Designing Resiliency: Availability Sets vs Availability Zones

Introduction

In cloud computing, ensuring high availability (HA) is paramount for business continuity and resilience. Azure Availability Sets and Availability Zones are two essential features offered by Microsoft Azure to help achieve this goal. They play a crucial role in the AZ-104 certification exam, as understanding their differences, use cases, and configurations is vital for designing resilient Azure applications.

Availability Sets group virtual machines (VMs) to protect against localized hardware failures, distributing them across fault and update domains. In contrast, Availability Zones provide a higher level of redundancy by isolating VMs across multiple physically separate datacenters within a region. Both methods aim to minimize downtime and maintain service level agreements (SLAs).

In this tutorial, we will explore the core concepts, configurations, and practical examples of both Availability Sets and Availability Zones, helping you understand when to use each and how they fit into your Azure architecture.

Prerequisites

To follow this tutorial, you will need the following:

• An Azure subscription.
• Proper RBAC/permissions to create and manage resources.
• Tools: Azure CLI, PowerShell, or use the Azure Portal.
• Azure Virtual Machines service enabled in your subscription.

Core Concepts

Availability Sets

• Definition: A logical grouping of VMs that allows Azure to understand how your application is built for redundancy and availability.
• Architecture: VMs in an availability set are split across multiple fault domains (hardware) and update domains (planned maintenance).
• Use Cases: Ideal for applications that require high availability but do not need to span multiple datacenters.
• Limitations: Vulnerable to datacenter-wide failures; does not provide the highest level of redundancy.
• Pricing: No additional cost; you only pay for each VM instance created.

Availability Zones

• Definition: Physically separate datacenters within an Azure region, each with independent power, cooling, and networking.
• Architecture: Protects applications against datacenter-wide failures by deploying resources across multiple zones.
• Use Cases: Recommended for mission-critical applications requiring high availability and disaster recovery.
• Limitations: Higher latency between VMs in different zones; may incur additional data transfer costs.
• Pricing: No cost to use zones; charges are based on the resources deployed within them.

Feature	Availability Sets	Availability Zones
Scope of Failure	Rack	Datacenter
SLA	99.95%	99.99%
Network Latency	Very low	Low
Cost	Pay per VM	Pay per VM

Syntax/Configuration

Azure CLI

Create an Availability Set

az vm availability-set create --resource-group <ResourceGroupName> --name <AvailabilitySetName> --platform-fault-domain-count 2 --platform-update-domain-count 2

Create a VM in an Availability Set

az vm create --resource-group <ResourceGroupName> --name <VMName> --image <Image> --availability-set <AvailabilitySetName>

PowerShell

Create an Availability Set

New-AzAvailabilitySet -ResourceGroupName <ResourceGroupName> -Name <AvailabilitySetName> -Location <Location> -Sku Aligned

Create a VM in an Availability Set

New-AzVM -ResourceGroupName <ResourceGroupName> -Location <Location> -VMName <VMName> -Image <Image> -AvailabilitySet <AvailabilitySetName>

Azure Portal

1. Navigate to "Create a resource" > "Compute" > "Availability set".
2. Fill in details (name, region, fault domain count).
3. Click "Create".

Practical Examples

Example 1: Create an Availability Set

Using Azure CLI, create an availability set in a resource group:

az vm availability-set create --resource-group MyResourceGroup --name MyAvailabilitySet --platform-fault-domain-count 2 --platform-update-domain-count 2

This command creates an availability set with two fault and update domains.

Example 2: Create VMs in an Availability Set

Create two VMs in the availability set:

az vm create --resource-group MyResourceGroup --name MyVM1 --image UbuntuLTS --availability-set MyAvailabilitySet
az vm create --resource-group MyResourceGroup --name MyVM2 --image UbuntuLTS --availability-set MyAvailabilitySet

Example 3: Create an Availability Zone

Create a VM in an Availability Zone:

az vm create --resource-group MyResourceGroup --name MyZoneVM --image UbuntuLTS --zone 1

This command deploys a VM in Zone 1 of the specified region.

Example 4: Create a Virtual Machine Scale Set in Availability Zones

az vmss create --resource-group MyResourceGroup --name MyScaleSet --image UbuntuLTS --upgrade-policy-mode automatic --zones 1 2 3

Deploys a scale set spanning three availability zones for high availability.

Example 5: Check SLA for VMs

Using Azure CLI, you can check the SLA for your VMs:

az vm show --resource-group MyResourceGroup --name MyVM1 --query "provisioningState"

Example 6: Add VMs to an Existing Availability Set

az vm create --resource-group MyResourceGroup --name MyNewVM --image UbuntuLTS --availability-set MyAvailabilitySet

Example 7: Scale Set with Autoscale

Set up autoscaling for your scale set:

az monitor autoscale create --resource-group MyResourceGroup --resource MyScaleSet --resource-type Microsoft.Compute/virtualMachineScaleSets --name MyAutoscale

Example 8: Deploying across Availability Zones

Deploying VMs across zones can be tested by simulating a zone failure, ensuring the VMs in other zones remain available.

Real-World Scenarios

Scenario 1: E-commerce Application

An e-commerce application can use Availability Zones to ensure that the website remains operational during heavy traffic or maintenance. VMs in different zones can handle user requests, maintaining uptime.

Scenario 2: Financial Services

For a financial application requiring high compliance and availability, deploying VMs across Availability Sets ensures that even during software updates, one instance remains operational, meeting the 99.95% SLA.

Scenario 3: Disaster Recovery

A company can deploy its primary application in one region using Availability Zones, while a backup instance in another region with a paired zone ensures that data is safe even in a complete regional failure.

Best Practices

1. Use Availability Zones for Critical Apps: Always opt for Availability Zones for applications with high uptime requirements.
2. Combine with Load Balancers: Utilize Azure Load Balancers to distribute traffic effectively across VMs in different availability sets or zones.
3. Monitor SLAs: Regularly check the SLAs and adjust your architecture to meet compliance.
4. Regular Testing: Conduct failover testing to ensure that your setup can handle outages gracefully.
5. Use Infrastructure as Code: Implement solutions using ARM templates or Terraform for reproducible and manageable deployments.

Common Errors

1. Error: "VM cannot be created in an Availability Set"

   • Cause: Attempting to add a VM to an availability set that has reached its limit.
   • Fix: Ensure the availability set has available capacity.

2. Error: "Zone not available for selected VM size"

   • Cause: The selected VM size is not available in the specified zone.
   • Fix: Choose a different size or check for available zones.

3. Error: "Insufficient quota"

   • Cause: Insufficient resources in the selected region.
   • Fix: Request a quota increase from Azure support.

4. Error: "Failed to create load balancer"

   • Cause: Network settings are incorrect.
   • Fix: Verify configuration settings for the load balancer.

Related Services/Commands

Service/Command	Description
Azure Load Balancer	Distributes traffic among VMs to enhance availability.
Virtual Machine Scale Sets	Manages groups of VMs, automatically scaling based on demand.
Azure Traffic Manager	Provides DNS-based traffic load balancing across regions.

Automation Script

Here’s a PowerShell script to automate the creation of an availability set with VMs:

# Define parameters
$resourceGroupName = "MyResourceGroup"
$location = "East US"
$availabilitySetName = "MyAvailabilitySet"

# Create a resource group
New-AzResourceGroup -Name $resourceGroupName -Location $location

# Create an Availability Set
$availabilitySet = New-AzAvailabilitySet -ResourceGroupName $resourceGroupName -Name $availabilitySetName -Location $location -Sku Aligned

# Create VMs in the Availability Set
for ($i=1; $i -le 2; $i++) {
    New-AzVM -ResourceGroupName $resourceGroupName -Location $location -VMName "MyVM$i" -ImageName "UbuntuLTS" -AvailabilitySet $availabilitySet
}

This script creates an availability set and deploys two Ubuntu VMs in it.

Conclusion

Understanding the differences between Availability Sets and Availability Zones is critical for designing resilient Azure architectures. Utilizing these features correctly can enhance your application's availability and ensure compliance with SLAs. As you prepare for the AZ-104 exam, focus on practical applications of these concepts, and consider how they fit into real-world scenarios.

For further reading and hands-on practice, explore the Azure documentation and consider engaging in labs that focus on high availability solutions.

References

• Manage the availability of Windows virtual machines in Azure
• Availability sets overview
• Availability zones overview
• Service Level Agreements (SLA) for Virtual Machines