Each availability zone is isolated from other availability zones to stop the propagation of failure or outage across zone boundaries.
Together, multiple availability zones provide continuous availability through redundancy, helping to avoid outages and improve SLAs. An outage that is caused by external factors (such as power, cooling, and physical integrity) affects only one zone. Those factors most likely do not lead to an outage in other zones except in the case of major disasters.
Each availability zone runs on its own physically distinct, independent infrastructure, and is engineered to be highly reliable. Each zone should have independent power supply, cooling system, network, and security. Common points of failures within a physical data center, like generators and cooling equipment, should not be shared across availability zones. Additionally, these zones should be physically separate so that even uncommon disasters affect only a single availability zone. Availability zones are usually either two distinct data centers within metro distance (latency in the single digit range) or two safety/fire sectors (data halls) within the same large scale data center.
Multiple availability zones (usually two) belong to a single region. The physical distance between availability zones can be up to approximately 50 kilometers (30 miles), which offers low, single-digit latency and large bandwidth by using dark fiber between the zones. This architecture allows the SDDC equipment across the availability zone to operate in an active/active manner as a single virtual data center or region.
You can operate workloads across multiple availability zones within the same region as if they were part of a single virtual data center. This supports an architecture with very high availability that is suitable for mission critical applications. When the distance between two locations of equipment becomes too large, these locations can no longer function as two availability zones within the same region and need to be treated as separate regions.