What are Availability Zones in Azure?

Availability Zones is a high-availability offering that protects your applications and data from datacenter failures. Availability Zones are unique physical locations within an Azure region. Each zone is made up of one or more datacenters equipped with independent power, cooling, and networking. To ensure resiliency, there’s a minimum of three separate zones in all enabled regions. The physical separation of Availability Zones within a region protects applications and data from datacenter failures. Zone-redundant services replicate your applications and data across Availability Zones to protect from single-points-of-failure. With Availability Zones, Azure offers industry best 99.99% VM uptime SLA. The full Azure SLA explains the guaranteed availability of Azure as a whole.

An Availability Zone in an Azure region is a combination of a fault domain and an update domain. For example, if you create three or more VMs across three zones in an Azure region, your VMs are effectively distributed across three fault domains and three update domains. The Azure platform recognizes this distribution across update domains to make sure that VMs in different zones are not updated at the same time.

Build high-availability into your application architecture by co-locating your compute, storage, networking, and data resources within a zone and replicating in other zones. Azure services that support Availability Zones fall into two categories:

  • Zonal services – you pin the resource to a specific zone (for example, virtual machines, managed disks, Standard IP addresses), or
  • Zone-redundant services – platform replicates automatically across zones (for example, zone-redundant storage, SQL Database).

To achieve comprehensive business continuity on Azure, build your application architecture using the combination of Availability Zones with Azure region pairs. You can synchronously replicate your applications and data using Availability Zones within an Azure region for high-availability and asynchronously replicate across Azure regions for disaster recovery protection.

conceptual view of one zone going down in a region

Important

The Availability Zone identifiers (the numbers 1, 2 and 3 in the picture above) are logically mapped to the actual physical zones for each subscription independently. That means that Availability Zone 1 in a given subscription might refer to a different physical zone than Availability Zone 1 in a different subscription. As a consequence, it's recommended to not rely on Availability Zone IDs across different subscriptions for virtual machine placement.

Services support by region

The combinations of Azure services and regions that support Availability Zones are:

Americas Europe Asia Pacific
Central US East US East US 2 West US 2 France Central North Europe UK South West Europe Japan East Southeast Asia
Compute
Linux Virtual Machines
Windows Virtual Machines
Virtual Machine Scale Sets
Storage
Managed Disks
Zone-redundant Storage
Networking
Standard IP Address
Standard Load Balancer
VPN Gateway
ExpressRoute Gateway
Application Gateway(V2)
Azure Firewall
Databases
Azure Data Explorer
SQL Database ✓ (Preview)
Azure Cache for Redis
Azure Cosmos DB
Analytics
Event Hubs
Integration
Service Bus (Premium Tier Only)
Event Grid
Identity
Azure AD Domain Services

Services resiliency

All Azure management services are architected to be resilient from region-level failures. In the spectrum of failures, one or more Availability Zone failures within a region have a smaller failure radius compared to an entire region failure. Azure can recover from a zone-level failure of management services within the region or from another Azure region. Azure performs critical maintenance one zone at a time within a region, to prevent any failures impacting customer resources deployed across Availability Zones within a region.

Pricing

There is no additional cost for virtual machines deployed in an Availability Zone. 99.99% VM uptime SLA is offered when two or more VMs are deployed across two or more Availability Zones within an Azure region. There will be additional inter-Availability Zone VM-to-VM data transfer charges. For more information, review the Bandwidth pricing page.

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