[jira] [Created] (KAFKA-6392) Do not permit message down-conversion for replicas

["Jason Gustafson (JIRA)" <ji...@apache.org>]

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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