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https://issues.apache.org/jira/browse/KAFKA-5716?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16120577#comment-16120577
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Per Steffensen commented on KAFKA-5716:
---------------------------------------
{quote}
Commit the offsets, up to the offsets that have been returned by \{@link
#poll()}
*and confirmed by a call to \{@link #commitRecord(SourceRecord)}*.
{quote}
Actually I am pretty sure that is not true either, so I guess when
task.commit() is called, you really do not know exactly for which of the
SourceRecords you returned from poll
* a) the derived outgoing ProducerRecord has been sent to Kafka
* b) the derived connector-offsets has been written/flushed
Guess the only thing I can think of, where you do not have to introduce
significantly more synchronization (we do not want that), but where you as the
SourceTask-developer knows "enough" for most cases, is to change
SourceTask.commit() to take a parameter of type SourceRecord. This record will
be the last record given by WorkerSourceTask to its offsetWriter. The
SourceTask-developer will know upon call to commit, that
* a) Outgoing ProducerRecords derived from any SourceRecord returned from poll
prior to the given SourceRecord (included) have been sent to Kafka. Outgoing
ProducerRecords derived from any SourceRecord returned from poll after the
given SourceRecord may or may not have been sent to Kafka (conservative
guarantee)
* b) Connector-offsets derived from any SourceRecord returned from poll prior
to the given SourceRecord (included) have been written and flushed to
offset-storage. Outgoing connector-offsets derived from any SourceRecord
returned from poll after the given SourceRecord have NOT been written to
offset-storage (strict guarantee)
Sorry, just thinking loud
> 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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