In Kafka HA, why minimum number of brokers required are 3 and not 2

Question

We are trying to implement Kafka HA using kafka cluster. While doing R&D, we found that minimum number of nodes recommended for zookeeper & kafka brokers are 3.

We understand that why zookeeper should have minimum 3 nodes, because for leader election minimum (n+1)/2 nodes should be up & running.

But its not clear, why minimum 3 kafka brokers are required. Why can't we implement HA with 2 kafka brokers & 3 zookeepr nodes?

Answer 1

The minimum number of nodes of Zookeeper is 3 because of the quorum attribute. It should be odd because the even number of nodes is no used. e.g: Zookeeper with total nodes of 8 can be downgraded to 7. Many nodes in Zookeepers also isn't good because of the consensus algorithm. (e.g: Paxos)

For the Kafka cluster, personally I think it is okay for setting 2 brokers. But it is better with 3 brokers. The reason because of maintaining the ISR - In Sync Replicas.

Let say your Kafka cluster has 2 brokers. To maintain the high availability and the consistency of the data, we will set the replicas and the ISR both to 2. The interesting part is the min-ISR attribute. If you set the min-ISR to 1 then the leader fails, likely you don't have any remaining replicas. If you set the min-ISR to 2, when either the leader or the follower fails, nor the producer and consumer can work.

If our Kafka cluster has 3 brokers and we set the ISR equals to 3, the min-ISR equals to 2. With this configuration, we accept the risk of losing 1 replica (either leader or follower) while working. For instance, if we lose the leader, there has at least one follower that in-sync for switching. If we lose one of the followers, we still have a remaining follower to keep the min-ISR to 2.

Answer 2

In addition to @hqt answer:

You can setup a Kafka HA Cluster with only 2 brokers, but the recommended replication-factor for production is 3, so you need 3 brokers in order to achieve this.

Also you should consider that Confluent are working at migrating the leader election to Kafka, so in the future you will not need Zookeeper anymore, which will possibly implies to have an odd number of Kafka brokers.