🏗️ Fault Domain vs Update Domain in Azure – Ensuring High Availability

 


When building cloud-native applications in Microsoft Azure, ensuring high availability and fault tolerance is critical—especially for production workloads. Two key concepts that help you achieve this in Azure are Fault Domains and Update Domains.

While these may sound technical at first, understanding them is essential for deploying resilient applications in the cloud. Let’s break them down in simple terms.

🚨 What is a Fault Domain?

A Fault Domain (FD) represents a group of Azure infrastructure that shares a common power source and network switch—essentially, a physical rack in a data center.

Purpose: To protect your application from hardware failures.

When you place your virtual machines (VMs) in an Availability Set, Azure ensures they are distributed across different fault domains. That means if one rack experiences a hardware issue (e.g., power outage or switch failure), the other VMs in different racks will remain unaffected.

🛠️ Example:
You have 2 VMs deployed across 2 Fault Domains. If Rack 1 fails, only the VM in that rack is affected. The other continues to run without interruption.

🔄 What is an Update Domain?

An Update Domain (UD) is a logical grouping used by Azure to roll out updates (like patches or maintenance tasks) without downtime.

Purpose: To protect your application from planned maintenance interruptions.

When updates are required, Azure performs them one update domain at a time. That way, your application remains available even while updates are happening.

🛠️ Example:
You have 3 VMs across 3 Update Domains. Azure will update one domain at a time, ensuring at least 2 of your VMs are always running.

🧠 Key Differences at a Glance

FeatureFault Domain (FD)Update Domain (UD)
DefinitionPhysical separation (rack, power, network)Logical separation for updates
PurposeResilience against hardware failuresResilience during platform updates
Impacted byRack-level issuesOS patches, maintenance
Customizable?Limited (usually 2–3 per region)Yes (up to 20 per Availability Set)
Used WithAvailability SetsAvailability Sets

🧱 Availability Set = FD + UD

When you create an Availability Set in Azure and place your VMs inside it:

  • Azure spreads them across Fault Domains to avoid hardware failures

  • And across Update Domains to avoid downtime during maintenance

This combination ensures your application stays online through both unplanned and planned disruptions.

🔍 Quick Recap

  • Fault Domain = Protects against hardware failure

  • Update Domain = Protects against planned updates

  • Used together in Availability Sets to increase uptime

💡 When to Use

Use Availability Sets with FD & UD when:

  • Hosting multiple VM instances of a critical application

  • Uptime is crucial, and downtime (even for updates) is unacceptable

  • You want cost-effective high availability without using full Availability Zones

Azure offers a rich toolbox for high availability, and understanding how Fault Domains and Update Domains work will help you make smarter infrastructure decisions.

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