What is min.insync.replicas and when does it matter?

min.insync.replicas (minISR) is the minimum number of in-sync replicas required for a produce request with acks=all to succeed. If ISR count drops below this threshold (due to broker failures), the leader rejects writes with NotEnoughReplicasException. For example: RF=3, minISR=2, acks=all. If 2 brokers fail, ISR=1 < minISR=2, so writes are rejected. This prevents data loss by refusing writes that can't meet the replication guarantee. It only applies with acks=all — acks=1 ignores minISR.

Expected key concepts: min.insync.replicas, minISR, acks=all, NotEnoughReplicasException, ISR, broker failures, RF=3, writes rejected, data loss