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https://issues.apache.org/jira/browse/BEAM-653?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17145642#comment-17145642
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Beam JIRA Bot commented on BEAM-653:
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This issue was marked "stale-P2" and has not received a public comment in 14
days. It is now automatically moved to P3. If you are still affected by it, you
can comment and move it back to P2.
> Refine specification for WindowFn.isCompatible()
> -------------------------------------------------
>
> Key: BEAM-653
> URL: https://issues.apache.org/jira/browse/BEAM-653
> Project: Beam
> Issue Type: New Feature
> Components: beam-model
> Reporter: Kenneth Knowles
> Priority: P3
>
> {{WindowFn#isCompatible}} doesn't really have a spec. In practice, it is used
> primarily when flattening together multiple PCollections. All of the
> WindowFns must be compatible, and then just a single WindowFn is selected
> arbitrarily for the output PCollection.
> In consequence, downstream of the Flatten, the merging behavior will be taken
> from this WindowFn.
> Currently, there are some mismatches:
> - Sessions with different gap durations _are_ compatible today, but probably
> shouldn't be since merging makes little sense. (The use of tiny proto-windows
> is an implementation detail anyhow)
> - SlidingWindows and FixedWindows _could_ reasonably be compatible if they
> had the same duration, though it might be odd.
> Either way, we should just nail down what we actually mean so we can arrive
> at a verdict in these cases.
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