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  1. Kafka
  2. KAFKA-7408

Truncate to LSO on unclean leader election



    • Improvement
    • Status: Open
    • Major
    • Resolution: Unresolved
    • None
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      If an unclean leader is elected, we may lose committed transaction data. That alone is expected, but what is worse is that a transaction which was previously completed (either committed or aborted) may lose its marker and become dangling. The transaction coordinator will not know about the unclean leader election, so will not know to resend the transaction markers. Consumers with read_committed isolation will be stuck because the LSO cannot advance.

      To keep this scenario from occurring, it would be better to have the unclean leader truncate to the LSO so that there are no dangling transactions. Truncating to the LSO is not alone sufficient because the markers which allowed the LSO advancement may be at higher offsets. What we can do is let the newly elected leader truncate to the LSO and then rewrite all the markers that followed it using its own leader epoch (to avoid divergence from followers).

      The interesting cases when an unclean leader election occurs are are when a transaction is ongoing.

      1. If a producer is in the middle of a transaction commit, then the coordinator may still attempt to write transaction markers. This will either succeed or fail depending on the producer epoch in the unclean leader. If the epoch matches, then the WriteTxnMarker call will succeed, which will simply be ignored by the consumer. If the epoch doesn't match, the WriteTxnMarker call will fail and the transaction coordinator can potentially remove the partition from the transaction.

      2. If a producer is still writing the transaction, then what happens depends on the producer state in the unclean leader. If no producer state has been lost, then the transaction can continue without impact. Otherwise, the producer will likely fail with an OUT_OF_ORDER_SEQUENCE error, which will cause the transaction to be aborted by the coordinator. That takes us back to the first case.

      By truncating the LSO, we ensure that transactions are either preserved in whole or they are removed from the log in whole. For an unclean leader election, that's probably as good as we can do. But we are ensured that consumers will not be blocked by dangling transactions. The only remaining situation where a dangling transaction might be left is if one of the transaction state partitions has an unclean leader election.


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              jagsancio Jose Armando Garcia Sancio
              hachikuji Jason Gustafson
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