[jira] [Commented] (BEAM-653) Refine specification for WindowFn.isCompatible()

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

    [ https://issues.apache.org/jira/browse/BEAM-653?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15953781#comment-15953781 ] 

Daniel Halperin commented on BEAM-653:
--------------------------------------

Is it important to resolve this before first stable release?

> 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, beam-model-runner-api
>            Reporter: Kenneth Knowles
>
> {{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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