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 than '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 than '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 than 'Z1' and an asset id other than '0001' and a category of 'perishable'. In this case the output event populates property 'c' but not properties 'a' and 'b'.

When your filter expression provides the name of a named window then the filter expression matches each time an event is inserted into the named window that matches the filter conditions.

For example, assume a named window that holds the last order event per order id:

create window LastOrderWindow#unique(orderId) as OrderEvent

Assume that all order events are inserted into the named window using insert-into:

insert into LastOrderWindow select * from OrderEvent

This sample pattern fires 10 seconds after an order event with a price greater then 100 was inserted:

select * from pattern [every o=LastOrderWindow(price >= 100) -> timer:interval(10 sec)]

The pattern above fires only for events inserted-into the LastOrderWindow named window and does not fire when an order event was updated using on-update or merged using on-merge.

If your application would like to have the pattern fire for any change to the named window events including updates and merges, you must select from the named window as follows:

insert into OrderWindowChangeStream select * from LastOrderWindow

select * from pattern [every o=OrderWindowChangeStream(price >= 100) -> timer:interval(10 sec)]

A table cannot be listed as part of a pattern filter, however any filter EPL expressions can have tables access expressions and subqueries against tables.

Assuming that MyTable is a table, the following is not allowed:

// not allowed
select * from pattern [every MyTable -> timer:interval(10 sec)]

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

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)

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

Similar to the Java “||” operator the or operator requires either one of the expressions to turn true before the whole expression turns true.

Look for either an A event or a B event. As always, A and B can itself be nested expressions as well.

A or B

The next EPL outputs all A and B events:

every A or every B

Elaborating further, the expression every A or every B is equivalent to every (A or B). We prefer every A or every B as the every keys lets the engine know that filters for A and B can remain active. Consider the expression every A or every timer:interval(10 sec) which is not equivalent to every (A or timer:interval(10 sec)). This is because in the latter expression when an A event arrives the interval restarts.

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 timer:at pattern 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 x_th 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 timer:schedule observer is a flexible observer for scheduling.