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https://issues.apache.org/jira/browse/BEAM-32?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15995636#comment-15995636
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Kenneth Knowles commented on BEAM-32:
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Yea, this seems like a really problematic situation.
This only happens when there was some data (to make the window active) and it
was all emitted in an early pane.
- In accumulating mode, we will always re-emit the full result, which is fine.
A user may at some point in their pipeline filter for just the on-time result
and that will work.
- In discarding mode, it will be like a punctuation that says "you've got all
the data". In this case the user would have to filter out late data, rather
than have a positive filter for on-time. That also seems fine.
The impact of this is that users cannot safely use identity-less {{CombineFn}}
in discarding mode with early triggering. It is easy to adjoin an identity
element to any type so it isn't quite as critical as I had feared.
Noting also BEAM-210 that lets users turn it off, but I'm a bit unsure about
the best default.
> Consider not emitting empty ON_TIME pane unless requested
> ---------------------------------------------------------
>
> Key: BEAM-32
> URL: https://issues.apache.org/jira/browse/BEAM-32
> Project: Beam
> Issue Type: New Feature
> Components: beam-model, runner-core
> Reporter: Kenneth Knowles
> Priority: Minor
> Labels: Triggers, Windowing, backward-incompatible
> Fix For: First stable release
>
>
> Today, the ReduceFnRunner sets a timers and emits an empty ON_TIME pane as
> long as the trigger allows it. This could be controlled in a manner analogous
> to the empty final pane at window expiration (also owned by the
> ReduceFnRunner).
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