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Failback enables mirroring to be reversed after a failover, allowing the original source cluster to become the destination and vice versa. This is critical for scenarios where you want to fail back to the original cluster after recovering from an outage or planned maintenance. When failback is initiated on the old source cluster, it needs to determine where to truncate its log before starting to fetch from the new source cluster. If the new API is supported, the broker sends a LastMirrorredEpochsLastMirrorEpochs request to the new source cluster asking for the LME, and then truncates its local log to the last offset of the returned epoch. If the new API is not supported, the broker truncates to zero and starts mirroring from scratch.

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