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From "Daniel Halperin (JIRA)" <>
Subject [jira] [Commented] (BEAM-696) Document: Side-Inputs non-deterministic with merging main-input windows
Date Thu, 30 Mar 2017 17:17:41 GMT


Daniel Halperin commented on BEAM-696:

Status update:

* Community has agreed that the model definition is correct, and that runners should not make
performance improvements like "combine-before-GBK" where those changes could result in incorrect

* At the time, documentation was needed to explain this issue -- it was present on the old
Dataflow website but not the Beam website.

A lot of documentation has been moved to Beam. I found what I believe is the content [~amitsela]

If the main input element exists in more than one window, then processElement gets called
multiple times, once for each window. Each call to processElement projects the “current”
window for the main input element, and thus might provide a different view of the side input
each time.

If the side input has multiple trigger firings, Beam uses the value from the latest trigger
firing. This is particularly useful if you use a side input with a single global window and
specify a trigger.

[~amitsela] do you think we can close this now?

> Document: Side-Inputs non-deterministic with merging main-input windows
> -----------------------------------------------------------------------
>                 Key: BEAM-696
>                 URL:
>             Project: Beam
>          Issue Type: Task
>          Components: beam-model
>            Reporter: Ben Chambers
> 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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