Kafka min.insync.replicas < replication.factor

Question

Suppose I have a cluster with 3 kafka brokers. I set:

min.insync.replicas=2
default.replication.factor=3

• All brokers are up, ISR is fine, I get a message where ack=all. Since ISR=2, two copies of the message are for sure stored. 1) Will one more copy (because replication=3) be made in the background? 2) If it fails - it does not matter, correct? Cluster health is just fine.

• One broker is down, ISR=2 can be maintained and the message is saved to two brokers. After some time that broker that was down comes up again. 3) Since replication=3, will it try to catch up with the others in the back-ground?

I am trying to figure out of a practical example where setting replication factor to be bigger than ISR would make sense. A real example I could "touch" and understand. If this is a duplicate, please refer me to it. Thank you.

Answer 1

Yes, one replica is made in the background.

Yes, the broker will catch up all out of sync replicas upon restarts.

If you ever have in-sync replicas <= replication factor, then you cannot lose any brokers more than the difference between the values (due to maintenance or failure). Therefore, replication factor should always be greater