Multiple App Service Composite SLA calculation

Question

Multiple App Service Composite SLA calculation

Wouter Slob 6

This page describes how composite SLAs can be calculated:
https://learn.microsoft.com/en-us/azure/architecture/framework/resiliency/business-metrics#understand-service-level-agreements.

The SLA for App Service is defined on https://azure.microsoft.com/en-us/support/legal/sla/app-service/v1_4/ as:
"We guarantee that Apps running in a customer subscription will be available 99.95% of the time. "

I am wondering how this works if you have multiple Web Apps on different App Service Plans in the same subscription.
How would we calculate the composite SLA for the following scenarios:

Scenario 1: Sequential, eg: ... -> WebApp A -> WebApp B -> ...

Would the composite SLA of Web App A + Web App B be 99.95% * 99.95% = 99.90%?
Or is it just 99.95%, because App Service is used?

Scenario 2: Parallel, eg: ... -> (WebApp A or WebApp B) -> ... (Eg with a load balancer in front of the Web Apps).

Would the composite SLA Web App A + Web App B be 1.0 − (0.05% × 0.05%) = 99.9999%
Or is it just 99.95%, because App Service is used?

SnehaAgrawal-MSFT 22,706 Reputation points Moderator

2021-11-22T09:43:46.433+00:00

Thanks for asking question!

To elaborate on Availability Zone:

An Availability Zone is a high-availability offering that protects your applications and data from datacenter failures. Availability Zones are unique physical locations within an Azure region. Each zone is made up of one or more datacenters equipped with independent power, cooling, and networking. To ensure resiliency, there’s a minimum of three separate zones in all enabled regions. Build high-availability into your application architecture by co-locating your compute, storage, networking, and data resources within a zone and replicating in other zones.

To learn more about Availability Zones, check here

Further Azure App Service can be deployed into Availability Zones (AZ) to help you achieve resiliency and reliability for your business-critical workloads. This architecture is also known as zone redundancy.

An app lives in an App Service plan (ASP), and the App Service plan exists in a single scale unit. When an App Service is configured to be zone redundant, the platform automatically spreads the VM instances in the App Service plan across all three zones in the selected region.

Also, Availability Zone known as zone redundancy, is a property of the App Service Plan (ASP) and not Web App.

Check this link for more details on App Service Support for Availability Zones and How to Deploy a Zone Redundant App Service

Hope this helps, let us know if further query on this.
Wouter Slob 6 Reputation points

2021-11-25T07:53:45.827+00:00

Thanks for your reply, but unfortunately this does not help me with my composite SLA calculations. Do you have any insights on that?

Maybe the SLA is tight to the scale unit? If so can we enforce deployments of different ASP to different scale units?

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SnehaAgrawal-MSFT 22,706 Reputation points Moderator

2021-11-22T09:43:46.433+00:00

Thanks for asking question!

To elaborate on Availability Zone:

An Availability Zone is a high-availability offering that protects your applications and data from datacenter failures. Availability Zones are unique physical locations within an Azure region. Each zone is made up of one or more datacenters equipped with independent power, cooling, and networking. To ensure resiliency, there’s a minimum of three separate zones in all enabled regions. Build high-availability into your application architecture by co-locating your compute, storage, networking, and data resources within a zone and replicating in other zones.

To learn more about Availability Zones, check here

Further Azure App Service can be deployed into Availability Zones (AZ) to help you achieve resiliency and reliability for your business-critical workloads. This architecture is also known as zone redundancy.

An app lives in an App Service plan (ASP), and the App Service plan exists in a single scale unit. When an App Service is configured to be zone redundant, the platform automatically spreads the VM instances in the App Service plan across all three zones in the selected region.

Also, Availability Zone known as zone redundancy, is a property of the App Service Plan (ASP) and not Web App.

Check this link for more details on App Service Support for Availability Zones and How to Deploy a Zone Redundant App Service

Hope this helps, let us know if further query on this.
Wouter Slob 6 Reputation points

2021-11-25T07:53:45.827+00:00

Thanks for your reply, but unfortunately this does not help me with my composite SLA calculations. Do you have any insights on that?

Maybe the SLA is tight to the scale unit? If so can we enforce deployments of different ASP to different scale units?

Answer 1

Thanks for reply! As its mentioned in the document that" Composite SLAs involve multiple services"

In Azure, the Service Level Agreement describes Microsoft's commitments for uptime and connectivity. If the SLA for a particular service is 99.9%, you should expect the service to be available 99.9% of the time. As example the SLA for App Service running in a customer subscription will be available 99.95% of the time.

Different services have different SLAs.

In case of App Service deploying to more than one region than the composite SLA for a multiregion deployment is calculated as mentioned here.

Check this blog here Calculate the availability and SLA for your Azure solution might be helpful.

Disclaimer: This response contains a reference to a third-party World Wide Web site. Microsoft is providing this information as a convenience to you. Microsoft does not control these sites and has not tested any software or information found on these sites; therefore, Microsoft cannot make any representations regarding the quality, safety, or suitability of any software or information found there.

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Multiple App Service Composite SLA calculation

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