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https://issues.apache.org/jira/browse/KAFKA-9307?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16997885#comment-16997885
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Dhruvil Shah commented on KAFKA-9307:
-------------------------------------
Become-follower transition ignored during step 7:
{code:java}
[2019-12-12 03:08:18,241] TRACE [Broker id=3] Received LeaderAndIsr request
PartitionState(controllerEpoch=16, leader=4, leaderEpoch=112, isr=2,4,1,
zkVersion=208, replicas=3,4,2,1, isNew=false) correlation id 1 from controller
2 epoch 16 for partition __transaction_state-41 (state.change.logger)
[2019-12-12 03:08:18,247] WARN [Broker id=3] Ignoring LeaderAndIsr request from
controller 2 with correlation id 1 epoch 16 for partition
__transaction_state-41 since its associated leader epoch 112 is not higher than
the current leader epoch 112 (state.change.logger)
{code}
> Transaction coordinator could be left in unknown state after ZK session
> timeout
> -------------------------------------------------------------------------------
>
> Key: KAFKA-9307
> URL: https://issues.apache.org/jira/browse/KAFKA-9307
> Project: Kafka
> Issue Type: Bug
> Components: core
> Reporter: Dhruvil Shah
> Assignee: Dhruvil Shah
> Priority: Major
>
> We observed a case where the transaction coordinator could not load
> transaction state from __transaction-state topic partition. Clients would
> continue seeing COORDINATOR_LOAD_IN_PROGRESS exceptions until the broker
> hosting the coordinator is restarted.
> This is the sequence of events that leads to the issue:
> # The broker is the leader of one (or more) transaction state topic
> partitions.
> # The broker loses its ZK session due to a network issue.
> # Broker reestablishes session with ZK, though there are still transient
> network issues.
> # Broker is made follower of the transaction state topic partition it was
> leading earlier.
> # During the become-follower transition, the broker loses its ZK session
> again.
> # The become-follower transition for this broker fails in-between, leaving
> us in a partial leader / partial follower state for the transaction topic.
> This meant that we could not unload the transaction metadata. However, the
> broker successfully caches the leader epoch of associated with the
> LeaderAndIsrRequest.
> # Later, when the ZK session is finally established successfully, the broker
> ignores the become-follower transition as the leader epoch was same as the
> one it had cached. This prevented the transaction metadata from being
> unloaded.
> # Because this partition was a partial follower, we had setup replica
> fetchers. The partition continued to fetch from the leader until it was made
> part of the ISR.
> # Once it was part of the ISR, preferred leader election kicked in and
> elected this broker as the leader.
> # When processing the become-leader transition, the transaction state load
> operation failed as we already had transaction metadata loaded at a previous
> epoch.
> # This meant that this partition was left in the "loading" state and we thus
> returned COORDINATOR_LOAD_IN_PROGRESS errors.
> Restarting the broker that hosts the transaction state coordinator is the
> only way to recover from this situation.
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