How does Kafka achieve fault tolerance?

Fault tolerance comes from partition replication across multiple brokers (so losing one broker doesn't lose data), the ISR (in-sync replica) mechanism that only promotes a follower that's fully caught up to leader status, and configurable acknowledgment levels (acks=all waits for all in-sync replicas to confirm a write before considering it successful) — together these ensure the cluster keeps serving data correctly even when individual brokers fail.