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Chapter 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

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.

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.

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 4.2.6.1, “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

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] [@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, optionally followed by @now and and followed by 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 do not need to declare a context to partition data windows, aggregation values or patterns themselves individually. You may mix-and-match partitioning as needed.

The table below outlines other partitioning syntax supported by EPL:


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.