# A rack just lost power. The shop kept selling.

> Why Azure places your VMs on different physical racks, and what breaks the moment it does not.

- **Format:** short video, 16 steps, ~53 seconds
- **Topic:** Azure Fault Domain — how spreading VMs across separate physical racks keeps an e-commerce site online when a rack loses power.
- **Author:** Suthahar Jegatheesan (MSDEVBUILD)
- **Category:** Azure · Azure
- **Tags:** azure, faultdomain, availabilityset, highavailability, azurevm, loadbalancer, microsoftazure, azurecloud, cloudarchitecture, devops, sre, systemdesign, msdevbuild
- **Canonical URL:** https://blog.msdevbuild.com/shorts/azure-fault-domain/

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## What you'll learn

- What a rack shares — one power feed, one switch, one blast radius
- How an availability set spreads VMs across fault domains for you
- Why two VMs in the same fault domain is not redundancy

## Understand it one step at a time

### 1. 12,000 shoppers. One checkout

A live e-commerce site on Azure. Orders are landing, nobody is thinking about hardware.

### 2. Two servers, two racks

VM1 sits in rack A, VM2 in rack B. The load balancer splits every request between them.

### 3. Rack A just lost power

Not a reboot. Not a patch. The power feed to an entire cabinet is gone.

### 4. Four servers gone at once

A rack is a shared blast radius. One power feed, one switch, one cooling loop — and everything in it drops together.

### 5. This is where most sites die

Half the capacity is gone mid-checkout. You would expect the error page right about now.

### 6. The health probe fails

Azure Load Balancer probes every backend on a fixed interval. VM1 stops answering, so it leaves the pool.

### 7. Every request goes to Rack B

Within seconds the surviving backend is carrying 100% of the traffic. Same URL, same cart, same session.

### 8. The shop never went down

An entire rack is dark and the checkout is still taking money. That is the whole trick.

### 9. Azure Fault Domain

That rack has a name. A fault domain is a group of hardware that shares one power source and one network switch.

### 10. Separate power, switch and cooling

Two fault domains share nothing physical. A failure in one has no path into the other.

### 11. Azure picks the racks for you

Put the VMs in an availability set and Azure spreads them across 2–3 fault domains. You never choose a cabinet.

### 12. Now put both VMs in one rack

Two VMs, same app, same load balancer — but both landed in fault domain 0. It looks identical from the portal.

### 13. Same power cut. Site is gone

One cabinet, one failure, zero surviving backends. The load balancer has nowhere left to send anyone.

### 14. Fault domains fix that at deploy time

Nothing about the VMs changed. Only where Azure put them — and that is decided before the outage, not during it.

### 15. Fault Domain = Hardware Failure Protection

Rack power, top-of-rack switch, a dead host — spread across fault domains, none of them is your outage.

### 16. Follow for Azure & Cloud Engineering Tips

One architecture idea per reel, from the failure backwards. Save this one for your next design review.

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## The takeaway

**Fault Domain = Hardware Failure Protection**

Azure spreads the VMs in an availability set across separate racks, so a dead power feed or switch takes one of them and the load balancer sends everyone to the other. Follow for Azure & Cloud Engineering tips.