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From "Daniel Halperin (JIRA)" <>
Subject [jira] [Updated] (BEAM-696) Side-Inputs non-deterministic with merging main-input windows
Date Thu, 30 Mar 2017 17:13:42 GMT


Daniel Halperin updated BEAM-696:
    Issue Type: Improvement  (was: Bug)

> Side-Inputs non-deterministic with merging main-input windows
> -------------------------------------------------------------
>                 Key: BEAM-696
>                 URL:
>             Project: Beam
>          Issue Type: Improvement
>          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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