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From "Kenneth Knowles (JIRA)" <>
Subject [jira] [Commented] (BEAM-2447) Reintroduce DoFn.ProcessContinuation
Date Wed, 21 Jun 2017 17:29:00 GMT


Kenneth Knowles commented on BEAM-2447:

Is "After a failed tryClaim() call, the ProcessElement method MUST return stop()" necessary?
What is the pitfall of the runner ignoring the value and treating everything as stop()?

> Reintroduce DoFn.ProcessContinuation
> ------------------------------------
>                 Key: BEAM-2447
>                 URL:
>             Project: Beam
>          Issue Type: Bug
>          Components: sdk-java-core
>            Reporter: Eugene Kirpichov
>            Assignee: Eugene Kirpichov
> ProcessContinuation.resume() is useful for tailing files - when we reach current EOF,
we want to voluntarily suspend the process() call rather than wait for runner to checkpoint
> In BEAM-1903, DoFn.ProcessContinuation was removed because there was ambiguity about
the semantics of resume() especially w.r.t. the following situation described in
: the runner has taken a checkpoint on the tracker, and then the ProcessElement call returns
resume() signaling that the work is still not done - then there's 2 checkpoints to deal with.
> Instead, the proper way to refine this semantics is:
> - After checkpoint() on a RestrictionTracker, the tracker MUST fail all subsequent tryClaim()
calls, and MUST succeed in checkDone().
> - After a failed tryClaim() call, the ProcessElement method MUST return stop()
> - So ProcessElement can return resume() only *instead* of doing tryClaim()
> - Then, if the runner has already taken a checkpoint but tracker has returned resume(),
we do not need to take a new checkpoint - the one already taken already accurately describes
the remainder of the work.

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