W
- The type of Windows
on which this Trigger
can operate.public abstract class Trigger<W extends Window> extends Object implements Serializable
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 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
.
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 and Description |
---|
Trigger() |
Modifier and Type | Method and Description |
---|---|
boolean |
canMerge()
Returns true if this trigger supports merging of trigger state and can therefore.
|
abstract void |
clear(W window)
Clears any state that the trigger might still hold for the given window.
|
abstract boolean |
onElement(Object element,
long timestamp,
W window)
Called for every element that gets added to a pane.
|
abstract boolean |
onEventTime(long time,
W window)
Called when an event-time timer that was set using the trigger context fires.
|
void |
onMerge(W window,
Trigger.OnMergeContext mergeContext)
Called when several windows have been merged into one window by the
WindowAssigner . |
abstract boolean |
onProcessingTime(long time,
W window)
Called when a processing-time timer that was set using the trigger context fires.
|
abstract void |
open(Trigger.TriggerContext ctx)
Initialization method for the trigger.
|
public abstract void open(Trigger.TriggerContext ctx) throws Exception
ctx
- A context object that can be used to get states.Exception
public abstract boolean onElement(Object element, long timestamp, W window) throws Exception
element
- The element that arrived.timestamp
- The timestamp of the element that arrived.window
- The window to which the element is being added.Exception
public abstract boolean onProcessingTime(long time, W window) throws Exception
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.
time
- The timestamp at which the timer fired.window
- The window for which the timer fired.Exception
public abstract boolean onEventTime(long time, W window) throws Exception
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.
time
- The timestamp at which the timer fired.window
- The window for which the timer fired.Exception
public boolean canMerge()
If this returns true
you must properly implement onMerge(Window,
OnMergeContext)
public void onMerge(W window, Trigger.OnMergeContext mergeContext) throws Exception
WindowAssigner
.window
- The new window that results from the merge.Exception
public abstract void clear(W window) throws Exception
Trigger.TriggerContext.registerEventTimeTimer(long)
and
Trigger.TriggerContext.registerProcessingTimeTimer(long)
should be deleted here as well as
state acquired using TriggerContext#getPartitionedState(StateDescriptor)
.Exception
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