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https://issues.apache.org/jira/browse/BEAM-696?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15546263#comment-15546263
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Kenneth Knowles commented on BEAM-696:
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The SDK needs to lay out a spec, which I think is what Pei is saying, and the
runner comes up with the execution plan. To clarify - are you suggesting that
{{CombineFn}} should only be allowed side input access in {{extractOutput}}, or
are you suggesting that runners be required to wait until {{extractOutput}}
_will_ be called before running a sequence of {{addInput}}* {{mergeAccum}}*
{{extractoutput}} that accesses side inputs?
The latter sounds like it could be loosed to "give a consistent view of a side
input to the sequence of {{addInput}}* {{mergeAccum}}* {{extractOutput}}" and
your proposed execution plan is one obvious choice of how to achieve it.
> 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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