Kenneth Knowles created BEAM-653:
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Summary: Refine specification for WindowFn.isCompatible()
Key: BEAM-653
URL: https://issues.apache.org/jira/browse/BEAM-653
Project: Beam
Issue Type: New Feature
Components: beam-model
Reporter: Kenneth Knowles
{{WindowFn#isCompatible}} doesn't really have a spec. In practice, it is used
primarily when flattening together multiple PCollections. All of the WindowFns
must be compatible, and then just a single WindowFn is selected arbitrarily for
the output PCollection.
In consequence, downstream of the Flatten, the merging behavior will be taken
from this WindowFn.
Currently, there are some mismatches:
- Sessions with different gap durations _are_ compatible today, but probably
shouldn't be since merging makes little sense. (The use of tiny proto-windows
is an implementation detail anyhow)
- SlidingWindows and FixedWindows _could_ reasonably be compatible if they had
the same duration, though it might be odd.
Either way, we should just nail down what we actually mean so we can arrive at
a verdict in these cases.
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