[jira] [Commented] (BEAM-170) Session windows should not be identified by their bounds

["Kenneth Knowles (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/BEAM-170?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16274684#comment-16274684 ] 

Kenneth Knowles commented on BEAM-170:
--------------------------------------

I think there are some new thoughts on this. For a long time, we've wanted to stop triggers from "finishing" and closing the window. With that change implemented, two sessions with the same key and bounds cannot both exist - they will merge. So that part is solved in a simpler way.

The only other way that we would be comparing two sessions by their bounds (their encoded form) would be if we were doing another GBK on already-merged sessions, but today that will cause a pipeline to be rejected since you can't GBK after merging without re-windowing. I don't know if there is a use case that would convince us to change this, but we haven't seen it yet.

> Session windows should not be identified by their bounds
> --------------------------------------------------------
>
>                 Key: BEAM-170
>                 URL: https://issues.apache.org/jira/browse/BEAM-170
>             Project: Beam
>          Issue Type: Bug
>          Components: sdk-java-core
>            Reporter: Kenneth Knowles
>              Labels: backward-incompatible
>
> Today, if two session windows for the same key have the same bounds, they are considered the same window. This is an accident. It is not intended that any session windows are considered equal except via the operation of merging them into the same session.
> A risk associated with this behavior is that two windows that happen to coincide will share per-window-and-key state rather than evolving separately and having their separate state reconciled by state merging logic. These code paths are not required to be coherent, and in practice they are not.
> In particular, if the trigger for a session window ever finishes, then subsequent data in a window with the same bounds will be dropped, whereas if it had differed by a millisecond it would have created a new session, ignoring the previously closed session.



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