Jason Gustafson created KAFKA-7897:
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             Summary: Invalid use of epoch cache following message format 
downgrade
                 Key: KAFKA-7897
                 URL: https://issues.apache.org/jira/browse/KAFKA-7897
             Project: Kafka
          Issue Type: Bug
            Reporter: Jason Gustafson
            Assignee: Jason Gustafson


Message format downgrades are not supported, but they generally work as long as 
broker/clients at least can continue to parse both message formats. After a 
downgrade, the truncation logic should revert to using the high watermark, but 
currently we use the existence of any cached epoch as the requirement in order 
to leverage OffsetsForLeaderEpoch. This has the effect of causing a massive 
truncation after startup which causes re-replication.

I think our options to fix this are to either 1) clear the cache when we notice 
a downgrade, or 2) forbid downgrades and raise an error.



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