Availability sets are designed to ensure that VMs stagger updates and have varied power and network connectivity, preventing you from losing all your VMs with a single network or power failure.
Each vm perform the identical set of functionalitites
Availability sets do this by grouping VMs in two ways: update domain and fault domain.
An Availability Zone iscombination of a fault domain and an update domain
Update domain:
The update domain groups VMs that can be rebooted at the same time.
Apply updates while knowing that only one update domain grouping will be offline at a time.
All of the machines in one update domain will be updated.
An update group going through the update process is given a 30-minute time to recover before maintenance on the next update domain starts.
There are 5 (non-user-configurable) update domains
Fault domain:
The fault domain groups your VMs by common power source and network switch.
Automatically recognized by Azure
By default, an availability set will split your VMs across up to three fault domains.
This helps protect against a physical power or networking failure by having VMs in different fault domains (thus being connected to different power and networking resources).
