Explain Azure regions, availability zones and how you design for high availability.

The hierarchy:

• Region — a geographic area containing one or more datacentres, such as Central India or West Europe.
• Availability Zone — a physically separate datacentre within a region, with independent power, cooling, and networking. Zones protect against a datacentre failure.
• Region pair — each region is paired with another in the same geography, used for geo-redundant storage and coordinated platform updates.

Designing for availability, in increasing order of cost and protection:

• Multiple instances in one zone — protects against a single instance failing. This is the minimum; a single VM has a limited SLA.
• Zone-redundant deployment — instances spread across zones behind a load balancer, with zone-redundant storage and a zone-redundant database. Protects against losing a datacentre, usually at little or no extra cost. This is the sensible default.
• Multi-region — active-passive with failover, or active-active with Front Door or Traffic Manager routing. Protects against losing an entire region, but roughly doubles cost and introduces genuinely hard data replication and consistency problems.

Note: Tie the choice to RTO and RPO rather than picking a tier. And say that a disaster recovery plan you have never tested is not a plan — failover drills are what make it real.