Regions, availability zones and region pairs · AZ-900

Azure runs on physical hardware in data centres around the world. AZ-900 expects you to know how those data centres are organised, because the organisation is what gives you resilience.

Data centres

A data centre is a building full of servers, arranged in racks, with its own power, cooling and networking. Microsoft runs hundreds of them around the world. You can't choose or visit an individual data centre: they are grouped into availability zones and regions, and those are what you pick. Physical security of the buildings is always Microsoft's job.

Regions

A region is a geographical area containing at least one, and often several, data centres that are close together and connected by a low-latency network. Azure has more regions than most providers, across many countries.

When you create a resource, you choose its region. Things to consider:

Distance to users: closer means faster.
Data residency and compliance: some data must stay in a particular country.
Service availability: not every service or size is available in every region.
Price: the same resource can cost slightly different amounts in different regions.

Some services are global and do not belong to a region, for example Microsoft Entra ID and Azure DNS.

Availability zones

An availability zone is a physically separate location within a region, made of one or more data centres with independent power, cooling and networking. Regions that support zones have at least three of them.

If one zone fails (a fire, a power cut), the others keep running. The zones are connected by high-speed private fibre. Azure services fall into three groups:

Zonal services: you pin a resource to a specific zone, such as a VM in zone 1. For resilience you deploy copies in several zones yourself.
Zone-redundant services: Azure replicates automatically across zones, such as zone-redundant storage or SQL Database.
Non-regional services: always available from Azure geographies, resilient to zone-wide and region-wide outages.

Resilience across zones usually costs more than a single copy, simply because you run duplicates.

A zone is one or more datacenters with independent power, cooling and networking. Zones protect against a datacenter failure; region pairs against losing a region.

Region pairs

Most regions are paired with another region in the same geography, usually at least 300 miles (about 480 km) apart. For example, East US is paired with West US.

Region pairs help with disaster recovery:

some services replicate data to the paired region automatically (such as geo-redundant storage),
if there is a large outage, one region in each pair is prioritised for recovery,
planned platform updates roll out to one region of a pair at a time,
data stays within the same geography as its pair, for legal and tax reasons (with rare exceptions, such as Brazil South, which pairs with a region outside Brazil).

Most pairs work in both directions, but a few are one-way: West India's secondary is South India, but South India's secondary is Central India. And some newer regions have no pair at all: they rely on availability zones for resilience, and you choose where to replicate data yourself.

Sovereign regions

Sovereign regions are separate instances of Azure, isolated from the main public cloud for legal or compliance reasons:

Azure Government (US): for US government agencies and partners.
Azure operated by 21Vianet (China): Azure in China, operated by a Chinese company.