What is the replication factor?

The replication factor (RF) is the number of copies of each partition kept in the cluster, on different brokers; it is set when creating a topic (for example --replication-factor 3), and brokers default to 1 for auto-created topics. With RF=3 each partition has one leader and two followers, and the docs call a replication factor of 3 a common production setting: there will always be three copies of your data. Per the design docs, with f+1 replicas a topic can tolerate f failures without losing committed messages, so RF=3 tolerates two failures without losing committed data. Writes are acknowledged by the in-sync replicas, and with acks=all the min.insync.replicas setting sets how many must have the write; a common pairing is RF=3 with min.insync.replicas=2. More copies cost more disk and replication traffic.

Expected key concepts: replication factor, copies, partition, brokers, leader, followers, RF=3, tolerate, in-sync replicas, min.insync.replicas