I'm planning to run a Kafka cluster in a production environment. Before deploying the cluster, I try to find the best configuration to ensure HA and data consistency.
I read in the official doc that it is not possible to reduce the partition replication factor, but what about the min.insync.replicas? When I decrease the value on a test environment I don't see any differences when I look at topics description. After changing the value from 3 to 2, I still have 3 ISR. Is it because it's a min value or because the configuration change is not taken into account?
Yes, it is possible to reduce (or in general change) the min.insync.replicas configuration of a topic.
However, as you were looking for the best configuration to ensure high availablity and data consistency, it would be counter-intuitive to reduce the value.
There is a big difference between ISR (in sync replicas) and the setting min.insync.replicas. The ISR shown in the kafka-topics --describe just tells you how healthy the data in your topic is and if all the replicas keep up with the partitions leader.
On the other hand, the min.insync.replicas works together with a KafkaProducer writing to the topic with that setting. It is described on the official Kafka docs on TopicConfigs as:
When a producer sets
acksto "all" (or "-1"),min.insync.replicasspecifies the minimum number of replicas that must acknowledge a write for the write to be considered successful.
To emphasize again, if you are looking for high availabilty and consistency it is best to set acks = all in your producer while at the same time keeping the min.insync.replicas high.