org.apache.flink.streaming.api.windowing.triggers

Class Trigger<T,W extends Window>

java.lang.Object

org.apache.flink.streaming.api.windowing.triggers.Trigger<T,W>

Type Parameters:

T - The type of elements on which this Trigger works.

W - The type of Windows on which this Trigger can operate.

All Implemented Interfaces:

Serializable

Direct Known Subclasses:

ContinuousEventTimeTrigger, ContinuousProcessingTimeTrigger, CountTrigger, DeltaTrigger, EventTimeTrigger, GlobalWindows.NeverTrigger, ProcessingTimeTrigger, PurgingTrigger

@PublicEvolving
public abstract class Trigger<T,W extends Window>
extends Object
implements Serializable

A Trigger determines when a pane of a window should be evaluated to emit the results for that part of the window.

A pane is the bucket of elements that have the same key (assigned by the KeySelector) and same Window. An element can be in multiple panes if it was assigned to multiple windows by the WindowAssigner. These panes all have their own instance of the Trigger.

Triggers must not maintain state internally since they can be re-created or reused for different keys. All necessary state should be persisted using the state abstraction available on the Trigger.TriggerContext.

When used with a MergingWindowAssigner the Trigger must return true from canMerge() and onMerge(Window, OnMergeContext) most be properly implemented.

See Also:

Serialized Form

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
static interface	Trigger.OnMergeContext Extension of Trigger.TriggerContext that is given to onMerge(Window, OnMergeContext).
static interface	Trigger.TriggerContext A context object that is given to Trigger methods to allow them to register timer callbacks and deal with state.

Constructor Summary

Constructors

Constructor and Description
Trigger()

Method Summary

Modifier and Type	Method and Description
boolean	canMerge() Returns true if this trigger supports merging of trigger state and can therefore be used with a MergingWindowAssigner.
abstract void	clear(W window, Trigger.TriggerContext ctx) Clears any state that the trigger might still hold for the given window.
abstract TriggerResult	onElement(T element, long timestamp, W window, Trigger.TriggerContext ctx) Called for every element that gets added to a pane.
abstract TriggerResult	onEventTime(long time, W window, Trigger.TriggerContext ctx) Called when an event-time timer that was set using the trigger context fires.
void	onMerge(W window, Trigger.OnMergeContext ctx) Called when several windows have been merged into one window by the WindowAssigner.
abstract TriggerResult	onProcessingTime(long time, W window, Trigger.TriggerContext ctx) Called when a processing-time timer that was set using the trigger context fires.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

Trigger

public Trigger()

Method Detail

onElement

public abstract TriggerResult onElement(T element,
                                        long timestamp,
                                        W window,
                                        Trigger.TriggerContext ctx)
                                 throws Exception

Called for every element that gets added to a pane. The result of this will determine whether the pane is evaluated to emit results.

Parameters:

element - The element that arrived.

timestamp - The timestamp of the element that arrived.

window - The window to which the element is being added.

ctx - A context object that can be used to register timer callbacks.

Throws:

Exception

onProcessingTime

public abstract TriggerResult onProcessingTime(long time,
                                               W window,
                                               Trigger.TriggerContext ctx)
                                        throws Exception

Called when a processing-time timer that was set using the trigger context fires.

Note: This method is not called in case the window does not contain any elements. Thus, if you return PURGE from a trigger method and you expect to do cleanup in a future invocation of a timer callback it might be wise to clean any state that you would clean in the timer callback.

Parameters:

time - The timestamp at which the timer fired.

window - The window for which the timer fired.

ctx - A context object that can be used to register timer callbacks.

Throws:

Exception

onEventTime

public abstract TriggerResult onEventTime(long time,
                                          W window,
                                          Trigger.TriggerContext ctx)
                                   throws Exception

Called when an event-time timer that was set using the trigger context fires.

Note: This method is not called in case the window does not contain any elements. Thus, if you return PURGE from a trigger method and you expect to do cleanup in a future invocation of a timer callback it might be wise to clean any state that you would clean in the timer callback.

Parameters:

time - The timestamp at which the timer fired.

window - The window for which the timer fired.

ctx - A context object that can be used to register timer callbacks.

Throws:

Exception

canMerge

public boolean canMerge()

Returns true if this trigger supports merging of trigger state and can therefore be used with a MergingWindowAssigner.

If this returns true you must properly implement onMerge(Window, OnMergeContext)

onMerge

public void onMerge(W window,
                    Trigger.OnMergeContext ctx)
             throws Exception

Called when several windows have been merged into one window by the WindowAssigner.

Parameters:

window - The new window that results from the merge.

ctx - A context object that can be used to register timer callbacks and access state.

Throws:

Exception

clear

public abstract void clear(W window,
                           Trigger.TriggerContext ctx)
                    throws Exception

Clears any state that the trigger might still hold for the given window. This is called when a window is purged. Timers set using Trigger.TriggerContext.registerEventTimeTimer(long) and Trigger.TriggerContext.registerProcessingTimeTimer(long) should be deleted here as well as state acquired using Trigger.TriggerContext.getPartitionedState(StateDescriptor).

Throws:

Exception