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From "Randall Hauch (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (KAFKA-5716) Connect: When SourceTask.commit it is possible not everthing from SourceTask.poll has been sent
Date Sun, 10 Sep 2017 17:02:00 GMT

    [ https://issues.apache.org/jira/browse/KAFKA-5716?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16160407#comment-16160407
] 

Randall Hauch commented on KAFKA-5716:
--------------------------------------

[~steff1193], thanks for logging the issue and submitting a proposal for the change. You're
analysis does seem to be correct, though I need to spend a bit more time verifying it, and
I'd love to hear [~hachikuji]'s thoughts. 

Have you tried replicating this with a test case? It might not be straightforward considering
it is dependent upon the timing of the WorkerSourceTask implementation, but is seems like
it should be possible to replicate with a mock offset writer and consumer. Not only will this
help prove your conjecture, but it also will help verify that we can correctly fix it.

Also, please be aware that all changes to the Connect API *must* be binary compatible with
previous releases so that existing SourceTask implementations need not be changed or even
recompiled to run on newer versions of the framework. This has two implications for your proposed
patch.

First, we cannot change the signature of public API methods, so we need to avoid changing
the name of {{commitRecord(...)}}. Like it or not, though, we're stuck with that naming convention.
FWIW, I personally think the names of {{commitRecord(...)}}, {{commit()}, and even a new {{commitOffsets(SourceRecord)}}
on the SourceTask actually _do_ make sense since they reflect the action that the source task
should take. 

Second, we have to keep the existing {{commit()}} method, but we deprecate it and can add
a new method such as {{commitOffsets(SourceRecord)}} that has the behavior you state and that
should by default call the existing {{commit()}}. Doing this would allow existing SourceTask
implementations to continue to work, albeit potentially with the incorrect assumptions/behavior
as you argue above. However, any developer that wants to take advantage of the new/correct
behavior can change their {{SourceTask}} implementations to instead use {{commitOffsets(SourceRecord)}}
rather than {{commit()}} to switch to the correct behavior. Note that we should definitely
have thorough JavaDoc for both {{commit()}} and {{commitOffsets(SourceRecord)}} that should
say what they do by default and how implementations can take advantage of them.

Again, I *think* your analysis is correct, but I need to spend time looking into this and
hopefully get others to confirm it as well. In the meantime, it might be worth updating your
patch and, as [~yuzhihong@gmail.com] mentioned, creating a pull request with your changes
based the {{trunk}} branch. One advantage of maintaining binary compatibility is that we can
more easily backport the changes to older branches, though just because we can doesn't mean
we will. If the committers do backport and have any non-trivial merge conflicts, they'll ask
for a separate dedicated PR for the older branch(es).

Thanks again for all your work on this!

> 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
>          Components: KafkaConnect
>            Reporter: Per Steffensen
>            Priority: Minor
>         Attachments: KAFKA-5716.patch
>
>
> 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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