4. Replication, Leaders, and Fault Tolerance

Each Kafka partition has a leader and replicas on other brokers. Producers and consumers normally communicate with the leader, while followers replicate its log. The in-sync replica set contains replicas that are sufficiently caught up to become leaders.

Replication factor, min.insync.replicas, and producer acknowledgements work together. A higher replication factor improves durability but consumes more storage and network bandwidth.