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

["Ben Chambers (JIRA)" <ji...@apache.org>]

Ben Chambers created BEAM-696:
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             Summary: 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: Frances Perry


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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