[KAFKA-5716] Connect: When SourceTask.commit it is possible not everthing from SourceTask.poll has been sent

• Type: Bug
• Status: Patch Available
• Priority: Minor
• Resolution: Unresolved
• Component/s: connect
• Labels: kip-381 needs-kip
• Assignee: Per Steffensen
• Reporter: Per Steffensen
• Created: 09/Aug/17 10:16
• Updated: 14/Jun/23 16:25

Description

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

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.

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)

  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.

• 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.

Issue Links

• causes: KAFKA-15091 Javadocs for SourceTask::commit are incorrect
• links to: GitHub Pull Request #3872