Type of Maintenance

planned maintenance (update domain)

unplanned maintenance (fault domain) = that deals with hardware failures

Availability set

Update Domain

Fault Domain

Scale Sets

How horizontal scaling should happen. Define the rules for each set. If traffic goes above 90%, provision new vms. A scale set can have 1000 vms (if these are standard azure vms).

Availability Zones

physically separate zone with in the same region

Multi region deployments

Active passive model with Hot standby

the other region is already booted up but not serving any requests

Active active model

the other region is also servicing requests for customers close to its geography

Active passive model with cold standby

the other region is boot up dynamically

good for balancing the cost vs. SLA

Hypervisor

planned maintenance vs unplanned maintenance (power, network or hardware failure)

planned maintenance = update domain

unplanned maintenance = fault domain

In every region, there are different number of update and fault domains available. Take them into consideration before designing HA solutions.