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The mirror leader replicates these compacted log segments exactly as they exist in the source cluster, maintaining the same offset assignments and gaps. After failover, when the mirror topic becomes writable, log compaction continues normally in the destination cluster according to the topic's compaction policy, and any new records produced locally will fill in after the highest mirrored offset.
When a destination cluster lags behind the source on a compacted topic, tombstone records may not yet be replicated at the time of failover. For example, if the source has a tombstone at offset 100 that deletes a key originally written at offset 3, but the destination has only replicated up to offset 50, that tombstone is never applied on the destination. After failover, offset 3 remains as a stale entry that will never be cleaned up. The problem is compounded on failback: truncating the former source to match the destination also discards the tombstone, so the orphaned key persists in both clusters permanently. This is an inherent limitation of asynchronous replication. Compaction correctness depends on the full sequence of tombstones being present, and any that fall beyond the replication watermark at failover time are lost. The same problem exists with MirrorMaker 2. The follow-up KIP for synchronous mirroring may address this by ensuring zero lag at switchover, guaranteeing all tombstones are replicated before failover occurs.
Topic Retention
Cluster Mirroring handles topic retention policies by periodically synchronizing the topic configurations from the source cluster, ensuring that the topic retention policies are consistent. When the source cluster applies retention policies, older log segments are deleted and the log start offset advances. For example, if a topic originally contained offsets 0-100 and retention deletes offsets 0-99, the source cluster's log start offset becomes 100. When the mirror leader fetches from the source, it discovers the new log start offset and updates its local log start offset to match, creating the same offset gap.
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