POM supports one partition per topic.

Replicas

The list of nodes that replicate the log for this partition regardless of whether they are the leader or even if they are currently alive.

The minimum replication factor recommended for each topic is three to support HA. Hence, we require a Kafka cluster with three nodes as depicted in the diagram, see Zookeeper Ensemble.

The replication factor has to be less than or equal to the total broker count.

ISR

Set of "in-sync" replicas. This is the subset of the replicas list that is currently alive and caught-up to the leader.