[jira] [Commented] (BEAM-1396) GABWVOBDoFn expects grouped values to be ordered by their timestamp but there is no such guarantee

["Daniel Halperin (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/BEAM-1396?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15853298#comment-15853298 ] 

Daniel Halperin commented on BEAM-1396:
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To add a little more flavor: I'm not sure what right thing to do with unused or little-used code in {{runners-core}}. These are meant to be illustrative and/or useful to show runners how key functionality can be implemented. So, among many courses of action:

1. More clearly document expectations in {{GroupAlsoByWindowsViaOutputBufferDoFn}}
1. Possibly add another version ({{GroupAlsoByWindowsUsingSortedInputDoFn}} or a flag that indicates whether input is already sorted. (ViaSortedOutputBuffer)?
1 .....

> GABWVOBDoFn expects grouped values to be ordered by their timestamp but there is no such guarantee
> --------------------------------------------------------------------------------------------------
>
>                 Key: BEAM-1396
>                 URL: https://issues.apache.org/jira/browse/BEAM-1396
>             Project: Beam
>          Issue Type: Bug
>          Components: sdk-java-core
>            Reporter: Amit Sela
>            Assignee: Kenneth Knowles
>
> GABWVOBDoFn relies on the grouped values to be ordered by their timestamp but nothing in the SDK guarantees this: 
> https://github.com/apache/beam/blob/master/runners/core-java/src/main/java/org/apache/beam/runners/core/GroupAlsoByWindowsViaOutputBufferDoFn.java#L86
> If such a chunk of timestamped values will be processed out-of-order I assume we'd end up with an {{IllegalStateException}} thrown here:
> https://github.com/apache/beam/blob/master/runners/core-java/src/main/java/org/apache/beam/runners/core/InMemoryTimerInternals.java#L191
> I suggest we go ahead and add sorting before processing the bundle in chunks - this might prove expensive in extreme cases where a very large bundle with very few keys is processed, but it seems that timestamp order is necessary.
> As for runners who provide order guarantee, since GABW is optional I don't see an issue here, though [~dhalperi@google.com] suggested we add a "shouldSort" flag.
> Also, probably worth creating a test for this, though it would prove difficult since we would have to preset the order which is the problem to begin with :-)



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