Kenneth Knowles created BEAM-3568:
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Summary: Overlapping sessions with zero allowed lateness due to
window expiry rules
Key: BEAM-3568
URL: https://issues.apache.org/jira/browse/BEAM-3568
Project: Beam
Issue Type: Bug
Components: beam-model, runner-core
Reporter: Kenneth Knowles
Assignee: Kenneth Knowles
Consider this sequence, with session gap durations of 5:
- element arrives with timestamp 0, assigned to proto-window [0, 5)
- watermark advances to 6, emitting the session and discarding it
- element arrives with timestamp 3, assigned to proto-window [3, 8) so it is
not dropped as the window is not expired
- watermark advances to 8+, emitting that session
While "technically correct" according to spec, this seems undesirable. It was
introduced when late data dropping was tied to window expiry. I think either
dropping the second element or including it and emitting a merged window would
be OK.
In the case of sessions, we could just retain the window until it cannot
possibly merge with other non-expired data. Even with allowed lateness zero
this is double the gap duration. The window would be in an interesting state
where it would be expired and ineligible for further output but could still
merge and the greater window could be output.
The challenge is that sessions are just one kind of merging window - the
merging logic has to be assumed opaque. So we cannot simply reason about how
sessions work. The other, more drastic option, is to rethink how late data
dropping is defined for merging windows, particularly in the "proto-window"
phase.
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