Github user fhueske commented on a diff in the pull request:
https://github.com/apache/flink/pull/3191#discussion_r98027639
--- Diff: docs/dev/windows.md ---
@@ -692,30 +708,39 @@ input
## Triggers
-A `Trigger` determines when a window (as formed by the `WindowAssigner`)
is ready to be
+A `Trigger` determines when a window (as formed by the *window assigner*)
is ready to be
processed by the *window function*. Each `WindowAssigner` comes with a
default `Trigger`.
If the default trigger does not fit your needs, you can specify a custom
trigger using `trigger(...)`.
-The trigger interface provides five methods that react to different
events:
+The trigger interface has five methods that allow a `Trigger` to react to
different events:
* The `onElement()` method is called for each element that is added to a
window.
* The `onEventTime()` method is called when a registered event-time timer
fires.
* The `onProcessingTime()` method is called when a registered
processing-time timer fires.
* The `onMerge()` method is relevant for stateful triggers and merges the
states of two triggers when their corresponding windows merge, *e.g.* when
using session windows.
* Finally the `clear()` method performs any action needed upon removal of
the corresponding window.
-Any of these methods can be used to register processing- or event-time
timers for future actions.
+Two things to notice about the above methods are:
+
+1) The first three can return a `TriggerResult`, *i.e.* take action as a
response to their corresponding event. The action can be one of the following:
+* `CONTINUE`: do nothing,
+* `FIRE`: trigger the computation,
+* `PURGE`: clear the elements in the window, and
+* `FIRE_AND_PURGE`: take both previous actions.
--- End diff --
"trigger the computation and clear the elements in the window afterwards."?
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