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  1. Kafka
  2. KAFKA-7164

Follower should truncate after every leader epoch change

    Details

    • Type: Bug
    • Status: Resolved
    • Priority: Major
    • Resolution: Fixed
    • Affects Version/s: None
    • Fix Version/s: 1.1.2, 2.0.1, 2.1.0
    • Component/s: None
    • Labels:
      None

      Description

      Currently we skip log truncation for followers if a LeaderAndIsr request is received, but the leader does not change. This can lead to log divergence if the follower missed a leader change before the current known leader was reelected. Basically the problem is that the leader may truncate its own log prior to becoming leader again, so the follower would need to reconcile its log again.

      For example, suppose that we have three replicas: r1, r2, and r3. Initially, r1 is the leader in epoch 0 and writes one record at offset 0. r3 replicates this successfully.

      r1: 
        status: leader
        epoch: 0
        log: [{id: 0, offset: 0, epoch:0}]
      r2: 
        status: follower
        epoch: 0
        log: []
      r3: 
        status: follower
        epoch: 0
        log: [{id: 0, offset: 0, epoch:0}]
      

      Suppose then that r2 becomes leader in epoch 1. r1 notices the leader change and truncates, but r3 for whatever reason, does not.

      r1: 
        status: follower
        epoch: 1
        log: []
      r2: 
        status: leader
        epoch: 1
        log: []
      r3: 
        status: follower
        epoch: 0
        log: [{offset: 0, epoch:0}]
      

      Now suppose that r2 fails and r1 becomes the leader in epoch 2. Immediately it writes a new record:

      r1: 
        status: leader
        epoch: 2
        log: [{id: 1, offset: 0, epoch:2}]
      r2: 
        status: follower
        epoch: 2
        log: []
      r3: 
        status: follower
        epoch: 0
        log: [{id: 0, offset: 0, epoch:0}]
      

      If the replica continues fetching with the old epoch, we can have log divergence as noted in KAFKA-6880. However, if r3 successfully receives the new LeaderAndIsr request which updates the epoch to 2, but skips the truncation, then the logs will stay inconsistent.

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              • Assignee:
                bob-barrett Bob Barrett
                Reporter:
                hachikuji Jason Gustafson
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                Dates

                • Created:
                  Updated:
                  Resolved: