Major Description
We have observed the below errors in some cluster:
Uncaught exception in scheduled task 'handleTxnCompletion-902667' exception.message:Trying to complete a transactional offset commit for producerId *** and groupId *** even though the offset commit record itself hasn't been appended to the log.
When a transaction is completed, the transaction coordinator sends a WriteTxnMarkers request to all the partitions involved in the transaction to write the markers to them. When the broker receives it, it writes the markers and if markers are written to the __consumer_offsets partitions, it informs the group coordinator that it can materialize the pending transactional offsets in its main cache. The group coordinator does this asynchronously since Apache Kafka 2.0, see this patch.
The above error appends when the asynchronous operation is executed by the scheduler and the operation finds that there are pending transactional offsets that were not written yet. How come?
There is actually an issue is the steps described above. The group coordinator does not wait until the asynchronous operation completes to return to the api layer. Hence the WriteTxnMarkers response may be send back to the transaction coordinator before the async operation is actually completed. Hence it is possible that the next transactional produce to be started also before the operation is completed too. This could explain why the group coordinator has pending transactional offsets that are not written yet.
There is a similar issue when the transaction is aborted. However on this path, we don't have any checks to verify whether all the pending transactional offsets have been written or not so we don't see any errors in our logs. Due to the same race condition, it is possible to actually remove the wrong pending transactional offsets.
PS: The new group coordinator is not impacted by this bug.