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https://issues.apache.org/jira/browse/BEAM-32?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15995613#comment-15995613
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Ben Chambers commented on BEAM-32:
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http://stackoverflow.com/questions/43765921/combine-perkey-receives-empty-groups-when-a-repeatedly-trigger-is-used/43769182#43769182
Emmitting an empty creates a problem for a CombineFn. Specifically, it requires
an identity. For some classes of combine Fn there is no natural identity. For
example, "the smallest string combineFn" would have "the largest string" as its
identity. But we shouldn't actually instantiate the largest string, so then the
combineFn needs to have an artificial identity (such as "null") created, and
handled.
> 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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