An availability zone is a collection of infrastructure components. Each availability zone is isolated from other availability zones to prevent the propagation of failure or outage across the data center. An availability zone runs on its own physically distinct, independent infrastructure, and is engineered to be highly reliable. Each zone should have independent power, cooling, network, and security. Additionally, these zones should be physically separate so that even uncommon disasters affect only one zone.
The physical distance between availability zones is short enough to offer low, single-digit latency (less than 5 ms) and large bandwidth (10 Gbps) between the zones. Hence, availability zones can either be two distinct data centers in a metro distance, or two safety or fire sectors (data halls) in the same large-scale data center.
The recommended minimum number of hosts in each availability zone is 4 hosts and the maximum is 15 hosts. If you are expanding a cluster, you must add hosts in pairs. Each host in the pair must have the same CPU, memory, and storage.