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From "Eugene Kirpichov (JIRA)" <j...@apache.org>
Subject [jira] [Created] (BEAM-2447) Reintroduce DoFn.ProcessContinuation
Date Tue, 13 Jun 2017 23:56:00 GMT
Eugene Kirpichov created BEAM-2447:
--------------------------------------

             Summary: Reintroduce DoFn.ProcessContinuation
                 Key: BEAM-2447
                 URL: https://issues.apache.org/jira/browse/BEAM-2447
             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 us.

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 https://docs.google.com/document/d/1BGc8pM1GOvZhwR9SARSVte-20XEoBUxrGJ5gTWXdv3c/edit
: 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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