Availability Sets vs Availability Zones
Tags: beginners, azure, cloud, microsoft
Published: 2024-06-27 19:48:45

An availability set protects your Azure resources from failures within data centres, whereas an availability zone protects from entire data centre failures.
Understanding the differences between Azure Availability Sets and Availability Zones is crucial for designing resilient and highly available architectures in the Azure cloud environment. Let's explore each concept and highlight their key distinctions:


### Availability Sets:

1. **Purpose**: 
   - **Availability Sets** in Azure are used to ensure high availability of applications by distributing virtual machines across multiple physical servers within a datacenter.
   
2. **Fault Domain Isolation**:
   - VMs within an Availability Set are placed in separate **Fault Domains**, which means they are physically isolated from each other. This isolation ensures that if a hardware failure or maintenance event affects one Fault Domain, VMs in other Fault Domains remain unaffected.

3. **Update Domain Isolation**:
   - Additionally, VMs in an Availability Set are distributed across **Update Domains**, which helps in minimizing downtime during planned maintenance events. Azure updates or reboots VMs in one Update Domain at a time, ensuring that a portion of VMs are always available.

4. **Scope**:
   - Availability Sets are scoped to a single Azure region. They are designed to protect against failures within that region but do not provide protection against a regional outage.

5. **Use Cases**:
   - Recommended for traditional applications that require high availability within a single region but can tolerate brief downtimes during Azure platform updates or maintenance.

![Image description](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/37ytidna7od3zvqbhamm.jpg)
### Availability Zones:

1. **Purpose**:
   - **Availability Zones** are a newer Azure feature designed to provide high availability and resiliency to applications by distributing them across multiple datacenters (Availability Zones) within a single Azure region.

2. **Physical Isolation**:
   - Each Availability Zone is a physically separate datacenter with independent power, cooling, and networking. This isolation significantly reduces the likelihood of a single event impacting all zones simultaneously.

3. **Scalability and Redundancy**:
   - By distributing application components across different Availability Zones, Azure provides redundancy and ensures that applications remain operational even if one zone goes down due to a catastrophic event.

4. **Scope**:
   - Availability Zones are available in select Azure regions and provide protection against both datacenter-level and potentially regional-level failures.

### Key Differences Summary:

- **Fault and Update Domain Isolation**: Availability Sets provide fault and update domain isolation within a single datacenter, whereas Availability Zones offer fault isolation across multiple datacenters within the same region.
  
- **Scope**: Availability Sets are limited to a single Azure region, while Availability Zones operate within a single region but span across multiple physically separate datacenters.

- **Resiliency**: Availability Zones provide higher resiliency and protection against both datacenter-level and potentially regional-level failures compared to Availability Sets.

_In conclusion, choosing between Availability Sets and Availability Zones depends on the specific requirements of your application in terms of availability, fault tolerance, and scalability within the Azure cloud ecosystem. Understanding these differences enables you to architect solutions that meet your application's resilience and availability goals effectively._