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Copy file name to clipboardExpand all lines: articles/reliability/reliability-app-service.md
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---
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title: Reliability in Azure App Service
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description: Find out about reliability in Azure App Service, including availability zones and multi-region deployments.
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description: Find out about reliability in Azure App Service, including availability zones and multi-region deployments.
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author: anaharris-ms
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ms.author: anaharris
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ms.topic: reliability-article
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# Reliability in Azure App Service
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Azure App Service is an HTTP-based service for hosting web applications, REST APIs, and mobile back ends. App Service integrates with Microsoft Azure to provide security, load balancing, autoscaling, and automated management for applications. This article describes reliability support in [Azure App Service](../app-service/overview.md), covering intra-regional resiliency via [availability zones](#availability-zone-support) and [multi-region deployments](#multi-region-support).
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Azure App Service is an HTTP-based service for hosting web applications, REST APIs, and mobile back ends. App Service integrates with Microsoft Azure to provide security, load balancing, autoscaling, and automated management for applications. This article describes reliability support in [App Service](../app-service/overview.md). It covers intra-regional resiliency via [availability zones](#availability-zone-support) and [multiple-region deployments](#multi-region-support).
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If you are using App Service Environment, see [Reliability in Azure App Service Environment](./reliability-app-service-environment.md) for more information about reliability support in that environment.
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For more information about reliability support in App Service Environment, see [Reliability in App Service Environment](./reliability-app-service-environment.md).
To learn about how to deploy Azure App Service to support your solution's reliability requirements, and how reliability affects other aspects of your architecture, see [Architecture best practices for Azure App Service (Web Apps) in the Azure Well-Architected Framework](/azure/well-architected/service-guides/app-service-web-apps).
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For more information about App Service support for reliability requirements and how reliability affects other aspects of your architecture, see [Architecture best practices for the Web Apps feature of App Service](/azure/well-architected/service-guides/app-service-web-apps).
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## Reliability architecture overview
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When you create an Azure App Service web app, you define which[App Service plan](../app-service/overview-hosting-plans.md)the app runs on.
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When you create an App Service web app, you specify the[App Service plan](../app-service/overview-hosting-plans.md)that runs the app.
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[!INCLUDE [App Service reliability architecture - plan description](includes/app-service/reliability-architecture-plans.md)]
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[!INCLUDE [Availability zone support description](includes/reliability-availability-zone-description-include.md)]
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For **Premium v2-v4 tiers**, App Service can be configured as *zone redundant*, which means that your resources are distributed across multiple availability zones. Distribution across multiple zones helps your production workloads achieve resiliency and reliability. When you configure zone redundancy on App Service plans, all apps that use the plan are made zone redundant.
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For **Premium v2 to v4 tiers**, you can configure App Service as *zone redundant*, which means that your resources are distributed across multiple availability zones. Distribution across multiple zones helps your production workloads achieve resiliency and reliability. When you configure zone redundancy on App Service plans, all apps that use the plan become zone redundant.
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### Region support
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Zone-redundant App Service **Premium v2-v4** plans can be deployed in [any region that supports availability zones](./regions-list.md).
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You can deploy zone-redundant App Service **Premium v2 to v4** plans in [any region that supports availability zones](./regions-list.md).
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### Requirements
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To enable zone-redundancy you must:
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To enable zone-redundancy, you must meet the following requirements:
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- Use [Premium v2-4 plan types](/azure/app-service/overview-hosting-plans).
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- Use [Premium v2 to v4 plan types](/azure/app-service/overview-hosting-plans).
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- Deploy a minimum of two instances in your plan.
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-Be located on a scale unit that supports availability zones. When you create an App Service plan, the plan is assigned to a scale unit. The scale unit that you're assigned to is based on the resource group that you deploy an App Service plan to. If your scale unit doesn't support availability zones, you need to create a new plan in a new resource group.
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-Use a scale unit that supports availability zones. When you create an App Service plan, the plan is assigned to a scale unitbased on the resource group where the plan resides. If your scale unit doesn't support availability zones, you need to create a new plan in a new resource group.
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To learn whether or not the scale unit that your App Service plan is on supports zone redundancy, see [Check for zone redundancy support for an App Service plan](../app-service/configure-zone-redundancy.md#check-for-zone-redundancy-support-on-an-app-service-plan).
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To determine whether the scale unit for your App Service plan supports zone redundancy, see [Check for zone redundancy support for an App Service plan](../app-service/configure-zone-redundancy.md#check-for-zone-redundancy-support-on-an-app-service-plan).
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### Instance distribution across zones
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[!INCLUDE [Instance distribution across zones description](includes/app-service/reliability-instance-distribution-across-zones-include.md)]
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### Considerations
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For **Premium v2-4** plans, during an availability zone outage, some aspects of Azure App Service might be affected, even though the application continues to serve traffic. These behaviors include App Service plan scaling, application creation, application configuration, and application publishing.
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For **Premium v2 to v4** plans, an availability zone outage might affect some aspects of Azure App Service, even though the application continues to serve traffic. These behaviors include App Service plan scaling, application creation, application configuration, and application publishing.
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When you enable zone redundancy on your App Service **Premium v2-4** plan, you also improve your resiliency to updates that the App Service platform rolls out. To learn more, see [Reliability during service maintenance](#reliability-during-service-maintenance).
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When you enable zone redundancy on your App Service **Premium v2 to v4** plan, you also improve resiliency during platform updates. For more information, see [Reliability during service maintenance](#reliability-during-service-maintenance).
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For App Service plans that aren't configured as zone redundant, the underlying VM instances aren't resilient to availability zone failures. They can experience downtime during an outage in any zone in that region.
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For App Service plans that aren't configured as zone redundant, the underlying virtual machine (VM) instances aren't resilient to availability zone failures. They can experience downtime during an outage in any zone in that region.
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### Cost
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When you use App Service **Premium v2-v4 plans**, there's no extra cost associated with enabling availability zones as long as you have two or more instances in your App Service plan. You're charged based on your App Service plan SKU, the capacity you specify, and any instances that you scale to based on your autoscale criteria.
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When you use App Service **Premium v2 to v4 plans**, enabling availability zones doesn't add cost if you have two or more instances. Charges are based on your App Service plan SKU, the capacity that you specify, and any instances that you scale to based on your autoscale criteria.
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If you enable availability zones but specify a capacity of less than two, the platform enforces a minimum instance count of two. The platform charges you for those two instances.
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### Configure availability zone support
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-**Create a new zone-redundant App Service plan.**To learn how to create a new zone-redundant App Service plan, see [Create a new App Service plan with zone redundancy](../app-service/configure-zone-redundancy.md#create-a-new-app-service-plan-that-includes-zone-redundancy).
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-**Create a new zone-redundant App Service plan.**For more information, see [Create a new App Service plan that includes zone redundancy](../app-service/configure-zone-redundancy.md#create-a-new-app-service-plan-that-includes-zone-redundancy).
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-**Enable or disable zone redundancy on an existing App Service plan.**To learn how to enable or disable an existing App Service plan to zone redundancy, see [Enable or disable an existing App Service plan to zone redundancy](../app-service/configure-zone-redundancy.md#set-zone-redundancy-for-an-existing-app-service-plan).
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-**Enable or disable zone redundancy on an existing App Service plan.**For more information, see [Set zone redundancy for an existing App Service plan](../app-service/configure-zone-redundancy.md#set-zone-redundancy-for-an-existing-app-service-plan).
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### Capacity planning and management
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App Service is a single-region service. If the region becomes unavailable, your application is also unavailable.
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### Alternative multi-region approaches
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### Alternative multiple-region approaches
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To reduce the risk of a single-region failure affecting your application, you can deploy plans across multiple regions. The following steps help strengthen resilience:
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- Deploy your application to the plans in each region.
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- Configure load balancing and failover policies.
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- Replicate your data across regions so that you can recover your last application state.
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The following resources are related to this approach:
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-[Reference architecture: Highly available multi-region web application](/azure/architecture/web-apps/guides/enterprise-app-patterns/reliable-web-app/dotnet/guidance)
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Consider the following related resources:
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-[Reference architecture: Highly available multiple-region web application](/azure/architecture/web-apps/guides/enterprise-app-patterns/reliable-web-app/dotnet/guidance)
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-[Approaches to consider](/azure/architecture/web-apps/guides/multi-region-app-service/multi-region-app-service?tabs=paired-regions#approaches-to-consider)
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-[Tutorial: Create a highly available multi-region app in App Service](/azure/app-service/tutorial-multi-region-app)
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-[Tutorial: Create a highly available multiple-region app in App Service](/azure/app-service/tutorial-multi-region-app)
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## Backups
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When you use Basic tier or higher, you can back up your App Service apps to a file by using the App Service backup and restore capabilities.
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When you use the Basic tier or higher, you can back up your App Service apps to a file by using the App Service backup and restore capabilities.
[!INCLUDE [Reliability during service maintenance description](includes/app-service/reliability-maintenance-include.md)]
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To learn more, see [Routine planned maintenance for Azure App Service](/azure/app-service/routine-maintenance) and [Routine maintenance for Azure App Service, restarts, and downtime](/azure/app-service/routine-maintenance-downtime).
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For more information, see [Routine planned maintenance for App Service](/azure/app-service/routine-maintenance) and [Routine maintenance for App Service, restarts, and downtime](/azure/app-service/routine-maintenance-downtime).
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