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Terms:
Last Mirrored Leader Epoch
(LMLE): This is the leader epoch that the source cluster recognizes in this destination cluster. Or we can say, below this leader epoch (inclusive), the source cluster and destination cluster is in sync in the perspective of leader epoch.
For example:
1. Cluster B creates a mirror and starts mirroring from the source cluster A. At this point, because the source cluster stores nothing about the LMLE for this partition, -1 will be returned. The "-1" means the source cluster (A) doesn't recognize any leader epoch in destination cluster (B), so all the logs should be truncated.
2. Revere mirroring. Cluster B becomes the source cluster. And before cluster B stops the mirroring from A, it stores the LMLE in the internal topic, which is 1 in this case. When cluster A mirrors from cluster B, it will first query the LMLE from cluster B, and then truncate every record beyond this leader epoch. That is, for cluster A and B, leader epochs for [0, 1] are in sync.
3. Reverse mirroring again. Cluster A stores LMLE 3 in the internal topic, and cluster B queries this LMLE and truncate records beyond leader epoch 3.
