Availability set
What it is
An availability set is a logical grouping of VMs that spreads them across fault domains (distinct racks/datacenter fault hardware) and update domains (reboot groups during planned maintenance) so that a single hardware failure or maintenance window doesn’t take down the whole workload. Also called availability set across FDs/UDs in a region.
Why it exists
A single VM sits on one physical host — any host/rack failure or Azure patch cycle could kill it. Availability sets arrange VMs across independent failure boundaries for 99.95% service-level availability, without needing multiple regions.
Key ideas
- Fault domain (FD): a set of hardware that shares a common power/network/rack — whole FDs fail together. Put replicas in different FDs.
- Update domain (UD): reboot groups applied sequentially during planned maintenance. VMs in different UDs are rebooted one group at a time (up to 20 UDs).
- Put 2+ VMs into an availability set to qualify for the SLA; place each VM in its own FD/UD.
- Zonal vs regional: availability sets are regional; availability zones are per-datacenter-isolated — newer, higher granularity.
- Combine with an internal load balancer to distribute traffic across the set’s VMs.
How it fits (diagram)
Diagrams courtesy of Microsoft Learn / Azure docs: virtual-machines/availability-set-overview
Exam notes
- Availability sets and availability zones are mutually exclusive for a VM — choose one for a given VM/scale set. (Zones give higher isolation; sets give rack-level within a region.)
- You must have ≥2 VMs in a set to get the availability SLA.
- Classic exam question: “spread VMs across racks + maintenance reboots” → availability set (fault + update domains).
- AZ-104 emphasizes knowing fault domain vs update domain vs zone semantics.
Related
Path MOC · azure-vm · availability-zones · virtual-machine-scale-set
📘 Source: Microsoft Learn — Availability Set