Jason Gustafson created KAFKA-6392:
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Summary: Do not permit message down-conversion for replicas
Key: KAFKA-6392
URL: https://issues.apache.org/jira/browse/KAFKA-6392
Project: Kafka
Issue Type: Improvement
Reporter: Jason Gustafson
We have seen several cases where down-conversion caused replicas to diverge
from the leader in subtle ways. Generally speaking, even if we addressed all of
the edge cases so that down-conversion worked correctly as far as consistency
of offsets, it would probably still be a bad idea to permit down-conversion.
For example, this can cause message timestamps to be lost if down-converting
from v1 to v0, or transactional data could be lost if down-converting from v2
to v1 or v0.
With that in mind, it would better to forbid down-conversion for replica
fetches. Following the normal upgrade procedure, down-conversion is not needed
anyway, but users often skip updating the inter-broker version. It is probably
better in these cases to let the ISR shrink until the replicas have been
updated as well.
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