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From "Pei He (JIRA)" <j...@apache.org>
Subject [jira] [Comment Edited] (BEAM-696) Side-Inputs non-deterministic with merging main-input windows
Date Mon, 10 Oct 2016 21:27:21 GMT

    [ https://issues.apache.org/jira/browse/BEAM-696?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15563569#comment-15563569
] 

Pei He edited comment on BEAM-696 at 10/10/16 9:26 PM:
-------------------------------------------------------

re: to "Sessions is not allowed in side inputs"
There are two WindowFns involved, one from main input PCollection and the other from side
input view.
For example, you can have Sessions in the main input window, and looks up values in a fix
windowed side input.
Sessions.java#L84 is saying side inputs cannot be windowed by Sessions.

This jira [BEAM-696] is about when the main input is windowed by Sessions, how Combine with
side input should be executed.


was (Author: peihe0@gmail.com):
re: to "Sessions is not allowed in side inputs"
There are two WindowFns involved, one from main input PCollection and the other from side
input view.
Now, you can have Sessions in the main input window, and looks up values in a fix windowed
side input.
Sessions.java#L84 is saying side inputs cannot be windowed by Sessions.

This jira [BEAM-696] is about when the main input is windowed by Sessions, how Combine with
side input should be executed.

> 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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