# Azure rebooted the server. The bank never noticed.

> How update domains let Microsoft patch the hosts under your VMs without dropping a single customer.

- **Format:** short video, 16 steps, ~57 seconds
- **Topic:** Azure Update Domain — how Azure reboots your VMs one group at a time for planned maintenance while a load balancer keeps a banking app online.
- **Author:** Suthahar Jegatheesan (MSDEVBUILD)
- **Category:** Azure · Azure
- **Tags:** azure, updatedomain, availabilityset, highavailability, azurevm, loadbalancer, microsoftazure, azurecloud, cloudarchitecture, devops, sre, systemdesign, msdevbuild
- **Canonical URL:** https://blog.msdevbuild.com/shorts/azure-update-domain/

---
## What you'll learn

- How Azure reboots hosts one update domain at a time
- Why the load balancer always has a healthy VM to use
- Update domain versus fault domain, in one line

## Understand it one step at a time

### 1. 9,400 customers. One payments app

A retail banking app on Azure. Transfers are clearing, nobody is thinking about servers.

### 2. Two VMs behind one load balancer

VM1 and VM2 sit in the same availability set. Every request is split between them.

### 3. Azure schedules host maintenance

Not a crash. A planned platform update — the physical host under your VM needs patching and a reboot.

### 4. VM1 starts installing updates

Azure takes the first update domain offline. VM1 stops answering and begins the host patch.

### 5. Everyone expects the app to drop

Half the capacity is mid-reboot at peak hour. This is the moment the error page usually shows up.

### 6. The load balancer already moved

The health probe on VM1 stops answering, so Azure Load Balancer drops it from the backend pool.

### 7. VM2 takes 100% of the traffic

Same URL, same session, same balance on screen. The surviving VM simply absorbs the load.

### 8. VM1 reboots and comes back

The host restarts, the VM boots, the probe answers again — and the load balancer puts VM1 straight back in.

### 9. Only now does Azure touch VM2

Azure waits for the first update domain to recover — up to 30 minutes — before starting the next one.

### 10. VM1 serves while VM2 patches

The roles swap cleanly. The freshly patched VM carries every request while the other one reboots.

### 11. Both VMs patched. Nobody noticed

Two host reboots inside one maintenance window, and the payments counter never dropped to zero.

### 12. Azure Update Domain

That sequence has a name. An update domain is a group of VMs Azure reboots together during planned maintenance.

### 13. One update domain at a time

Five by default, up to twenty. Azure walks them in order and waits about 30 minutes after each one.

### 14. Update domain vs fault domain

Update domains cover the maintenance Azure schedules. Fault domains cover the hardware failure nobody schedules.

### 15. Update Domain = Planned Maintenance Without Downtime

Spread across update domains, a platform reboot becomes a rolling restart instead of an 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.

---

## The takeaway

**Update Domain = Planned Maintenance Without Downtime**

Azure reboots one update domain at a time, so the load balancer always has a healthy VM to send traffic to. Follow for Azure & Cloud Engineering tips.