An availability set is a cluster of virtual machines that enables Azure to understand how an application offers redundancy and high accessibility. To provide a highly available application and achieve the 99.95 percent Azure SLA, creating two or more VMs within an availability set is safer. The two domains assigned to every VM in an availability set are- an update domain and a fault domain.
Update domains combine virtual machines and the underlying hardware that can reboot simultaneously. Before the maintenance on a different update domain, a restarted update domain takes 30 minutes to recover.
Fault domains are the set of virtual machines that share a common power source and a common network switch. Virtual machines specified in your availability set are split into up to three failure domains by default. Integrating virtual machines into an availability set helps mitigate the impact of hardware malfunctions, network outages, and power outages.
