An availability set is a logical grouping of Azure virtual machines that you can use to ensure that the virtual machines you place within it are isolated from each other when they are deployed within an Azure datacenter. The virtual machines that you place within an availability set are spread across multiple physical servers, racks, storage units, and network switches within Azure. If a failure occurs (whether it's related to Azure or your hardware), only a subset of the virtual machines is impacted. As such, the overall application remains up and running and availability is unaffected. Availability sets are therefore essential to a reliable cloud solution.

When configuring an availability set, a number of fault domains and a number of update domains need to be set. Fault domains define a group of VMs that share a common power source and network switch within the Azure datacenter. Update domains refer to groups of VMs that can be taken down at a single time. Configuring fault and update domains ensures that an application remains running during periods of planned maintenance of the underlying Azure hardware, as well as during unplanned hardware issues.

Use availability sets when you want to deploy reliable virtual machine solutions within Azure.