[jira] [Commented] (BEAM-175) Leak garbage collection timers in GlobalWindow

["Mark Shields (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/BEAM-175?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15281929#comment-15281929 ] 

Mark Shields commented on BEAM-175:
-----------------------------------

Here's the proposed solution, which pull/139 prototypes:
(Note that pull/139 also fixes https://issues.apache.org/jira/browse/BEAM-32 and https://issues.apache.org/jira/browse/BEAM-122 since they are all in the same space.)

- Window.ClosingBehavior becomes Window.EmptyPaneBehavior
- New property WindowingStrategy.onTimeBehavior of type Window.EmptyPaneBehavior, which is the analogue of closingBehavior but for the ON_TIME pane. Default is FIRE_ALWAYS.
- New enum Window.PaneIndexBehavior { SEQUENTIAL, ZERO }
- New property WindowingStrategy.paneIndexBehavior of type Window.PaneIndexBehavior which controls how PaneInfo.getIndex and friends are determined. Default is SEQUENTIAL.
- In ReduceFnRunner.emitIfApproriate we delete any outstanding end-of-window or garbage-collection timer if we can be sure the corresponding onTimer would be a no-op. This means:
    - No element state, either combiner or iterable.
    - No end-of-window/garbage-collection holds, which will have been elided if the onTimeBehavior/closingBehavor is FIRE_IF_NON_EMPTY
    - No trigger finished bits (ie the trigger must be a Repeatably which never closes).
    - No previous pane info, which will have been elided if paneIndexBehavior is ZERO.
    - No active window state, ie the windows are not merging or there's no currently active merging window.

The upshot is the following classic pattern will never leave behind state per key:
  Window.<KV<Long, Void>>into(new GlobalWindows())
            .triggering(Repeatedly.forever(AfterProcessingTime.pastFirstElementInPane().plusDelayOf(Duration.standardSeconds(10))))
            .discardingFiredPanes()
            .onTimeBehavior(EmptyPaneBehavior.FIRE_IF_NON_EMPTY)
            .paneIndexBehavior(PaneIndexBehavior.ZERO)
            .withAllowedLateness(Duration.ZERO))




> Leak garbage collection timers in GlobalWindow
> ----------------------------------------------
>
>                 Key: BEAM-175
>                 URL: https://issues.apache.org/jira/browse/BEAM-175
>             Project: Beam
>          Issue Type: Bug
>          Components: runner-core
>            Reporter: Mark Shields
>            Assignee: Mark Shields
>
> Consider the  transform:
>   Window
>     .into(new GlobalWindows())
>     .triggering(
>       Repeatedly.forever(
>         AfterProcessingTime.pastFirstElementInPane().plusDelayOf(...)))
>     .discardingFiredPanes()
> This is a common idiom for 'process elements bunched by arrival time'.
> Currently we create an end-of-window timer per key, which clearly will only fire if the pipeline is drained.
> Better would be to avoid creating end-of-window timers if there's no state which needs to be processed at end-of-window (ie at drain if the Global window).



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