Replication and Acknowledgments

A replica is one copy of a partition, stored on its own broker. A topic with a replication factor of 3 keeps every partition on three brokers, so that one machine can fail without taking the data with it. One replica, the leader, accepts every write for the partition, and the other replicas, the followers, copy what the leader has written.

Copies alone do not make a write safe. What matters is how many copies exist at the moment the producer is told "stored", and what Kafka does when too few brokers are left to make those copies.

Consider an insurer whose mobile app publishes a