How cost calculation is done for Zone redundancy enabled App service Plan

Ninad Pawar 1 Reputation point
2021-09-29T05:16:30.407+00:00

I have enabled Zone redundant property in App service plan ARM template and App Service plan created is now having Zoneredundant= enabled. I can see now 3 Instances in Scale Out section. I wanted to understand if I will be billed for 3 Instances or only primary zone where actual App service is created. Basically I wanted to understand when we enable zone redundancy, does Azure creates additional 2 instances in different zones at start? And how billing exactly happens

Azure App Service
Azure App Service
Azure App Service is a service used to create and deploy scalable, mission-critical web apps.
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  1. Alan Kinane 16,586 Reputation points MVP
    2021-09-29T09:55:32.663+00:00

    Yes, this feature requires a minimum of 3 instances deployed (one per availability zone). Therefore, you will be charged for 3 instances of the App Service instance size that you choose. More details here: https://learn.microsoft.com/en-us/azure/app-service/how-to-zone-redundancy

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  2. SnehaAgrawal-MSFT 12,546 Reputation points Microsoft Employee
    2021-10-01T17:07:07.34+00:00

    Thanks for asking question! You might be aware that Azure App Service can be deployed into Availability Zones (AZ) which enables high availability for your apps and this architecture is also known as zone redundancy.

    Zone redundancy is a property of the App Service plan and the App Service platform automatically allocates compute across zones ensuring applications stay up and running. Combined with Availability Zone support across Azure for storage, networking and data, you can create highly available end-to-end application architectures.

    According to limitations for enabling zone redundancy is this requires either Premium v2 or Premium v3 App Service plans and Minimum of 3 instance count and if you specify less than 3 platform will enforce for 3.

    This is due to the platform automatically spreading these VMs across 3 zones when zone redundancy is enabled so you will pay for the redundant nodes because to have the AZ setup you need at least three nodes.

    Further If you want to decide instance capacity, you can use the following calculation:

    Since the platform spreads VMs across 3 zones and you need to account for at least the failure of 1 zone, multiply peak workload instance count by a factor of zones/(zones-1), or 3/2. For example, if your typical peak workload requires 4 instances, you should provision 6 instances: (2/3 * 6 instances) = 4 instances.

    Reference: https://azure.github.io/AppService/2021/08/25/App-service-support-for-availability-zones.html

    Let us know if you have further query on this.

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