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Let’s take an example. Imagine we have two brokers A & B. B is the leader initially as in the below figure. (A) fetches message m2 from the leader (B). So the follower (A) has message m2, but has not yet got confirmation from the leader (AB) that m2 has been committed (the second round of replication, which lets (A) move forward its high watermark past m2, has yet to happen). At this point the follower (A) restarts. It truncates its log to the high watermark and issues a fetch request to the leader (B). (B) then fails and A becomes the new leader. Message m2 has been lost permanently (regardless of whether B comes back or not).

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NB: It should be noted that this proposal does not protect against log divergence for clusters that use the setting unclean.leader.election.enable=true. (See the appendix for more details.)

Proposed Changes

Leader Epoch

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