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From "Kenneth Knowles (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (BEAM-1283) DoFn.Context.output spec for startBundle/finishBundle is a mess
Date Thu, 19 Jan 2017 21:12:26 GMT

     [ https://issues.apache.org/jira/browse/BEAM-1283?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Kenneth Knowles updated BEAM-1283:
----------------------------------
    Description: 
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.

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.

  was:
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 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.

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.


> DoFn.Context.output spec for startBundle/finishBundle is a mess
> ---------------------------------------------------------------
>
>                 Key: BEAM-1283
>                 URL: https://issues.apache.org/jira/browse/BEAM-1283
>             Project: Beam
>          Issue Type: Bug
>          Components: beam-model, sdk-java-core
>            Reporter: Kenneth Knowles
>            Assignee: Kenneth Knowles
>
> 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.
> 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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