Why Can't I Select Two AZs?

A Kafka cluster uses 3 ZooKeeper nodes regardless of instance specifications. Kafka uses the ZooKeeper cluster to manage configurations. If the ZooKeeper cluster is faulty, Kafka will not work properly.

At least two ZooKeepers are required for the cluster to run properly.

Assume that you select only two AZs. AZ1 has one ZooKeeper node, and AZ2 has two. If AZ1 is faulty, the instance can be used properly. If AZ2 is faulty, the cluster cannot be used. In this case, the availability rate is just 50%. Therefore, do not select 2 AZs.