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https://issues.apache.org/jira/browse/BEAM-696?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15546538#comment-15546538
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Amit Sela commented on BEAM-696:
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So it runs once, on the merged window ?
That happens in the bundle level, correct ? Do bundles always behave the same ?
in terms of #elements they hold on to ?
If not, and a side input is called on the merged windows, won't the sideInput
value be affected from things like network or something else that may affect
the bundle's contents ? and if the bundle always holds just one element, then
merging never happens, correct ?
I find this a bit confusing, I think the problem here has to do with the fact
that applying a CombineFn with SideInputs on any PCollection is problematic.
Sessions seem to be handled as any other BoundedWindow for that matter, but
they are not..
BTW, isn't this:
https://github.com/apache/incubator-beam/blob/master/sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/windowing/Sessions.java#L84
saying that sideInputs are not allowed on Sessions ? Fact is that they are
allowed, but what does that mean ?
> Side-Inputs non-deterministic with merging main-input windows
> -------------------------------------------------------------
>
> Key: BEAM-696
> URL: https://issues.apache.org/jira/browse/BEAM-696
> Project: Beam
> Issue Type: Bug
> Components: beam-model
> Reporter: Ben Chambers
> Assignee: Pei He
>
> Side-Inputs are non-deterministic for several reasons:
> 1. Because they depend on triggering of the side-input (this is acceptable
> because triggers are by their nature non-deterministic).
> 2. They depend on the current state of the main-input window in order to
> lookup the side-input. This means that with merging
> 3. Any runner optimizations that affect when the side-input is looked up may
> cause problems with either or both of these.
> This issue focuses on #2 -- the non-determinism of side-inputs that execute
> within a Merging WindowFn.
> Possible solution would be to defer running anything that looks up the
> side-input until we need to extract an output, and using the main-window at
> that point. Specifically, if the main-window is a MergingWindowFn, don't
> execute any kind of pre-combine, instead buffer all the inputs and combine
> later.
> This could still run into some non-determinism if there are triggers
> controlling when we extract output.
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