Why Can't I Select Two AZs?

To improve the reliability of a Kafka instance, you are advised to select three AZs or more when creating the instance. You cannot select two AZs.

Each Kafka instance contains three ZooKeeper nodes. The ZooKeeper cluster manages Kafka instance configurations. If the ZooKeeper cluster is faulty, the Kafka instance cannot run properly. At least two ZooKeepers are required for the cluster to run properly.

Assume that you select only two AZs. AZ 1 has one ZooKeeper node, and AZ 2 has two. If AZ 1 is faulty, the instance can be used properly. If AZ 2 is faulty, the cluster cannot be used. In this case, the availability rate of the Kafka instance is just 50%. Therefore, do not select 2 AZs.