# Azure is rebooting your server. Nobody notices.

> How an Azure Availability Set keeps an online shop selling straight through planned server maintenance.

- **Format:** short video, 14 steps, ~64 seconds
- **Topic:** Azure Availability Set — how fault domains, update domains and a load balancer keep an e-commerce site online while Azure patches the servers under it.
- **Author:** Suthahar Jegatheesan (MSDEVBUILD)
- **Category:** Azure · Azure
- **Tags:** azure, availabilityset, highavailability, azurevm, loadbalancer, microsoftazure, azurecloud, cloudarchitecture, devops, sre, systemdesign, msdevbuild
- **Canonical URL:** https://blog.msdevbuild.com/shorts/azure-availability-set/

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## What you'll learn

- Why one VM cannot survive Azure patching the host under it
- How an availability set spreads VMs across fault and update domains
- What the load balancer health probe does during a reboot

## Understand it one step at a time

### 1. One server. Every order goes through it

The shop is live and selling. Right now nothing about this looks like a problem.

### 2. Azure has to patch the host

Security updates land on the physical host underneath. Your VM reboots — and it is not optional.

### 3. Server Unavailable

Problem: server maintenance causes downtime. Every shopper hits an error page and leaves.

### 4. Put two VMs in an Availability Set

VM1 and VM2 run the same shop. The set is Azure’s promise that it will never take both down at once.

### 5. One address in front of both

Azure Load Balancer owns the public IP and health-probes each VM before it sends a single request there.

### 6. Fault domains and update domains

Different rack, power and switch. And Azure only ever reboots one update domain at a time.

### 7. Requests start arriving

Shoppers hit one public address. They have no idea how many servers are behind it.

### 8. The load balancer splits the traffic

Roughly half to VM1, half to VM2 — and it keeps probing both the entire time.

### 9. Both green. Orders going through

Two servers doing the work of one. The spare capacity is a bonus, not the point.

### 10. Azure starts maintenance on VM1

Update domain 0 goes first. VM1 drains and reboots — update domain 1 is not touched.

### 11. Traffic Automatically Redirected

The health probe fails, the load balancer drops VM1 from the pool, and VM2 takes 100%.

### 12. The shopper never finds out

Same URL, same cart, same checkout. No error page, no support ticket, no abandoned basket.

### 13. VM1 rejoins. Only then does UD 1 go

The probe passes, traffic rebalances — and Azure waits for that before touching VM2 at all.

### 14. High Availability Achieved

Two VMs, one availability set, one load balancer. Azure backs it with a 99.95% SLA.

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## The takeaway

**Azure reboots the server. Your shop keeps selling**

An availability set spreads your VMs across fault and update domains, so planned maintenance takes one out at a time while the load balancer sends every shopper to the other.