What is the ISR (In-Sync Replicas) set?

The In-Sync Replica set contains all partition replicas that are currently fully caught up with the leader. A replica is in the ISR if it has fetched from the leader within replica.lag.time.max.ms (default 30s). The leader and all healthy followers are in the ISR. When a follower lags too far behind, it is removed from the ISR. The ISR is the source of eligible candidates for leader election — ensuring the next leader has all committed data.

Expected key concepts: in-sync, caught up, replica.lag.time.max.ms, 30s, leader, followers, eligible, leader election, committed data