Kenneth Knowles created BEAM-1283:
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Summary: 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 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.
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