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In KAFKA-18723, we identified a race condition where a late-arriving fetch response could contain corrupted or inconsistent records. The fix ensures that only record batches whose partition leader epoch is less than or equal to the leader epoch in the Fetch request are appended. The destination cluster stores batches using the leader epoch from the source cluster. For the mirroring leader in the destination cluster, this works naturally: the leader epoch in the Fetch request is set to the latest source cluster leader epoch, so the existing validation applies without issue. For followers in the destination cluster, however, the situation is different. During mirroring, the local leader epoch diverges from the batch leader epoch. The fetched batch may carry a leader epoch of X while the local leader epoch is Y, where X > Y or X < Y. In either case, the KAFKA-18723 fix no longer applies correctly. To address this, we introduce the MirrorLeaderEpoch field in the Fetch request and response.

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