In-Sync Replicas and Acknowledgements

Kafka’s in-sync replica set includes the partition leader and followers meeting its catch-up criteria. A follower need not contain every newly appended record at every instant to be in that set. Replication factor describes the configured copies; ISR describes the currently qualifying replicas.

With acks=1, the producer waits for the leader’s append acknowledgement. With acks=all, it waits for the required ISR acknowledgement, and min.insync.replicas constrains whether that write is accepted. For replication factor three and minimum ISR two, losing one replica can still permit writes if the other requirements for leadership and availability hold.

A successful acknowledgement does not mean every disk has fsynced or a warehouse has loaded the event. A timeout can follow a successful append, so retries need appropriate producer settings. Assess election policy, surviving replicas, and controller quorum separately; these settings are not a promise of uninterrupted requests.

Reference: Kafka fundamentals lab.