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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 LastMirrorEpochs 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 LastMirrorEpochs API is not supported, the broker truncates to zero and starts mirroring from scratch.

Before transitioning a mirror partition from PREPARING to MIRRORING, the MirrorCoordinator must ensure that all in-sync replicas in the destination cluster have truncated their logs to the correct offset. If less than min ISR are available, we will skip and retry in the following fetch. This coordination step validates that every ISR member has completed truncation before the partition is allowed to begin actively fetching from the source cluster. Without it, the mirror leader could start appending new data from the source while local followers still hold divergent log segments, causing inconsistencies within the destination cluster. After truncation, reverse mirroring begins normally. Note that the log truncation on everse reverse mirroring may cause the data loss if there are records that didn't get mirrored to the old destination cluster.

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