[jira] [Closed] (BEAM-696) Document: Side-Inputs non-deterministic with merging main-input windows

["Amit Sela (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/BEAM-696?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Amit Sela closed BEAM-696.
--------------------------
       Resolution: Not A Problem
    Fix Version/s: Not applicable

Resolved by providing better documentation. See comments for details. 

> Document: 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: Task
>          Components: beam-model
>            Reporter: Ben Chambers
>            Assignee: Amit Sela
>             Fix For: Not applicable
>
>
> 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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