---
title: "What is a failure domain?"
canonical: https://enum.co/learn/what-is-a-failure-domain
locale: en
topic: Operations
publishedAt: 2026-10-24
updatedAt: 2026-10-24
author: "Max Heyer"
summary: "A failure domain is a group of hardware that can fail together, such as a disk, server, rack or site, so copies of data must sit in separate ones."
---

# What is a failure domain?

A failure domain is a group of hardware that can fail together, such as a disk, server, rack or site, so copies of data must sit in separate ones.

After reading this you will be able to:

- Define a failure domain and name the common levels
- Explain why each copy or fragment needs its own domain
- Spot shared parts that silently merge two domains into one

## What is a failure domain?

A failure domain is a set of hardware that one fault can take down at the same time. A disk, a server, a rack and a whole site are each a failure domain. Redundancy only works when the copies of data, or the replicas of a service, sit in different failure domains, so one fault cannot reach all of them.

Think of carrying eggs to a party. Two cartons in one bag do not help if the bag tears. Each carton needs its own bag, or its own person carrying it.

## What are the common levels?

Failure domains nest inside each other. A larger domain holds many smaller ones, and losing it takes all of them down at once.

### Disk and server

A dead disk loses only its own data. A failed server takes every disk inside it offline, so two copies on two disks in one server are still in one failure domain.

### Rack and site

Servers in a rack often share power distribution and a top-of-rack switch. A site shares the building and usually its power, cooling and network entry. Spreading copies across racks or sites protects against faults at those levels.

## Why does the choice matter?

Storage and scheduling systems place copies by failure domain. With the server as the domain, a storage system puts each replica or erasure-coded fragment on a different server. With the rack as the domain, each one goes into a different rack.

The same idea applies to services. A three-member etcd cluster keeps working with one member down, because two of three still form a majority. That only holds if no two members share a server. In Kubernetes, topology spread constraints spread pods across nodes or zones using node labels.

Hidden shared parts are the usual trap:

- Two servers on the same power strip or switch.
- Virtual machines that land on the same physical host.
- A single network uplink for a rack that looks redundant inside.
- One storage pool backing every replica of a database.

## How enum handles failure domains

[enum Object Storage](/object-storage) uses erasure coding 4+2 with each fragment on its own server, six servers in one rack. Block storage acknowledges a write once it is on three failure domains, and each Kubernetes control plane is replicated across three. All of this sits in one zone, so keep a second copy elsewhere of data you cannot lose.

## FAQ

**Is a failure domain the same as an availability zone?**
No. An availability zone is one large kind of failure domain: one or more data centres in a region with their own power, cooling and network. Disks, servers and racks are smaller domains inside it.

**How many failure domains do I need?**
One per copy or fragment, so no fault takes out two at once. Three replicas need three domains and a 4+2 erasure code needs six. A three-member etcd cluster in three domains survives one loss.

**Does spreading across failure domains replace backups?**
No. It protects against broken hardware, not against deleted or overwritten data. A bad delete reaches every copy at once, so you still need versioning and a backup in another location.