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https://issues.apache.org/jira/browse/BEAM-1372?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17546263#comment-17546263
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Kenneth Knowles commented on BEAM-1372:
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This issue has been migrated to https://github.com/apache/beam/issues/18145
> OutputTimeFn and Accumulating Mode is Confusing
> -----------------------------------------------
>
> Key: BEAM-1372
> URL: https://issues.apache.org/jira/browse/BEAM-1372
> Project: Beam
> Issue Type: Bug
> Components: beam-model
> Reporter: Thomas Groh
> Priority: P3
>
> See [here|
> https://github.com/tgroh/beam/commit/2238df334a368ce1a41e14ee616be954c5430c73]
> for an example pipeline
> The Timestamp used by a pane does not change based on the accumulation mode
> of the windowing strategy - as a result, elements which have associated
> timestamps can not be safely reassigned to those timestamps after a
> GroupByKey if more than one pane could have been produced, regardless of the
> {{OutputTimeFn}}. The first example pipeline demonstrates two PCollections
> where the elements within the last PCollection cannot be reassigned to their
> timestamps, even though we are using
> {{OutputTimeFn#outputAtEarliestInputTimestamp}} and
> When using a more complex windowing strategy like sessions, this is even more
> confusing - a session that spans more than one of the downstream windows but
> that is produced in multiple panes will over time be assigned to later and
> later windows as more panes are produced - thus, a pipeline that produces
> session windows and wishes to group the sessions by the point at which they
> started must only ever produce a single pane per session.
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