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https://issues.apache.org/jira/browse/BEAM-1283?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Davor Bonaci reassigned BEAM-1283:
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Assignee: Thomas Groh
> DoFn finishBundle should be required to specify the window for output
> ---------------------------------------------------------------------
>
> Key: BEAM-1283
> URL: https://issues.apache.org/jira/browse/BEAM-1283
> Project: Beam
> Issue Type: Bug
> Components: beam-model, sdk-java-core, sdk-py
> Reporter: Kenneth Knowles
> Assignee: Thomas Groh
> Labels: backward-incompatible
> Fix For: First stable release
>
>
> The spec is here in Javadoc:
> https://github.com/apache/beam/blob/master/sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/DoFn.java#L128
> "If invoked from {{@StartBundle}} or {{@FinishBundle}}, this will attempt to
> use the {{WindowFn}} of the input {{PCollection}} to determine what windows
> the element should be in, throwing an exception if the {{WindowFn}} attempts
> to access any information about the input element. The output element will
> have a timestamp of negative infinity."
> This is a collection of caveats that make this method not always technically
> wrong, but quite a mess. Ideas that reasonable folks have suggested lately:
> - The {{WindowFn}} cannot actually be applied because {{WindowFn}} is
> allowed to see the element type. The spec just avoids this by limiting which
> {{WindowFn}} can be used.
> - There is no natural output timestamp, so it should always be provided. The
> spec avoids this by specifying an arbitrary and fairly useless timestamp.
> - If it is a merging {{WindowFn}} like sessions that has already been merged
> then you'll just have a bogus proto window regardless of explicit timestamp
> or not.
> The use cases for these methods are best addressed by state plus window
> expiry callback, so we should revisit this spec and probably just wipe it.
> There are some rare case where you might need to output from {{FinishBundle}}
> in a way that is not _actually_ sensitive to bundling (perhaps modulo some
> downstream notion of equivalence) in which case you had better know what
> window you are outputting to. Often it should be the global window.
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