Availability sets

High availability in Azure is primarily achieved through redundancy. Redundancy means that there is more than one resource instance, such that in the event of a failure of a resource, the other takes over. But just having more similar resources does not make them highly available. For example, having more than one VM does not make these VMs highly available. Azure provides a resource known as an availability set, and any VM that is associated with it becomes highly available. All VMs in the availability set become highly available because they are placed on separate physical racks in the Azure data center and each VM is updated one at a time, instead of all at the same time. Availability sets provide a fault domain and updates domain to achieve this and this is discussed in the next section. In short, availability sets provide redundancy at a data center level, similar to locally redundant storage.

It is important to note that availability sets provide high availability within a data center. If the entire data center is down, then the availability of the application will be impacted. To ensure that applications are still available even when a data center goes down, Azure introduced a new feature known as availability zones, which we will understand in a little while.