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ISR is a set of replicas that are fully sync-ed up with the leader. In other words, every replica in ISR has all messages that are committed. In an ideal system, ISR should always include all replicas unless there is a real failure. A replica will be dropped out of ISR if it diverges from the leader. This is controlled by two parameters: replica.lag.time.max.ms and replica.lag.max.messages. The former is typically set to a value that reliably detects the failure of a broker. We have a min fetch rate JMX in the broker. If that rate is n, set the former to a value larger than 1/n * 1000. The latter is typically set to the observed max lag (there is a JMX bean per partition) in the follower. Note that if replica.lag.max.messages is too large, it can increase the time to commit a message. If latency becomes a problem, you can increase the number of partitions in a topic.