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• Last Mirrored Epoch (LME): The greatest leader epoch of a given partition that a destination cluster recognizes from the source cluster and stores in the __mirror_state internal topic. It represents the synchronization point between source and destination. After the log is truncated to LME, the destination cluster does not contain any record with a leader epoch beyond the LME. Only the source cluster owns leader epochs exceeding the LME. This ensures the source leader epoch remains the source of truth, even when epoch histories diverge across clusters during asynchronous mirroring.
• Leader Epoch Bump (LEB): The leader epoch in the destination cluster remains unchanged during replication. That means it is possible that the fetched batch carries a leader epoch of X that does not match with the local leader epoch. To ensure the leader epoch remains monotonically increasing, it is incremented when a partition becomes writable after failover. New records produced on the cluster will then carry an epoch higher than anything in the existing log.

A two-phase reconciliation process truncation protocol is applied by the mirror fetchers: 

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