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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. If a mirror follower attempts to fetch from an offset below the source cluster's log start offset (e.g. fetching offset 50 when log start offset is 100), the source broker returns an OffsetOutOfRangeException. The mirror leader handles this by truncating its local log to the source's current log start offset and resuming fetching from that point. This ensures the destination cluster mirrors the current retention state of the source cluster without attempting to replicate already-deleted data.

Group

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Offsets

Cluster Mirroring synchronizes group offsets from the source cluster to the destination cluster, enabling consumers to resume consumption from their last position after failover. Both traditional consumer groups and share consumer groups (Kafka Queue functionality) are supported, with offset synchronization running in two separate phases to prevent cross-type conflicts. For consumer groups, committed offsets are periodically fetched from the source and applied to the destination. For share groups, which use a different offset management model based on Share-Partition Start Offset (SPSO) and Share-Partition End Offset (SPEO), the current SPSO is retrieved from the source and applied to the destination, which also initializes the group state in both the group coordinator and share coordinator. This means a share group can be initialized in the destination cluster even if it doesn't exist yet, eliminating the need for pre-creation or complex state management.

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