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https://issues.apache.org/jira/browse/KAFKA-7786?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Anna Povzner reassigned KAFKA-7786:
-----------------------------------
Assignee: Anna Povzner
> Fast update of leader epoch may stall partition fetching due to
> FENCED_LEADER_EPOCH
> -----------------------------------------------------------------------------------
>
> Key: KAFKA-7786
> URL: https://issues.apache.org/jira/browse/KAFKA-7786
> Project: Kafka
> Issue Type: Bug
> Affects Versions: 2.1.0
> Reporter: Anna Povzner
> Assignee: Anna Povzner
> Priority: Critical
>
> KIP-320/KAFKA-7395 Added FENCED_LEADER_EPOCH error response to a
> OffsetsForLeaderEpoch request if the epoch in the request is lower than the
> broker's leader epoch. ReplicaFetcherThread builds a OffsetsForLeaderEpoch
> request under _partitionMapLock_, sends the request outside the lock, and
> then processes the response under _partitionMapLock_. The broker may receive
> LeaderAndIsr with the same leader but with the next leader epoch, remove and
> add partition to the fetcher thread (with partition state reflecting the
> updated leader epoch) – all while the OffsetsForLeaderEpoch request (with the
> old leader epoch) is still outstanding/ waiting for the lock to process the
> OffsetsForLeaderEpoch response. As a result, partition gets removed from
> partitionStates and this broker will not fetch for this partition until the
> next LeaderAndIsr which may take a while. We will see log message like this:
> [2018-12-23 07:23:04,802] INFO [ReplicaFetcher replicaId=3, leaderId=2,
> fetcherId=0] Partition test_topic-17 has an older epoch (7) than the current
> leader. Will await the new LeaderAndIsr state before resuming fetching.
> (kafka.server.ReplicaFetcherThread)
> We saw this happen with
> kafkatest.tests.core.reassign_partitions_test.ReassignPartitionsTest.test_reassign_partitions.bounce_brokers=True.reassign_from_offset_zero=True.
> This test does partition re-assignment while bouncing 2 out of 4 total
> brokers. When the failure happen, each bounced broker was also a controller.
> Because of re-assignment, the controller updates leader epoch without
> updating the leader on controller change or on broker startup, so we see
> several leader epoch changes without the leader change, which increases the
> likelihood of the race condition described above.
> Here is exact events that happen in this test (around the failure):
> We have 4 brokers Brokers 1, 2, 3, 4. Partition re-assignment is started for
> test_topic-17 [2, 4, 1] —> [3, 1, 2]. At time t0, leader of test_topic-17 is
> broker 2.
> # clean shutdown of broker 3, which is also a controller
> # broker 4 becomes controller, continues re-assignment and updates leader
> epoch for test_topic-17 to 6 (with same leader)
> # broker 2 (leader of test_topic-17) receives new leader epoch:
> “test_topic-17 starts at Leader Epoch 6 from offset 1388. Previous Leader
> Epoch was: 5”
> # broker 3 is started again after clean shutdown
> # controller sees broker 3 startup, and sends LeaderAndIsr(leader epoch 6)
> to broker 3
> # controller updates leader epoch to 7
> # broker 2 (leader of test_topic-17) receives LeaderAndIsr for leader epoch
> 7: “test_topic-17 starts at Leader Epoch 7 from offset 1974. Previous Leader
> Epoch was: 6”
> # broker 3 receives LeaderAndIsr for test_topic-17 and leader epoch 6 from
> controller: “Added fetcher to broker BrokerEndPoint(id=2) for leader epoch 6”
> and sends OffsetsForLeaderEpoch request to broker 2
> # broker 3 receives LeaderAndIsr for test_topic-17 and leader epoch 7 from
> controller; removes fetcher thread and adds fetcher thread + executes
> AbstractFetcherThread.addPartitions() which updates partition state with
> leader epoch 7
> # broker 3 receives FENCED_LEADER_EPOCH in response to
> OffsetsForLeaderEpoch(leader epoch 6), because the leader received
> LeaderAndIsr for leader epoch 7 before it got OffsetsForLeaderEpoch(leader
> epoch 6) from broker 3. As a result, it removes partition from
> partitionStates and it does not fetch until controller updates leader epoch
> and sends LeaderAndIsr for this partition to broker 3. The test fails,
> because re-assignment does not finish on time (due to broker 3 not fetching).
>
> One way to address this is possibly add more state to PartitionFetchState.
> However, we may introduce other race condition. A cleaner way, I think, is to
> return leader epoch in the OffsetsForLeaderEpoch response with
> FENCED_LEADER_EPOCH error, and then ignore the error if partition state
> contains a higher leader epoch. The advantage is less state maintenance, but
> disadvantage is it requires bumping inter-broker protocol.
> h1.
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