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The main problem from a compatibility perspective is dealing with the existing producers which reset the sequence number to 0 but continue to use the same epoch. We believe that caching the producer state even after it is no longer retained in the log will make the UNKNOWN_PRODUCER_ID error unlikely in practice. Furthermore, even if the sequence number is reset, the fencing check should still be valid. So we expect the behavior to continue to work as expected even with the additional protection.
The new CheckProducerEpoch GetTransactionState API and the new version of the transaction state message will not be used until the inter-broker version supports it. We expect the usual two rolling bounce process for updating the cluster.
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