When we deploy applications in Azure, one of the first things we think about is availability. Nobody wants an application to go down because a physical server fails or Azure performs planned maintenance. That’s where Azure’s high availability features come in.
Two options you’ll frequently come across are Availability Sets and Availability Zones. At first glance, they seem to solve the same problem, but they actually protect your applications in different ways.
I used to think they were just two names for the same feature until I started deploying production workloads. Understanding the difference became important because choosing the wrong option can either increase costs unnecessarily or leave your application vulnerable during an outage.
In this blog, I’ll explain both features in simple terms, compare them, and share when I would choose one over the other based on real-world scenarios.
What is an Availability Set?
An Availability Set is designed to keep your virtual machines running even if there’s a hardware issue within an Azure datacenter.
Imagine you have two web servers running the same application. If both virtual machines are placed on the same physical server and that server fails, your application goes offline.
To avoid this, Azure spreads the virtual machines across different physical hardware using Fault Domains and Update Domains. This way, if one piece of hardware fails or Azure performs maintenance, at least one VM remains available.
For smaller production workloads or legacy applications, Availability Sets are still a reliable and cost-effective option.
Example:
Suppose your company hosts an internal HR application with two virtual machines.
- HR-VM01
- HR-VM02
Both VMs are placed in an Availability Set.
One day, Azure performs planned maintenance on the host running HR-VM01. During the reboot, HR-VM02 continues serving users, so employees can still access the HR portal without interruption.
