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https://issues.apache.org/jira/browse/KAFKA-7897?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16760382#comment-16760382
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ASF GitHub Bot commented on KAFKA-7897:
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hachikuji commented on pull request #6232: KAFKA-7897; Clear leader epoch cache
after message format downgrade
URL: https://github.com/apache/kafka/pull/6232
If the message format is downgraded, we should clear the leader epoch cache
so that it is not mistakenly used for truncation. We want to revert to
truncation by high watermark.
### Committer Checklist (excluded from commit message)
- [ ] Verify design and implementation
- [ ] Verify test coverage and CI build status
- [ ] Verify documentation (including upgrade notes)
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> 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
> Priority: Major
>
> 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 sole
> prerequisite 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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