Kenneth Knowles created BEAM-653:

             Summary: Refine specification for WindowFn.isCompatible() 
                 Key: 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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