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https://issues.apache.org/jira/browse/KAFKA-5716?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16172057#comment-16172057
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Randall Hauch commented on KAFKA-5716:
--------------------------------------
{quote}
I am! And couldn't live without it I cannot afford to acknowledge data going
into my source-connector before the corresponding outgoing records AND their
offset-changes has been written, flushed and acknowledged. Today I pretend that
everything polled has been written to offset-storage when task.commit() is
called, even though that is not always entirely true, but it is close. But not
knowing anything about when offsets have been written to offset-storage would
definitely leave me in the blind.
{/quote}
The idiomatic pattern for source connectors is to rely upon the source
partition and source offsets included in each source record, and to have
Connect periodically commit those offsets to offset storage (e.g., Kafka in
distributed mode) as well as during graceful shutdown of the connector. Connect
then uses the committed offsets upon restart to inform the connector of the
last persisted offsets the connector provided, and then the connector can
restart from that point. In this scenario, the connector doesn't need to
acknowledge or be notified of the committed offsets since Connect is completely
managing the offsets for the connector.
I'd be interested in hearing more about your use case, where IIUC your
connector needs to notify the source system that progress has completed. Can
you describe the source system and why you're using this approach? Does the
source not allow you to replay from any point? Are you using this mechanism to
notify the source system that it can garbage collect events that your connector
is reading?
> 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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