[jira] [Created] (KAFKA-12558) MM2 may not sync partition offsets correctly

["Alan (Jira)" <ji...@apache.org>]

Alan created KAFKA-12558:
----------------------------

             Summary: MM2 may not sync partition offsets correctly
                 Key: KAFKA-12558
                 URL: https://issues.apache.org/jira/browse/KAFKA-12558
             Project: Kafka
          Issue Type: Bug
          Components: mirrormaker
    Affects Versions: 2.6.1, 2.7.0
            Reporter: Alan


There is a race condition in {{MirrorSourceTask}} where certain partition offsets may never be sent. The bug occurs when the [outstandingOffsetSync semaphore is full|https://github.com/apache/kafka/blob/trunk/connect/mirror/src/main/java/org/apache/kafka/connect/mirror/MirrorSourceTask.java#L207]. In this case, the sendOffsetSync [will silently fail|https://github.com/apache/kafka/blob/trunk/connect/mirror/src/main/java/org/apache/kafka/connect/mirror/MirrorSourceTask.java#L207].

This failure is normally acceptable since offset sync will retry frequently. However, {{maybeSyncOffsets}} has a bug where it will [mutate the partition state|https://github.com/apache/kafka/blob/trunk/connect/mirror/src/main/java/org/apache/kafka/connect/mirror/MirrorSourceTask.java#L199] prior to confirming the result of {{sendOffsetSync}}. The end result is that the partition state is mutated prematurely, and prevent future offset syncs to recover.

Since {{MAX_OUTSTANDING_OFFSET_SYNCS}} is 10, this bug happens when you assign more than 10 partitions to each task.

In my test cases where I had over 100 partitions per task, the majority of the offsets were wrong. Here's an example of such a failure. https://issues.apache.org/jira/browse/KAFKA-12468?focusedCommentId=17308308&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-17308308

During my troubleshooting, I customized the {{MirrorSourceTask}} to confirm that all partitions that has the wrong offset were failing to acquire the initial semaphore. The condition [can be trapped here|https://github.com/apache/kafka/blob/trunk/connect/mirror/src/main/java/org/apache/kafka/connect/mirror/MirrorSourceTask.java#L208].

*Possible Fix:*

A possible fix is to create a {{shouldUpdate}} method in {{PartitionState}}. This method should be read-only and return true if {{sendOffsetSync}} is needed. Once {{sendOffsetSync}} is successful, only then {{update}} should be called.

Here's some pseudocode
{code:java}
private void maybeSyncOffsets(TopicPartition topicPartition, long upstreamOffset,
        long downstreamOffset) {
    PartitionState partitionState =
        partitionStates.computeIfAbsent(topicPartition, x -> new PartitionState(maxOffsetLag));
    if (partitionState.shouldUpdate(upstreamOffset, downstreamOffset)) {
        if(sendOffsetSync(topicPartition, upstreamOffset, downstreamOffset)) {
            partitionState.update(upstreamOffset, downstreamOffset)
        }
    }
}
{code}
 

*Workaround:*

For those who are experiencing this issue, the workaround for this issue is to make sure you have less than or equal to 10 partitions per task. Set your `tasks.max` value accordingly.



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