Alan created KAFKA-12558:
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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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