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It may not be immediately obvious that the OffsetsForLeaderEpoch can be used to determine if an offset is still valid. The consumer will request from the leader the end offset for the epoch corresponding to its current fetch offset. If the fetch offset has been truncated in an unclean leader election, then the OffsetsForLeaderEpoch response will return either 1) the same epoch with an end offset smaller than the fetch offset, or 2) a smaller epoch with and a . Otherwisesmaller fetch offset. In either case, the returned end offset would be smaller than the fetch offset. Otherwise, if the offset exists, then the response will include the same epoch and an end offset which is larger than the fetch offset.    

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