damccorm opened a new issue, #17947:
URL: https://github.com/apache/beam/issues/17947
`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.
Imported from Jira
[BEAM-653](https://issues.apache.org/jira/browse/BEAM-653). Original Jira may
contain additional context.
Reported by: kenn.
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