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From "Per Steffensen (JIRA)" <j...@apache.org>
Subject [jira] [Created] (KAFKA-5716) Connect: When SourceTask.commit it is possible not everthing from SourceTask.poll has been sent
Date Wed, 09 Aug 2017 10:17:00 GMT
Per Steffensen created KAFKA-5716:
-------------------------------------

             Summary: Connect: When SourceTask.commit it is possible not everthing from SourceTask.poll
has been sent
                 Key: KAFKA-5716
                 URL: https://issues.apache.org/jira/browse/KAFKA-5716
             Project: Kafka
          Issue Type: Bug
            Reporter: Per Steffensen
            Priority: Minor


Not looking at the very latest code, so the "problem" may have been corrected recently. If
so, I apologize. I found the "problem" by code-inspection alone, so I may be wrong. Have not
had the time to write tests to confirm.

According to java-doc on SourceTask.commit
{quote}
Commit the offsets, up to the offsets that have been returned by \{@link #poll()}. This
method should block until the commit is complete.

SourceTasks are not required to implement this functionality; Kafka Connect will record offsets
automatically. This hook is provided for systems that also need to store offsets internally
in their own system.
{quote}

As I read this, when commit-method is called, the SourceTask-developer is "told" that everything
returned from poll up until "now" has been sent/stored - both the outgoing messages and the
associated connect-offsets. Looking at the implementation it also seems that this is what
it tries to "guarantee/achieve".

But as I see read the code, it is not necessarily true
The following threads are involved
* Task-thread: WorkerSourceTask has its own thread running WorkerSourceTask.execute.
* Committer-thread: From time to time SourceTaskOffsetCommitter is scheduled to call WorkerSourceTask.commitOffsets
(from a different thread)

The two thread synchronize (on the WorkerSourceTask-object) in sendRecord and commitOffsets
respectively, hindering the task-thread to add to outstandingMessages and offsetWriter while
committer-thread is marking what has to be flushed in the offsetWriter and waiting for outstandingMessages
to be empty. This means that the offsets committed will be consistent with what has been sent
out, but not necessarily what has been polled. At least I do not see why the following is
not possible:
* Task-thread polls something from the task.poll
* Before task-thread gets to add (all) the polled records to outstandingMessages and offsetWriter
in sendRecords, committer-thread kicks in and does its commiting, while hindering the task-thread
adding the polled records to outstandingMessages and offsetWriter
* Consistency will not have been compromised, but committer-thread will end up calling task.commit
(via WorkerSourceTask.commitSourceTask), without the records just polled from task.poll has
been sent or corresponding connector-offsets flushed.

If I am right, I guess there are two way to fix it
* Either change the java-doc of SourceTask.commit, to something a-la (which I do believe is
true)
{quote}
Commit the offsets, up to the offsets that have been returned by \{@link #poll()}
*and confirmed by a call to \{@link #commitRecord(SourceRecord)}*.
This method should block until the commit is complete.

SourceTasks are not required to implement this functionality; Kafka Connect will record offsets
automatically. This hook is provided for systems that also need to store offsets internally
in their own system.
{quote}
* or, fix the "problem" so that it actually does what the java-doc says :-)

If I am not right, of course I apologize for the inconvenience. I would appreciate an explanation
where my code-inspection is not correct, and why it works even though I cannot see it. I will
not expect such an explanation, though.



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