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With the recent release of our new Availability Zone features, a common question we’ve been getting from our customers is: “How do I know if my existing App Service supports Availability Zones?” While this information is documented in the App Service Resiliency Microsoft Learn Docs, in this blog post I will try and walk you through some essential aspects of enabling Availability Zones in Azure App Service.

This article outlines how to confirm if your App Service plan supports Availability Zones, guides you through upgrading from Free, Basic, or Standard tiers to Premium SKUs, and describes the process for enabling this feature, including creating new, properly configured plans. The overview also addresses specific considerations for App Service Environments, along with common issues and solutions, equipping you with the knowledge to enhance application resilience and high availability.

Before diving into how to check availability zone support, let’s understand the basics.

What Are Availability Zones?

Availability zones are physically separate groups of datacenters within each Azure region. When one zone fails, services can fail over to one of the remaining zones, providing high availability and resilience for your applications. Unlike in the past, now you only need a minimum of two instances to enable zone redundancy.

Key Benefit: Zone-redundant App Service plans automatically distribute your app instances across multiple availability zones, ensuring your application remains available even if an entire zone goes down.

Pre-requisites: Before diving into how to check availability zone support, let’s understand the basic requirements:

• Premium and Isolated SKUs: You must use Premium v2, v3, v4, or Isolated v2 App Service plans.


• Minimum 2 instances: Zone redundancy requires at least 2 instances in your App Service plan.


• Supported region: The region must support availability zones.


• Modern scale unit: Your App Service plan must be on a scale unit that supports availability zones.

In the next section, we will explore our options to make sure we meet these requirements.

How to Check if Your Existing App Service Supports Availability Zones

Here’s the step-by-step process to determine if your App Service can support availability zones:

Step 1: Check Your App Service Plan SKU

First, verify that you’re using a supported SKU. You can check this in the Azure portal or using Azure CLI.

Look for one of these supported SKUs:

• Premium v2


• Premium v3


• Premium v4


• Isolated v2

Step 2: Check maximumNumberofZones Property

This is the most important step! The maximumNumberOfZones property tells you if your App Service plan can support availability zones. This property was recently added to the “serverfarm” object (App Service plan) and can have a value of either 1, 2, or 3. This number indicates the number of zones the scale unit your plan is hosted on has available to it. This property is available in the Azure portal on the Scale out (App Service plan) blade for each of your plans and can also be found using the Azure CLI.

az appservice plan show -n -g –query properties.maximumNumberOfZones

✅ Zone Redundancy Supported: If the value is greater than 1 (typically 3), your App Service plan supports availability zones and you can enable zone redundancy. In some cases, the value is 2. This means that the scale unit you’re on only has two zones. While our standard approach prioritizes three zones, certain circumstances may limit availability to two. To maximize accessibility, we have extended Availability Zone support to scale units with both two and three zones. The service level agreement (SLA) remains consistent regardless of zone count, and platform-level measures are in place to ensure this standard is maintained.

❌ Zone Redundancy Not Supported: If the value equals 1, your App Service plan is on a scale unit that doesn’t support availability zones. You’ll need to create a new App Service plan following this guidance to ensure you land on a supported scale unit.

Step 3: Check Regional Support

Ensure your region supports availability zones. You can find the list of supported regions in the Azure documentation.

Step 4: Verify Current Configuration

Check if zone redundancy is already enabled. This is displayed on the Overview blade in the Azure portal for your App Service plan or can be found using the Azure CLI:

az appservice plan show -n -g –query properties.zoneRedundant

Upgrading from Basic or Standard to a supported plan

If you’re currently using Basic or Standard App Service plans (which don’t support availability zones), you’ll need to upgrade to one of the Premium plans such as Premium v2, v3, or v4. Here’s how to do it safely:

⚠️ Important: Avoid Quota Constraints

Don’t scale everything at once! If you try to scale up all of your plans at one time, you may run into quota limits for your subscription. If you have many App Service plans, upgrading them all simultaneously can hit your subscription’s quota limits and cause deployments to fail. For this reason, you should upgrade in smaller batches.

✅ Recommended Upgrade Strategy

Upgrade your App Service plans in small batches (2-3 plans at a time) rather than all at once:

# Upgrade first batch of App Service plans
az appservice plan update -g -n –sku P1V3
az appservice plan update -g -n –sku P1V3

# Wait and monitor, then proceed with next batch
az appservice plan update -g -n –sku P1V3

You can also do this plan by plan in the Azure portal.

Enabling Availability Zones

⚠️ Important: Ensure You Have Two Instances

Enabling zone redundancy is almost instantaneous and doesn’t cause downtime. However, you must specify at least 2 instances when enabling zone redundancy.

✅ Enabling availability zones

If your App Service plan supports availability zones, here’s how to enable it: 

az appservice plan update -g -n –set zoneRedundant=true sku.capacity=2

This feature is also now available in the Azure portal on the Scale out (App Service plan) blade.

New App Service Plan Creation

When creating App Service plans, ensure you meet the prerequisites: choose the correct SKU, use at least two instances, and select a region that supports availability zones. For supported regions, refer to the Azure regions list.

⚠️ Important: Ensure You Land on a Supported Region and Scale Set.

Here’s why this requirement matters: In the past, only some App Service scale units (VM clusters) had zones, and older units created in a single zone don’t support zone redundancy. Our current focus is building new multi-zone scale units, but updating all existing ones isn’t always possible due to limitations and priorities. As a result, we can’t guarantee every scale unit will have zones or provide ETAs for specific regions or scale units.

To increase the likelihood that a new App Service plan deployment is placed on a scale unit with zones, here are some steps you can follow:

• Deploy your App Service plan in a new resource group: Using a new resource group will ensure your new plan is put on a different scale unit than your existing plan(s) that may not currently have zones. Using a new resource group doesn’t guarantee that you will land on a scale unit with zones, but it improves your chances and there are other measures you can take to help guarantee that.


• Enable zone redundancy when creating your App Service plan: This ensures your plan is placed on a scale unit with zones. You can scale down to one instance and disable zone redundancy after setup if you don’t need it right away, but enabling it initially gives you the option to re-enable zone redundancy later.
  
  
  
  • If there are no zone-redundant scale units available, your deployment will fail. If this happens to you, feel free to open a support ticket to discuss your options.
  
  
  
  

App Service Environment Considerations

⚠️ Important: Isolated v2 Plans Require Configuration at Both the ASE and Plan levels

App Service Environment (ASE) v3 uses Isolated v2 plans. Unlike Premium plans, ASEv3 and these Isolated v2 plans support zone redundancy, but require configuration at both the ASE and plan levels. Enable zone redundancy on the ASE first; only then can individual App Service plans be made zone redundant. Once enabled, plans inherit the ability to be zone redundant, but you may have both zone redundant and non-zone redundant plans within the same ASE.

The “hostingEnvironment” resource (ASE) also has a maximumNumberOfZones property similar to the App Service plan. For ASE, the property at the ASE level is identical to the property at the plan level, meaning all plans in the ASE will have the same value as the ASE level property.

To enable zone redundancy on an ASEv3:

az resource update –ids /subscriptions/{subscriptionId}/resourceGroups/{resourceGroup}/providers/Microsoft.Web/hostingEnvironments/{aseName} –set properties.zoneRedundant=true

Beyond this consideration, enabling zone redundancy on the Isolated v2 plans follows the same process as the Premium SKUs we saw above.

Common Issues and Solutions

Issue 1: “Scale unit doesn’t support availability zones”

Problem: Your App Service plan shows maximumNumberOfZones = 1 even though you’re in a supported region with a supported SKU.

Solution: Create a new App Service plan in a new resource group. The scale unit assignment is based on the resource group, so a new resource group increases your chances of landing on a zone-capable scale unit.

Issue 2: “Region doesn’t support availability zones”

Problem: You’re using a region that doesn’t support availability zones.

Solution: Move to a supported region. Check the official list of regions that support availability zones.

Issue 3: “Using unsupported SKU”

Problem: You’re using Free, Basic or Standard App Service plans that don’t support availability zones.

Solution: Upgrade to a supported SKU (Premium v2-4 or Isolated v2).

Issue 4: “Quota constraints when upgrading multiple App Service plans”

Problem: You’re trying to upgrade multiple Free/Basic/Standard App Service plans to Premium tiers simultaneously and hitting quota limits.

Solution: Upgrade in batches and monitor quota usage.

Issue 5: “Scale unit supports zones but you are unable to enable zone redundancy”

Problem: You’re App Service plan shows maximumNumberOfZones > 1 but you’re unable to enable zone redundancy.

Solution: Create a support ticket for further investigation. There may be edge cases where additional support is needed to enable zone redundancy.

Issue 6: “ASE supports zone redundancy but you are unable to enable zone redundancy on your App Service plans”

Problem: You’re ASE and Isolated v2 plans show maximumNumberOfZones > 1 but you’re unable to enable zone redundancy.

Solution: Ensure you have enabled zone redundancy on the ASE first. Once that is enabled, you should be able to enable zone redundancy on the individual plans.

Additional Things to Know

Behind the Scenes Zone Balancing

Even if you don’t have zone redundancy explicitly enabled on your App Service plan, we may already be distributing your instances across zones where possible. This automatic distribution happens behind the scenes and has:

• No impact to performance: Your applications continue to run exactly as before.


• No impact to cost: You’re not charged any additional fees for this automatic distribution.


• Improved resilience: Your applications benefit from zone distribution even without explicit configuration on your part.

As a platform, we are committed to providing the most resilient products possible. This automatic zone distribution is a step toward that goal and aligns with the behavior of many other Azure services that are also adopting a zone-redundant-first (zone-redundant-by-default) architecture. So, even if your zoneRedundant property shows false, you might find that your instances are already running in multiple zones. This is not a mistake.

Checking Your Current Zone Distribution

You can verify if your instances are already distributed across zones by checking the physical zone placement:

az rest –method get –url https://management.azure.com/subscriptions/{subscriptionId}/resourceGroups/{resourceGroup}/providers/Microsoft.Web/sites/{appName}/instances?api-version=2024-04-01

This is also available in the Azure portal in the Health check blade for each of your App Service apps.

NOTE! The physical zone details and the Health check blade in the above screenshot is at the app level, not the plan level. Since all of the apps in a given App Service plan use all of the same instances, you can pick any app in your plan to view the physical zone details and it will be the same across all apps.

Aligning Property with Reality

If you find that your instances are already spread across multiple zones and want the zoneRedundant property to align with the actual state of your instances, you can update it yourself with no impact to your running apps.

Conclusion

To check if Azure App Service supports availability zones, look at the maximumNumberOfZones property: a value greater than 1 means zone redundancy is available; if it’s 1, create a new App Service plan in a different resource group to access zonal support.

Remember that availability zones provide significant resilience benefits, automatically handling zone failures and improving your application’s uptime. Upgrading to Premium or Isolated v2 SKUs and at least two instances are recommended for high-availability production workloads.

Next Steps:

For more detailed information, refer to the official Azure App Service reliability documentation.

For questions or feedback about this blog post, please use the comments below.