').std:lastevent()

// Don't have a unique-key data window if your filter specifies a single value
select * from MarketDataWindow(symbol = 'GE').std:unique(symbol)

The EventPropertyGetter interface is useful for obtaining an event property value without property name table lookup given an EventBean instance that is of the same event type that the property getter was obtained from.

When compiling a statement, the EPStatement instance lets us know the EventType via the getEventType() method. From the EventType we can obtain EventPropertyGetter instances for named event properties.

To demonstrate, consider the following simple statement:

select symbol, avg(price) from Market group by symbol

After compiling the statement, obtain the EventType and pass the type to the listener:

EPStatement stmt = epService.getEPAdministrator().createEPL(stmtText);
MyGetterUpdateListener listener = new MyGetterUpdateListener(stmt.getEventType());

The listener can use the type to obtain fast getters for property values of events for the same type:

public class MyGetterUpdateListener implements StatementAwareUpdateListener {
    private final EventPropertyGetter symbolGetter;
    private final EventPropertyGetter avgPriceGetter;

    public MyGetterUpdateListener(EventType eventType) {
        symbolGetter = eventType.getGetter("symbol");
        avgPriceGetter = eventType.getGetter("avg(price)");
    }

Last, the update method can invoke the getters to obtain event property values:

    public void update(EventBean[] eventBeans, EventBean[] oldBeans, EPStatement epStatement, EPServiceProvider epServiceProvider) {
        String symbol = (String) symbolGetter.get(eventBeans[0]);
        long volume = (Long) volumeGetter.get(eventBeans[0]);
        // some more logic here
    }

If your application is not a multithreaded application, or you application is not sensitive to the order of delivery of result events to your application listeners, then consider disabling the delivery order guarantees the engine makes towards ordered delivery of results to listeners:

Configuration config = new Configuration();
config.getEngineDefaults().getThreading().setListenerDispatchPreserveOrder(false);

If your application is not a multithreaded application, or your application uses the insert into clause to make results of one statement available for further consuming statements but does not require ordered delivery of results from producing statements to consuming statements, you may disable delivery order guarantees between statements:

Configuration config = new Configuration();
config.getEngineDefaults().getThreading().setInsertIntoDispatchPreserveOrder(false);

Additional configuration options are available and described in the configuration section that specify timeout values and spin or thread context switching.

Esper logging will log the following informational message when guaranteed delivery order to listeners is enabled and spin lock times exceed the default or configured timeout : Spin wait timeout exceeded in listener dispatch. The respective message for delivery from insert into statements to consuming statements is Spin wait timeout exceeded in insert-into dispatch.

If your application sees messages that spin lock times are exceeded, your application has several options: First, disabling preserve order is an option. Second, ensure your listener does not perform (long-running) blocking operations before returning, for example by performing output event processing in a separate thread. Third, change the timeout value to a larger number to block longer without logging the message.

A context partition is associated with certain context partition state that consists of current aggregation values, partial pattern matches, data windows or other view state depending on whether your statement uses such constructs. When an engine receives events it updates context partition state under locking such that context partition state remains consistent under concurrent multi-threaded access.

For high-volume streams, the locking required to protected context partition state may slow down or introduce blocking for very high arrival rates of events that apply to the very same context partition and its state.

Your first choice should be to utilize a context that allows for multiple context partitions, such as the hash segmented context. The hash segmented context usually performs better compared to the keyed segmented context since in the keyed segmented context the engine must check whether a partition exists or must be created for a given key.

Your second choice is to split the statement into multiple statements that each perform part of the intended function or that each look for a certain subset of the high-arrival-rate stream. There is very little cost in terms of memory or CPU resources per statement, the engine can handle larger number of statements usually as efficiently as single statements.

For example, consider the following statement:

// less effective in a highly threaded environment 
select venue, ccyPair, side, sum(qty)
from CumulativePrice
where side='O'
group by venue, ccyPair, side

The engine protects state of each context partition by a separate lock for each context partition, as discussed in the API section. In highly threaded applications threads may block on a specific context partition. You would therefore want to use multiple context partitions.

Consider creating a keyed segmented context, for example:

create context MyContext partition by venue, ccyPair, side from CumulativePrice(side='O')

The keyed segmented context instructs the engine to employ a context partition per venue, ccyPair, side key combination. As locking is on the level of context partition, the locks taken by the engine are very fine grained allowing for highly concurrent processing.

The new statement that refers to the context as created above is:

context MyContext select venue, ccyPair, side, sum(qty) from CumulativePrice

For testing purposes or if your application controls concurrency, you may disable context partition locking, see Section 15.4.23.3, “Disable Locking”.

When joining streams the engine builds a product of the joined data windows based on the where clause. It analyzes the where clause at time of statement compilation and builds the appropriate indexes and query strategy. Avoid using expressions in the join where clause that require evaluation, such as user-defined functions or arithmatic expressions.

When joining streams and not providing a where clause, consider using the std:unique data window or std:lastevent data window to join only the last event or the last event per unique key(s) of each stream.

The sample query below can produce up to 5,000 rows when both data windows are filled and an event arrives for either stream:

// Avoid joins between streams with data windows without where-clause
select * from StreamA.win:length(100), StreamB.win:length(50)

Consider using a subquery, consider using separate statements with insert-into and consider providing a where clause to limit the product of rows.

Below examples show different approaches, that are not semantically equivalent, assuming that an MyEvent is defined with the properties symbol and value:

// Replace the following statement as it may not perform well
select a.symbol, avg(a.value), avg(b.value) 
from MyEvent.win:length(100) a, MyEvent.win:length(50) b

// Join with where-clause
select a.symbol, avg(a.value), avg(b.value) 
from MyEvent.win:length(100) a, MyEvent.win:length(50) b 
where a.symbol = b.symbol

// Unidirectional join with where-clause
select a.symbol, avg(b.value) 
from MyEvent unidirectional, MyEvent.win:length(50) b 
where a.symbol = b.symbol

// Subquery
select 
  (select avg(value) from MyEvent.win:length(100)) as avgA, 
  (select avg(value) from MyEvent.win:length(50)) as avgB,
  a.symbol
from MyEvent

// Since streams cost almost nothing, use insert-into to populate and a unidirectional join 
insert into StreamAvgA select symbol, avg(value) as avgA from MyEvent.win:length(100)
insert into StreamAvgB select symbol, avg(value) as avgB from MyEvent.win:length(50)
select a.symbol, avgA, avgB from StreamAvgA unidirectional, StreamAvgB.std:unique(symbol) b
where a.symbol = b.symbol

A join is multidirectionally evaluated: When an event of any of the streams participating in the join arrive, the join gets evaluated, unless using the unidirectional keyword. Consider using a subquery instead when evaluation only needs to take place when a certain event arrives:

// Rewrite this join since we don't need to join when a LoginSucceededWindow arrives
// Also rewrite because the account number always is the value 123.
select * from LoginSucceededWindow as l, WithdrawalWindow as w
where w.accountNumber = '123' and w.accountNumber = l.accountNumber

// Rewritten as a subquery, 
select *, (select * from LoginSucceededWindow where accountNumber=’123’) 
from WithdrawalWindow(accountNumber=’123’) as w

The every and repeat operators in patterns control the number of sub-expressions that are active. Each sub-expression can consume memory as it may retain, depending on the use of tags in the pattern, the matching events. A large number of active sub-expressions can reduce performance or lead to out-of-memory errors.

During the design of the pattern statement consider the use of timer:within to reduce the amount of time a sub-expression lives, or consider the not operator to end a sub-expression.

The examples herein assume an AEvent and a BEvent event type that have an id property that may correlate between arriving events of the two event types.

In the following sample pattern the engine starts, for each arriving AEvent, a new pattern sub-expression looking for a matching BEvent. Since the AEvent is tagged with a the engine retains each AEvent until a match is found for delivery to listeners or subscribers:

every a=AEvent -> b=BEvent(b.id = a.id)

One way to end a sub-expression is to attach a time how long it may be active.

The next statement ends sub-expressions looking for a matching BEvent 10 seconds after arrival of the AEvent event that started the sub-expression:

every a=AEvent -> (b=BEvent(b.id = a.id) where timer:within(10 sec))

A second way to end a sub-expression is to use the not operator. You can use the not operator together with the and operator to end a sub-expression when a certain event arrives.

The next statement ends sub-expressions looking for a matching BEvent when, in the order of arrival, the next BEvent that arrives after the AEvent event that started the sub-expression does not match the id of the AEvent:

every a=AEvent -> (b=BEvent(b.id = a.id) and not BEvent(b.id != a.id))

The every-distinct operator can be used to keep one sub-expression alive per one or more keys. The next pattern demonstrates an alternative to every-distinct. It ends sub-expressions looking for a matching BEvent when an AEvent arrives that matches the id of the AEvent that started the sub-expression:

every a=AEvent -> (b=BEvent(b.id = a.id) and not AEvent(b.id = a.id))

This section describes common sources of out-of-memory problems.

If using the keep-all data window please consider the information above. If using pattern statements please consider pattern sub-expression instantiation and lifetime as discussed prior to this section.

When using the group-by clause or std:groupwin grouped data windows please consider the hints as described below. Make sure your grouping criteria are fields that don't have an unlimited number of possible values or specify hints otherwise.

The std:unique unique data window can also be a source for error. If your uniqueness criteria include a field which is never unique the memory use of the data window can grow, unless your application deletes events.

When using the every-distinct pattern construct parameterized by distinct value expressions that generate an unlimited number of distinct values, consider specifying a time period as part of the parameters to indicate to the pattern engine how long a distinct value should be considered.

In a match-recognize pattern consider limiting the number of optional events if optional events are part of the data reported in the measures clause. Also when using the partition clause, if your partitioning criteria include a field which is never unique the memory use of the match-recognize pattern engine can grow.

A further source of memory use is when your application creates new statements but fails to destroy created statements when they are no longer needed.

In your application design you may also want to be conscious when the application listener or subscriber objects retain output data.

An engine instance, uniquely identified by an engine URI is a relatively heavyweight object. Optimally your application allocates only one or a few engine instances per JVM. A statement instance is associated to one engine instance, is uniquely identified by a statement name and is a medium weight object. We have seen applications allocate 100,000 statements easily. A statement's context partition instance is associated to one statement, is uniquely identified by a context partition id and is a light weight object. We have seen applications allocate 5000 context partitions for 100 statements easily, i.e. 5,000,000 context partitions. An aggregation row, data window row, pattern etc. is associated to a statement context partition and is a very lightweight object itself.

The prev, prevwindow and prevtail functions access a data window directly. The engine does not need to maintain a separate data structure and grouping is based on the use of the std:groupwin grouped data window. Compare this to the use of event aggregation functions such as first, window and last which group according to the group by clause. If your statement utilizes both together consider reformulating to use prev instead.

Performance will also depend on your JVM (Sun HotSpot, BEA JRockit, IBM J9), your operating system and your hardware. A JVM performance index such as specJBB at spec.org can be used. For memory intensive statement, you may want to consider 64bit architecture that can address more than 2GB or 3GB of memory, although a 64bit JVM usually comes with a slow performance penalty due to more complex pointer address management.

The choice of JVM, OS and hardware depends on a number of factors and therefore a definite suggestion is hard to make. The choice depends on the number of statements, and number of threads. A larger number of threads would benefit of more CPU and cores. If you have very low latency requirements, you should consider getting more GHz per core, and possibly soft real-time JVM to enforce GC determinism at the JVM level, or even consider dedicated hardware such as Azul. If your statements utilize large data windows, more RAM and heap space will be utilized hence you should clearly plan and account for that and possibly consider 64bit architectures or consider EsperHA.

The number and type of statements is a factor that cannot be generically accounted for. The benchmark kit can help test out some requirements and establish baselines, and for more complex use cases a simulation or proof of concept would certainly works best. EsperTech' experts can be available to help write interfaces in a consulting relationship.

The @Hint annotation provides a single keyword or a comma-separated list of keywords that provide instructions to the engine towards statement execution that affect runtime performance and memory-use of statements. Also see Section 5.2.7.8, “@Hint”.

The hints for use with joins, outer joins, unidirectional joins, relational and non-relational join query planning are described in Section 5.12.5, “Hints Related to Joins”.

The hint for use with group by to specify how state for groups is reclaimed is described in Section 5.6.2.1, “Hints Pertaining to Group-By” and Section 12.3.2, “Grouped Data Window (std:groupwin)”.

The hint for use with group by to specify aggregation state reclaim for unbound streams and timestamp groups is described in Section 5.6.2.1, “Hints Pertaining to Group-By”.

The hint for use with match_recognize to specify iterate-only is described in Section 7.4.6, “Eliminating Duplicate Matches”.

To tune subquery performance when your subquery selects from a named window, consider the hints discussed in Section 5.11.8, “Hints Related to Subqueries”.

The @NoLock hint to remove context partition locking (also read caution note) is described at Section 14.7, “Engine Threading and Concurrency”.

The hint for influencing query planning is elaborated in Section 20.2.33, “Query Planning Index Hints” and query planning is described in Section 20.2.32, “Notes on Query Planning”.

Object-array events offer the best read access performance for access to event property values. In addition, object-array events use much less memory then Map-type events. They also offer the best write access performance.

A comparison of different event representations is in Section 2.13, “Comparing Event Representations”.

First, we recommend that your application sends object-array events into the engine, instead of Map-type events. See Section 2.7, “Object-array (Object[]) Events” for more information.

Second, we recommend that your application sets the engine-wide configuration of the default event representation to object array, as described in Section 15.4.11.1, “Default Event Representation”. Alternatively you can use the @EventRepresentation(array=true) annotation with individual statements.

Currently index hints are only supported for the following types of statements:

For the above statements, you may dictate to the engine which explicit index (created via create index syntax) to use.

Specify the name of the explicit index in parentheses following @Hint and the index literal.

The following example instructs the engine to use the UserProfileIndex if possible:

@Hint('index(UserProfileIndex)')

Add the literal bust to instruct the engine to use the index, or if the engine cannot use the index fail query planning with an exception and therefore fail statement creation.

The following example instructs the engine to use the UserProfileIndex if possible or fail with an exception if the index cannot be used:

@Hint('index(UserProfileIndex, bust)')

Multiple indexes can be listed separated by comma (,).

The next example instructs the engine to consider the UserProfileIndex and the SessionIndex or fail with an exception if either index cannot be used:

@Hint('index(UserProfileIndex, SessionIndex, bust)')

The literal explicit can be added to instruct the engine to use only explicitly created indexes.

The final example instructs the engine to consider any explicitly create index or fail with an exception if any of the explicitly created indexes cannot be used:

@Hint('index(explicit, bust)')

Whether aggregations of named window rows are computed incrementally or are recomputed from scratch depends on the type of query.

When the engine computes aggregation values incrementally, meaning it continuously updates the aggregation value as events enter and leave a named window, it means that the engine internally subscribes to named window updates and applies these updates as they occur. For some applications this is the desired behavior.

For some applications re-computing aggregation values from scratch when a certain condition occurs, for example when a triggering event arrives or time passes, is beneficial. Re-computing an aggregation can be less expensive if the number of rows to consider is small and/or when the triggering event or time condition triggers infrequently.

The next paragraph assumes that a named window has been created to hold some historical financial data per symbol and minute:

create window HistoricalWindow.win:keepall() as (symbol string, int minute, double price)
insert into HistoricalWindow select symbol, minute, price from HistoricalTick

For statements that simply select from a named window (excludes on-select) the engine computes aggregation values incrementally, continuously updating the aggregation, as events enter and leave the named window.

For example, the below statement updates the total price incrementally as events enter and leave the named window. If events in the named window already exist at the time the statement gets created, the total price gets pre-computed once when the statement gets created and incrementally updated when events enter and leave the named window:

select sum(price) from HistoricalWindow(symbol='GE')

The same is true for uncorrelated subqueries. For statements that sub-select from a named window, the engine computes aggregation values incrementally, continuously updating the aggregation, as events enter and leave the named window. This is only true for uncorrelated subqueries that don't have a where-clause.

For example, the below statement updates the total price incrementally as events enter and leave the named window. If events in the named window already exist at the time the statement gets created, the total price gets pre-computed once when the statement gets created and incrementally updated when events enter and leave the named window:

// Output GE symbol total price, incrementally computed
// Outputs every 15 minutes on the hour.
select (sum(price) from HistoricalWindow(symbol='GE')) 
from pattern [every timer:at(0, 15, 30, 45), *, *, *, *, 0)]

If instead your application uses on-select or a correlated subquery, the engine recomputes aggregation values from scratch every time the triggering event fires.

For example, the below statement does not incrementally compute the total price (use a plain select or subselect as above instead). Instead the engine computes the total price from scratch based on the where-clause and matching rows:

// Output GE symbol total price (recomputed from scratch) every 15 minutes on the hour
on pattern [every timer:at(0, 15, 30, 45), *, *, *, *, 0)]
select sum(price) from HistoricalWindow where symbol='GE'

Unidirectional joins against named windows also do not incrementally compute aggregation values.

Joins and outer joins, that are not unidirectional, compute aggregation values incrementally.

The benchmark application is basically an Esper event server build with Esper that listens to remote clients over TCP. Remote clients send MarketData(ticker, price, volume) streams to the event server. The Esper event server is started with 1000 statements of one single kind (unless otherwise written), with one statement per ticker symbol, unless the statement kind does not depend on the symbol. The statement prototype is provided along the results with a '$' instead of the actual ticker symbol value. The Esper event server is entirely multithreaded and can leverage the full power of 32bit or 64bit underlying hardware multi-processor multi-core architecture.

The kit also prints out when starting up the event size and the theoretical maximal throughput you can get on a 100 Mbit/s and 1 Gbit/s network. Keep in mind a 100 Mbit/s network will be overloaded at about 400 000 event/s when using our kit despite the small size of events.

Results are posted on our Wiki page at Performance Wiki. Reported results do not represent best ever obtained results. Reported results may help you better compare Esper to other solutions (for latency, throughput and CPU utilization) and also assess your target hardware and JVMs.

The Esper event server, client and statement prototypes are provided in the source repository esper/trunk/examples/benchmark/. Refer to http://xircles.codehaus.org/projects/esper/repo for source access.

A built is provided for convenience (without sources) as an attachment to the Wiki page at Performance Wiki. It contains Ant script to start client, server in simulation mode and server. For real measurement we advise to start from a shell script (because Ant is pipelining stdout/stderr when you invoke a JVM from Ant - which is costly). Sample scripts are provided for you to edit and customize.

If you use the kit you should:

  1. Choose the statement you want to benchmark, add it to etc/statements.properties under your own KEY and use the -mode KEY when you start the Esper event server.

  2. Prepare your runServer.sh/runServer.cmd and runClient.sh/runclient.cmd scripts. You'll need to drop required jar libraries in lib/ , make sure the classpath is configured in those script to include build and etc . The required libraries are Esper (any compatible version, we have tested started with Esper 1.7.0) and its dependencies as in the sample below (with Esper 2.1) :

    # classpath on Unix/Linux (on one single line)
    etc:build:lib/esper-4.11.0.jar:lib/commons-logging-1.1.1.jar:lib/cglib-nodep-2.2.jar
       :lib/antlr-runtime-3.2.jar:lib/log4j-1.2.16.jar
    @rem  classpath on Windows (on one single line)
    etc;build;lib\esper-4.11.0.jar;lib\commons-logging-1.1.1.jar;lib\cglib-nodep-2.2.jar
       ;lib\antlr-runtime-3.2.jar;lib\log4j-1.2.16.jar

    Note that ./etc and ./build have to be in the classpath. At that stage you should also start to set min and max JVM heap. A good start is 1GB as in -Xms1g -Xmx1g

  3. Write the statement you want to benchmark given that client will send a stream MarketData(String ticker, int volume, double price), add it to etc/statements.properties under your own KEY and use the -mode KEY when you start the Esper event server. Use '$' in the statement to create a prototype. For every symbol, a statement will get registered with all '$' replaced by the actual symbol value (f.e. 'GOOG')

  4. Ensure client and server are using the same -Desper.benchmark.symbol=1000 value. This sets the number of symbol to use (thus may set the number of statement if you are using a statement prototype, and governs how MarketData event are represented over the network. Basically all events will have the same size over the network to ensure predictability and will be ranging between S0AA and S999A if you use 1000 as a value here (prefix with S and padded with A up to a fixed length string. Volume and price attributes will be randomized.

  5. By default the benchmark registers a subscriber to the statement(s). Use -Desper.benchmark.ul to use an UpdateListener instead. Note that the subscriber contains suitable update(..) methods for the default proposed statement in the etc/statements.properties file but might not be suitable if you change statements due to the strong binding with statement results. Refer to Section 14.3.2, “Receiving Statement Results”.

  6. Establish a performance baseline in simulation mode (without clients). Use the -rate 1x5000 option to simulate one client (one thread) sending 5000 evt/s. You can ramp up both the number of client simulated thread and their emission rate to maximize CPU utilization. The right number should mimic the client emission rate you will use in the client/server benchmark and should thus be consistent with what your client machine and network will be able to send. On small hardware, having a lot of thread with slow rate will not help getting high throughput in this simulation mode.

  7. Do performance runs with client/server mode. Remove the -rate NxM option from the runServer script or Ant task. Start the server with -help to display the possible server options (listen port, statistics, fan out options etc). On the remote machine, start one or more client. Use -help to display the possible client options (remote port, host, emission rate). The client will output the actual number of event it is sending to the server. If the server gets overloaded (or if you turned on -queue options on the server) the client will likely not be able to reach its target rate.

    Usually you will get better performance by using server side -queue -1 option so as to have each client connection handled by a single thread pipeline. If you change to 0 or more, there will be intermediate structures to pass the event stream in an asynchronous fashion. This will increase context switching, although if you are using many clients, or are using the -sleep xxx (xxx in milliseconds) to simulate a listener delay you may get better performance.

    The most important server side option is -stat xxx (xxx in seconds) to print out throughput and latency statistics aggregated over the last xxx seconds (and reset every time). It will produce both internal Esper latency (in nanosecond) and also end to end latency (in millisecond, including network time). If you are measuring end to end latency you should make sure your server and client machine(s) are having the same time with f.e. ntpd with a good enough precision. The stat format is like:

    ---Stats - engine (unit: ns)
      Avg: 2528 #4101107
            0 <    5000:  97.01%  97.01% #3978672
         5000 <   10000:   2.60%  99.62% #106669
        10000 <   15000:   0.35%  99.97% #14337
        15000 <   20000:   0.02%  99.99% #971
        20000 <   25000:   0.00%  99.99% #177
        25000 <   50000:   0.00% 100.00% #89
        50000 <  100000:   0.00% 100.00% #41
       100000 <  500000:   0.00% 100.00% #120
       500000 < 1000000:   0.00% 100.00% #2
      1000000 < 2500000:   0.00% 100.00% #7
      2500000 < 5000000:   0.00% 100.00% #5
      5000000 <    more:   0.00% 100.00% #18
    ---Stats - endToEnd (unit: ms)
      Avg: -2704829444341073400 #4101609
            0 <       1:  75.01%  75.01% #3076609
            1 <       5:   0.00%  75.01% #0
            5 <      10:   0.00%  75.01% #0
           10 <      50:   0.00%  75.01% #0
           50 <     100:   0.00%  75.01% #0
          100 <     250:   0.00%  75.01% #0
          250 <     500:   0.00%  75.01% #0
          500 <    1000:   0.00%  75.01% #0
         1000 <    more:  24.99% 100.00% #1025000
    Throughput 412503 (active 0 pending 0 cnx 4)

    This one reads as:

    "Throughput is 412 503 event/s with 4 client connected. No -queue options 
    was used thus no event is pending at the time the statistics are printed. 
    Esper latency average is at 2528 ns (that is 2.5 us) for 4 101 107 events 
    (which means we have 10 seconds stats here). Less than 10us latency 
    was achieved for 106 669 events that is 99.62%. Latency between 5us 
    and 10us was achieved for those 2.60% of all the events in the interval."
    
    "End to end latency was ... in this case likely due to client clock difference
    we ended up with unusable end to end statistics."

    Consider the second output paragraph on end-to-end latency:

    ---Stats - endToEnd (unit: ms)
      Avg: 15 #863396
            0 <       1:   0.75%   0.75% #6434
            1 <       5:   0.99%   1.74% #8552
            5 <      10:   2.12%   3.85% #18269
           10 <      50:  91.27%  95.13% #788062
           50 <     100:   0.10%  95.22% #827
          100 <     250:   4.36%  99.58% #37634
          250 <     500:   0.42% 100.00% #3618
          500 <    1000:   0.00% 100.00% #0
         1000 <    more:   0.00% 100.00% #0

    This would read:

    "End to end latency average is at 15 milliseconds for the 863 396 events 
    considered for this statistic report. 95.13% ie 788 062 events were handled 
    (end to end) below 50ms, and 91.27% were handled between 10ms and 50ms."

This section specifies the output of a subset of EPL continuous queries, for two purposes: First, to help application developers understand streaming engine output in response to incoming events and in response to time passing. Second, to document and standardize output for EPL queries in a testable and trackable fashion.

The section focuses on a subset of features, namely the time window, aggregation, grouping, and output rate limiting. The section does not currently provide examples for many of the other language features, thus there is no example for other data windows (the time window is used here), joins, sub-selects or named windows etc.

Rather then just describe syntax and output, this section provides detailed examples for each of the types of queries presented. The input for each type of query is always the same set of events, and the same timing. Each event has three properties: symbol, volume and price. The property types are string, long and double, respectively.

The chapters are organized by the type of query: The presence or absence of aggregation functions, as well as the presence or absence of a group by clause change statement output as described in Section 3.7.2, “Output for Aggregation and Group-By”.

You will notice that some queries utilize the order by clause for sorting output. The reason is that when multiple output rows are produced at once, the output can be easier to read if it is sorted.

With output rate limiting, the engine invokes your listener even if there are no results to indicate when the output condition has been reached. Such is indicated as (empty result) in the output result columns.

The output columns show both insert and remove stream events. Insert stream events are delivered as an array of EventBean instances to listeners in the newData parameter, while remove stream events are delivered to the oldData parameter of listeners. Delivery to observers follows similar rules.

For the purpose of illustration and documentation, the example data set demonstrates input and remove streams based on a time window of a 5.5 second interval. The statement utilizing the time window could look as follows:

select symbol, volume, price from MarketData.win:time(5.5 sec)

We have picked a time window to demonstrate the output for events entering and leaving a data window with an expiration policy. The time window provides a simple expiration policy based on time: if an event resides in the time window more then 5.5 seconds, the engine expires the event from the time window.

The input events and their timing are below. The table should be read, starting from top, as "The time starts at 0.2 seconds. Event E1 arrives at 0.2 seconds with properties [S1, 100, 25]. At 0.8 second event E2 arrives with properties [S2, 5000, 9.0]" and so on.

                       Input                                 
-----------------------------------------------  
 Time Symbol  Volume   Price
  0.2                          
          S1     100    25.0   Event E1 arrives
  0.8                          
          S2    5000     9.0   Event E2 arrives
  1.0                          
  1.2                          
  1.5                          
          S1     150    24.0   Event E3 arrives
          S3   10000     1.0   Event E4 arrives
  2.0                          
  2.1                          
          S1     155    26.0   Event E5 arrives
  2.2                          
  2.5                          
  3.0                          
  3.2                          
  3.5                          
          S3   11000     2.0   Event E6 arrives
  4.0                          
  4.2                          
  4.3                          
          S1     150    22.0   Event E7 arrives
  4.9                          
          S3   11500     3.0   Event E8 arrives
  5.0                          
  5.2                          
  5.7                          Event E1 leaves the time window
  5.9                          
          S3   10500     1.0   Event E9 arrives
  6.0                          
  6.2                          
  6.3                          Event E2 leaves the time window
  7.0                          Event E3 and E4 leave the time window
  7.2                          

The event data set assumes a time window of 5.5 seconds. Thus at time 5.7 seconds the first arriving event (E1) leaves the time window.

The data set as above shows times between 0.2 seconds and 7.2 seconds. Only a couple of time points have been picked for the table to keep the set of time points constant between statements, and thus make the test data and output easier to understand.

This chapter provides sample output for queries that do not have aggregation functions and do not have a group by clause.

Using the SNAPSHOT keyword in the output clause, the engine posts data window contents when the output condition is reached.

The statement for this sample reads:

select irstream symbol, volume, price from MarketData.win:time(5.5 sec)
output snapshot every 1 seconds

The output is as follows:

                       Input                                  Output
                                                 Insert Stream    Remove Stream
-----------------------------------------------  ----------------------------------
 Time Symbol  Volume   Price
  0.2                          
         IBM     100    25.0   Event E1 arrives
  0.8                          
        MSFT    5000     9.0   Event E2 arrives
  1.0                          
  1.2                          
                                                 [IBM, 100, 25.0]                       
                                                 [MSFT, 5000, 9.0]                      
  1.5                          
         IBM     150    24.0   Event E3 arrives
         YAH   10000     1.0   Event E4 arrives
  2.0                          
  2.1                          
         IBM     155    26.0   Event E5 arrives
  2.2                          
                                                 [IBM, 100, 25.0]                       
                                                 [MSFT, 5000, 9.0]                      
                                                 [IBM, 150, 24.0]                       
                                                 [YAH, 10000, 1.0]                      
                                                 [IBM, 155, 26.0]                       
  2.5                          
  3.0                          
  3.2                          
                                                 [IBM, 100, 25.0]                       
                                                 [MSFT, 5000, 9.0]                      
                                                 [IBM, 150, 24.0]                       
                                                 [YAH, 10000, 1.0]                      
                                                 [IBM, 155, 26.0]                       
  3.5                          
         YAH   11000     2.0   Event E6 arrives
  4.0                          
  4.2                          
                                                 [IBM, 100, 25.0]                       
                                                 [MSFT, 5000, 9.0]                      
                                                 [IBM, 150, 24.0]                       
                                                 [YAH, 10000, 1.0]                      
                                                 [IBM, 155, 26.0]                       
                                                 [YAH, 11000, 2.0]                      
  4.3                          
         IBM     150    22.0   Event E7 arrives
  4.9                          
         YAH   11500     3.0   Event E8 arrives
  5.0                          
  5.2                          
                                                 [IBM, 100, 25.0]                       
                                                 [MSFT, 5000, 9.0]                      
                                                 [IBM, 150, 24.0]                       
                                                 [YAH, 10000, 1.0]                      
                                                 [IBM, 155, 26.0]                       
                                                 [YAH, 11000, 2.0]                      
                                                 [IBM, 150, 22.0]                       
                                                 [YAH, 11500, 3.0]                      
  5.7                          Event E1 leaves the time window
  5.9                          
         YAH   10500     1.0   Event E9 arrives
  6.0                          
  6.2                          
                                                 [MSFT, 5000, 9.0]                      
                                                 [IBM, 150, 24.0]                       
                                                 [YAH, 10000, 1.0]                      
                                                 [IBM, 155, 26.0]                       
                                                 [YAH, 11000, 2.0]                      
                                                 [IBM, 150, 22.0]                       
                                                 [YAH, 11500, 3.0]                      
                                                 [YAH, 10500, 1.0]                      
  6.3                          Event E2 leaves the time window
  7.0                          Event E3 and E4 leave the time window
  7.2                          
                                                 [IBM, 155, 26.0]                       
                                                 [YAH, 11000, 2.0]                      
                                                 [IBM, 150, 22.0]                       
                                                 [YAH, 11500, 3.0]                      
                                                 [YAH, 10500, 1.0] 			

This chapter provides sample output for queries that have aggregation functions, and that do not have a group by clause, and in which all event properties are under aggregation.

This chapter provides sample output for queries that have aggregation functions, and that do not have a group by clause, and in which there are event properties that are not under aggregation.

This chapter provides sample output for queries that have aggregation functions, and that have a group by clause, and in which all event properties are under aggregation or appear in the group by clause.

This chapter provides sample output for queries that have aggregation functions, and that have a group by clause, and in which some event properties are not under aggregation.

The statement for this sample reads:

select irstream symbol, volume, sum(price) from MarketData.win:time(5.5 sec) 
group by symbol 
output snapshot every 1 seconds

The output is as follows:

                       Input                                  Output
                                                 Insert Stream    Remove Stream
-----------------------------------------------  ----------------------------------
 Time Symbol  Volume   Price
  0.2                          
         IBM     100    25.0   Event E1 arrives
  0.8                          
        MSFT    5000     9.0   Event E2 arrives
  1.0                          
  1.2                          
                                                 [IBM, 100, 25.0]                       
                                                 [MSFT, 5000, 9.0]                      
  1.5                          
         IBM     150    24.0   Event E3 arrives
         YAH   10000     1.0   Event E4 arrives
  2.0                          
  2.1                          
         IBM     155    26.0   Event E5 arrives
  2.2                          
                                                 [IBM, 100, 75.0]                       
                                                 [MSFT, 5000, 9.0]                      
                                                 [IBM, 150, 75.0]                       
                                                 [YAH, 10000, 1.0]                      
                                                 [IBM, 155, 75.0]                       
  2.5                          
  3.0                          
  3.2                          
                                                 [IBM, 100, 75.0]                       
                                                 [MSFT, 5000, 9.0]                      
                                                 [IBM, 150, 75.0]                       
                                                 [YAH, 10000, 1.0]                      
                                                 [IBM, 155, 75.0]                       
  3.5                          
         YAH   11000     2.0   Event E6 arrives
  4.0                          
  4.2                          
                                                 [IBM, 100, 75.0]                       
                                                 [MSFT, 5000, 9.0]                      
                                                 [IBM, 150, 75.0]                       
                                                 [YAH, 10000, 3.0]                      
                                                 [IBM, 155, 75.0]                       
                                                 [YAH, 11000, 3.0]                      
  4.3                          
         IBM     150    22.0   Event E7 arrives
  4.9                          
         YAH   11500     3.0   Event E8 arrives
  5.0                          
  5.2                          
                                                 [IBM, 100, 97.0]                       
                                                 [MSFT, 5000, 9.0]                      
                                                 [IBM, 150, 97.0]                       
                                                 [YAH, 10000, 6.0]                      
                                                 [IBM, 155, 97.0]                       
                                                 [YAH, 11000, 6.0]                      
                                                 [IBM, 150, 97.0]                       
                                                 [YAH, 11500, 6.0]                      
  5.7                          Event E1 leaves the time window
  5.9                          
         YAH   10500     1.0   Event E9 arrives
  6.0                          
  6.2                          
                                                 [MSFT, 5000, 9.0]                      
                                                 [IBM, 150, 72.0]                       
                                                 [YAH, 10000, 7.0]                      
                                                 [IBM, 155, 72.0]                       
                                                 [YAH, 11000, 7.0]                      
                                                 [IBM, 150, 72.0]                       
                                                 [YAH, 11500, 7.0]                      
                                                 [YAH, 10500, 7.0]                      
  6.3                          Event E2 leaves the time window
  7.0                          Event E3 and E4 leave the time window
  7.2                          
                                                 [IBM, 155, 48.0]                       
                                                 [YAH, 11000, 6.0]                      
                                                 [IBM, 150, 48.0]                       
                                                 [YAH, 11500, 6.0]                      
                                                 [YAH, 10500, 6.0]  

The words in the following table are explicitly reserved in EPL, however certain keywords are allowed as event property names in expressions and as column names in the rename syntax of the select clause.

Most of the words in the table are forbidden by standard SQL as well. A few are reserved because EPL needs them.

Names of built-in functions and certain auxiliary keywords are permitted as identifiers for use either as event property names in expressions and for the column rename syntax. The second column in the table below indicates which keywords are acceptable. For example, count is acceptable.

An example of permitted use is:

select last, count(*) as count from MyEvent

This example shows an incorrect use of a reserved keyword:

// incorrect
select insert from MyEvent

The table of explicitly reserved keywords and permitted keywords:


A

after, Suppressing Output With After
aggregation functions
custom plug-in, Aggregation Function
overview, Aggregation Functions
and pattern operator, And
annotation, Annotation
application-provided, Application-Provided Annotations
builtin, Built-In Annotations
interrogating, Interrogating EPL Annotations
API
testing, Test and Assertion Support
arithmetic operators, Arithmetic Operators
array definition operator, Array Definition Operator

B

between operator, The 'between' Keyword
binary operators, Binary Operators

D

data types, Data Types
data window views
custom plug-in view, Derived-value and Data Window View
externally-time batch window, Externally-timed batch window (win:ext_timed_batch)
externally-timed window, Externally-timed window (win:ext_timed)
grouped data window, Grouped Data Window (std:groupwin)
keep-all window, Keep-All window (win:keepall)
last event window, Last Event (std:lastevent)
length batch window, Length batch window (win:length_batch)
length window, Length window (win:length)
overview, EPL Reference: Views
ranked window, Ranked Window View (ext:rank)
size window, Size (std:size)
sorted window, Sorted Window View (ext:sort)
time batch window, Time batch window (win:time_batch)
time length batch window, Time-Length combination batch window (win:time_length_batch)
time window, Time window (win:time)
time-accumulating window, Time-Accumulating window (win:time_accum)
time-order window, Time-Order View (ext:time_order)
unique window, Unique (std:unique)
dataflow, Introduction
decorated event, Decorated Events
delete, Deleting From Named Windows Using Fire-And-Forget Queries
deployment
EPDeploymentAdmin interface, The Deployment Administrative Interface
EPL module, EPL Modules
J2EE, J2EE Packaging and Deployment
derived-value views
correlation, Correlation (stat:correl)
overview, EPL Reference: Views
regression, Regression (stat:linest)
univariate statistics, Univariate statistics (stat:uni)
weighted average, Weighted average (stat:weighted_avg)
dot operator, Dot Operator
duck typing, Dot Operator
dynamic event properties, Dynamic Event Properties

E

enum types, Using Constants and Enum Types
EPAdministrator interface, The Administrative Interface
EPL
from clause, Specifying Event Streams: the From Clause
group by clause, Organizing statement results into groups: the Group-by clause
having clause, Selecting groups of events: the Having clause
inner join, Outer, Left and Right Joins
insert into clause, Merging Streams and Continuous Insertion: the Insert Into Clause
join, Joining Event Streams
join, unidirectional, Unidirectional Joins
joining non-relational data via method invocation, Accessing Non-Relational Data via Method Invocation
joining relational data via SQL, Accessing Relational Data via SQL
limit clause, Limiting Row Count: the Limit Clause
named window, Creating and Using Named Windows
deleting from, Deleting From Named Windows: the On Delete clause, Deleting From Named Windows Using Fire-And-Forget Queries
inserting into, Inserting Into Named Windows, Inserting Into Named Windows Using Fire-And-Forget Queries
merge, Triggered Upsert using the On-Merge Clause
populating from a named window, Populating a Named Window from an Existing Named Window
selecting from, Selecting From Named Windows
triggered select and delete using On Select Delete, Triggered Select+Delete on Named Windows: the On Select Delete clause
triggered select using On Select, Triggered Select on Named Windows: the On Select clause
updating, Updating Named Windows: the On Update clause, Updating Named Windows Using Fire-And-Forget Queries
upsert, Triggered Upsert using the On-Merge Clause
order by clause, Sorting Output: the Order By Clause
outer join, Outer, Left and Right Joins
outer join, unidirectional, Unidirectional Joins
output control and stabilizing, Stabilizing and Controlling Output: the Output Clause
select clause, Choosing Event Properties And Events: the Select Clause
subqueries, Subqueries
variable, Variables and Constants
where clause, Specifying Search Conditions: the Where Clause
EPRuntime interface, The Runtime Interface
EPServiceProviderManager class, The Service Provider Interface
EPStatement interface, The Administrative Interface
EPStatementObjectModel interface, Statement Object Model
escape, Escaping Strings
event
additional representations, Additional Event Representations
bulk, Coarse-Grained Events
coarse, Coarse-Grained Events
dynamic properties, Dynamic Event Properties
insert into, Event Objects Instantiated and Populated by Insert Into
Java object, Plain-Old Java Object Events
Map representation, java.util.Map Events
Object-array representation, Object-array (Object[]) Events
properties, Event Properties
underlying representation, Event Underlying Java Objects
update, Updating, Merging and Versioning Events
version, Updating, Merging and Versioning Events
XML representation, org.w3c.dom.Node XML Events
event as a property, Event as a Property
event object
custom, Event Type And Event Object
event type
declaring, Declaring an Event Type: Create Schema
EventBean interface, Introduction, Event and Event Type
EventType interface, Event and Event Type
every pattern operator, Every
every-distinct pattern operator, Every-Distinct
exists function, The Exists Function
expression
declaring, Declaring Global Expressions And Scripts: Create Expression
expression batch window, Expiry Expression Batch (win:expr_batch)
expression declaration, Expression Declaration
expression window, Expiry Expression (win:expr)
external time, Controlling Time-Keeping
externally-timed batch window, Externally-timed batch window (win:ext_timed_batch)
externally-timed window, Externally-timed window (win:ext_timed)

L

lambda
expression declaration, Expression Declaration
last event window, Last Event (std:lastevent)
length batch window, Length batch window (win:length_batch)
length window, Length window (win:length)
like operator, The 'like' Keyword
limit clause, Limiting Row Count: the Limit Clause
limiting output row count, Limiting Row Count: the Limit Clause
literals, Data Type of Constants
logical and comparison operators, Logical And Comparison Operators

M

map event representation, java.util.Map Events
match recognize
comparison, Comparison of Match Recognize and EPL Patterns
overview, Overview
match_recognize
operator precedences, Operator Precedence
max function, The Min and Max Functions
maximum-by, Maxby Aggregation Function
merge, Triggered Upsert using the On-Merge Clause
methods
date-time, Overview
enumeration, Overview
min function, The Min and Max Functions
minimum-by, Minby Aggregation Function
MVEL, Overview

P

packaging
EPDeploymentAdmin interface, The Deployment Administrative Interface
EPL module, EPL Modules
J2EE, J2EE Packaging and Deployment
pattern
filter event consumption, Controlling Event Consumption
filter expressions, Filter Expressions In Patterns
operator precedences, Operator Precedence
overview, Event Pattern Overview
pattern atom, Pattern Atoms
pattern guard, Pattern Guards
custom plug-in, Pattern Guard
timer-within, The timer:within Pattern Guard
timer-withinmax, The timer:withinmax Pattern Guard
while, The while Pattern Guard
pattern observer
custom plug-in, Pattern Observer
timer-at, timer:at
timer-interval, timer:interval
pattern operator
and, And
every, Every
every-distinct, Every-Distinct
followed-by, Followed-by
not, Not
or, Or
PERL, Overview
PHP, Overview
plug-in
custom aggregation function, Aggregation Function
custom event object, Event Type And Event Object
custom pattern guard, Pattern Guard
custom pattern observer, Pattern Observer
custom view, Derived-value and Data Window View
single-row function, Single-Row Function
plug-in loader, Plug-in Loader
previous count function, The Previous-Count Function
previous function, The Previous Function
previous tail function, The Previous-Tail Function
previous window function, The Previous-Window Function
prior function, The Prior Function
pull API, Using Iterators
Python, Overview

R

ranked window, Ranked Window View (ext:rank)
regexp operator, The 'regexp' Keyword
regression view, Regression (stat:linest)
relational databases, Accessing Relational Data via SQL
remove stream, Insert and Remove Stream
Ruby, Overview

S

safe iterator, Using Iterators
script
declaring, Declaring Global Expressions And Scripts: Create Expression
script declaration, Script Declaration, Overview
select clause, Choosing Event Properties And Events: the Select Clause
single-row functions
custom plug-in, Single-Row Function
size window, Size (std:size)
sorted, Sorted Aggregation Function
sorted window, Sorted Window View (ext:sort)
SQL, Accessing Relational Data via SQL
statement
receiving results, Receiving Statement Results
subscriber object, Setting a Subscriber Object
StatementAwareUpdateListener interface, Adding Listeners
static Java methods, Single-row Function Reference
subqueries, Subqueries
subscriber object, Setting a Subscriber Object
multi-row, Multi-Row Delivery
no-parameter, No-Parameter Update Method
row-by-row, Row-By-Row Delivery

T

testing, Test and Assertion Support
threading, Engine Threading and Concurrency
time
controlling, Controlling Time-Keeping
resolution, Time Resolution
time batch window, Time Batch, Time batch window (win:time_batch)
time length batch window, Time-Length combination batch window (win:time_length_batch)
time window, Time Window, Time window (win:time)
time-accumulating window, Time-Accumulating window (win:time_accum)
time-order window, Time-Order View (ext:time_order)
timer-at pattern observer, timer:at
timer-interval pattern observer, timer:interval
timer-within pattern guard, The timer:within Pattern Guard
timer-withinmax pattern guard, The timer:withinmax Pattern Guard
type-of function, The Type-Of Function

U

UDF
user-defined function, User-Defined Functions
unidirectional joins, Unidirectional Joins
unique window, Unique (std:unique)
univariate statistics view, Univariate statistics (stat:uni)
UnmatchedListener interface, Receiving Unmatched Events
update, Updating Named Windows Using Fire-And-Forget Queries
UpdateListener interface, Adding Listeners
upsert, Triggered Upsert using the On-Merge Clause
user-defined function, User-Defined Functions
user-defined single-row function, Single-row Function Reference

V

variable, Variables and Constants
variant stream, Merging Disparate Types of Events: Variant Streams
views
batch window processing, Batch Windows
correlation, Correlation (stat:correl)
custom plug-in view, Derived-value and Data Window View
expiry expression batch window, Expiry Expression Batch (win:expr_batch)
expiry expression window, Expiry Expression (win:expr)
externally-time batch window, Externally-timed batch window (win:ext_timed_batch)
externally-timed window, Externally-timed window (win:ext_timed)
first event, First Event (std:firstevent)
first length window, First Length (win:firstlength)
first time window, First Time (win:firsttime)
first unique window, First Unique (std:firstunique)
grouped data window, Grouped Data Window (std:groupwin)
keep-all window, Keep-All window (win:keepall)
last event window, Last Event (std:lastevent)
length batch window, Length batch window (win:length_batch)
length window, Length window (win:length)
overview, EPL Reference: Views
ranked window, Ranked Window View (ext:rank)
regression, Regression (stat:linest)
size window, Size (std:size)
sorted window, Sorted Window View (ext:sort)
time batch window, Time batch window (win:time_batch)
time length batch window, Time-Length combination batch window (win:time_length_batch)
time window, Time window (win:time)
time-accumulating window, Time-Accumulating window (win:time_accum)
time-order window, Time-Order View (ext:time_order)
unique window, Unique (std:unique)
univariate statistics, Univariate statistics (stat:uni)
weighted average, Weighted average (stat:weighted_avg)

X

XML event representation, org.w3c.dom.Node XML Events