ISR
In-Sync Replicas: the set of replicas (leader + followers) that are sufficiently caught up with the leader.
ISR (In-Sync Replicas) is the list of a partition's replicas — including the leader itself — that are caught up with confirmed data. Only followers in the ISR are eligible for election as the new leader in case of failure, preventing a lagging replica from taking over and causing loss of data already confirmed to producers.
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