Azure's physical foundation is the datacenter: a building full of servers, storage and networking equipment with its own power, cooling and physical security. You never choose an individual datacenter when you deploy a resource. Instead, Azure groups datacenters into availability zones, and zones into regions. Understanding that hierarchy, datacenter inside zone inside region, is the key to answering resilience questions on the Microsoft Azure Fundamentals exam (AZ-900). An availability zone is a physically separate location within an Azure region, made up of one or more datacenters with independent power, cooling and networking. Zones are far enough apart that a local failure, such as a fire, flood or power failure in one building, should not affect the others, yet close enough to be connected by a high-speed, low-latency private network, so data can be replicated between them quickly. Regions that support availability zones have a minimum of three zones. Not every region supports zones, and not every service supports them, so check before you design around them.
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