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When coordinator receives the OffsetCommitRequest, for each partition in the OffsetCommitRequest, it will additionally check whether the leader_epoch in the request >= leader epoch in the last commit. If not, the error for that partition in the OffsetCommitResponse will be INVALID_LEADER_EPOCH. 

3) Offset fetch

After consumer receives OffsetFetchResponse, it remembers the leader_epoch for each partition it needs to consume. Then the consumer needs to refresh metadata until the leader_epoch in the cached metadata >= the leader_epoch in OffsetFetchResponse for all partitions it wants to consume. Note that these logic are all hidden from user and the leader_epoch will not be exposed to user via consumer's public API (e.g. OffsetAndMetadata).

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