shunping commented on code in PR #40879:
URL: https://github.com/apache/beam/pull/40879#discussion_r4428366460
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model/pipeline/src/main/proto/org/apache/beam/model/pipeline/v1/beam_runner_api.proto:
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@@ -1361,20 +1364,52 @@ message TimeDomain {
// from a GroupByKey or CombinePerKey
//
// A trigger is described in terms of when it is _ready_ to permit output.
+//
+// Triggers are evaluated per key and window. Each time the root trigger is
+// ready, it fires and the current pane is output.
+//
+// Finishing triggers and closed windows:
+//
+// Some triggers may "finish", meaning they will never be ready again. When
+// the _root_ trigger of a window finishes, that window is closed for its key:
+//
+// * The firing that finished the trigger produces the final pane for the
+// window (its PaneInfo has is_last set).
+// * No further panes are produced for the window. In particular, there is no
+// additional pane when the watermark passes the end of the window or when
+// the window expires (end of window + allowed lateness), regardless of
+// ClosingBehavior or OnTimeBehavior.
+// * Any elements that arrive for the window after it has been closed are
+// dropped. This applies whether or not those elements are late relative to
+// the watermark.
+//
+// A root trigger that may finish before the window expires can therefore lose
+// data. SDKs typically call such triggers "unsafe". Wrapping a trigger in
+// Repeat makes it never finish. Runners must still implement the closed-window
+// semantics above for pipelines that use finishing triggers.
+//
+// A subtrigger that finishes does not by itself close the window; it only
+// affects its parent trigger, as described for each trigger type below.
message Trigger {
// Ready when all subtriggers are ready.
+ //
+ // Finishes after it fires once.
Review Comment:
Intersting. From the following implementations across SDKs, it seems there
is no specific guarantee. Particularly, the AfterAll firing invokes all firing
of its sub triggers, but in Python and Go, AfterAll `on_fire` can invoke the
firing of some sub triggers, but it only marks itself as finished after all sub
triggers are fired.
* Java
https://github.com/apache/beam/blob/b88840e0d3624dc2bec79a5b6f0292b640761ef2/runners/core-java/src/main/java/org/apache/beam/runners/core/triggers/AfterAllStateMachine.java#L107-L113
* Python
https://github.com/apache/beam/blob/b88840e0d3624dc2bec79a5b6f0292b640761ef2/sdks/python/apache_beam/transforms/trigger.py#L820-L829
* Go
https://github.com/apache/beam/blob/b88840e0d3624dc2bec79a5b6f0292b640761ef2/sdks/go/pkg/beam/runners/prism/internal/engine/strategy.go#L247-L258
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