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From "Kenneth Knowles (JIRA)" <>
Subject [jira] [Updated] (BEAM-1283) DoFn.Context.output spec for startBundle/finishBundle is a mess
Date Thu, 09 Mar 2017 19:24:38 GMT


Kenneth Knowles updated BEAM-1283:
    Fix Version/s: First stable release

> DoFn.Context.output spec for startBundle/finishBundle is a mess
> ---------------------------------------------------------------
>                 Key: BEAM-1283
>                 URL:
>             Project: Beam
>          Issue Type: Bug
>          Components: beam-model, sdk-java-core
>            Reporter: Kenneth Knowles
>            Assignee: Kenneth Knowles
>              Labels: backward-incompatible
>             Fix For: First stable release
> The spec is here in Javadoc:
> "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.

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