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From "Amit Sela (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (BEAM-696) Side-Inputs non-deterministic with merging main-input windows
Date Tue, 04 Oct 2016 09:47:21 GMT

    [ https://issues.apache.org/jira/browse/BEAM-696?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15544926#comment-15544926
] 

Amit Sela commented on BEAM-696:
--------------------------------

[~peihe0@gmail.com] shouldn't the SDK take care/enforce this behaviour and not let a runner
decide this ? It sounds like for merging windows the CombineFns should lookup sideInputs only
at extractOutput, otherwise I don't see how DataflowRunner is deferring, local-combine cannot
guarantee the state of the merging window (in a specific bundle), correct ?

I think this issue is about the SDK / Runner API and should be resolved by either limiting
the use of sideInputs with Sessions or enforcing/instructing runners to defer looking-up sideInput
until extractOutput executes.

Am I missing something here ?  
  

> 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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