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This behavior will apply only when answering requests from clients. Therefore, there will be no impact on ReplicaFetcherThread, or other parts of the broker that make ListOffsetsRequests to other brokers. We can distinguish broker requests from client requests by looking at the replicaId field of the RPC. (Hence, other brokers can still use ListOffsetsRequests to figure out which replicas has the longest log for a partition.)
When unclean leader elections are enabled, data loss is possible. So we cannot guarantee that offsets will always go forwards, even in theory, in this mode. Therefore, when unclean leader elections are enabled on the broker, the KIP-207 behavior will not apply.
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