Azure High Availability

Availability Sets

Just like on-premise, Microsoft have a need to update their environment and are subject to failures e.g. hardware. To protect against this you can use Availability Sets with Update and Fault Domains.

Update Domain

An update domain is basically like a deployment group. All computers in the update domain can be updated / restarted at the same time. Therefore, to prevent an outage to your application / service, you should span your virtual machines across multiple update domains.

Fault Domain

Each fault domain effectively shares the same fabric e.g. Network, Storage. In the event of a failure, all resources in that fault domain could be affected. To achieve Microsoft’s published SLA, you need to ensure that your virtual machines span across multiple fault domains.

Below is an example of an environment which consists of Web, File and Databases.

Fault Domain 1Fault Domain 2Fault Domain 3
Web 1
Update Domain 1
Web 2
Update Domain 2
File 1
Update Domain 2
File2
Update Domain 1
DB 1
Update Domain 3
DB2
Update Domain 2

If Fault Domain 1 was to go down, the service would continue to operate as there is a Web And File VM in another domain.

At the time of writing this post, Microsoft offer the following SLAs

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