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Azure App Service deployment slots are live apps with their own hostnames that function as isolated environments. These slots are available in the Standard, Premium, and Isolated service tiers. They allow developers to validate application changes in a secure staging environment before sending them to users. This architecture supports high availability and maintains a stable production environment during updates.
The core mechanism for updating applications is the swap operation, which exchanges content and configuration between a source slot and a target slot. During a swap, Azure applies production settings to the staging slot and restarts the instances to ensure the application works. It then performs a warm-up phase by sending HTTP requests to the application root to prevent latency for the first users. If the new production version encounters issues, teams can perform an immediate reverse swap to restore the last known good state, which helps protect the application's Service Level Agreement (SLA).
Deployment slots are provisioned as separate live apps within the same resource group as the main web app. Each slot has its own unique URL, allowing complete isolation of code and configuration changes. When setting up slots, it is important to keep the combined length of the parent app name and the slot name under 59 characters to avoid DNS limits. To set up slots, administrators can clone settings from an existing slot or start with an empty environment.
Administrators can create deployment slots using common command-line utilities. In the Azure CLI, the creation is handled using the following command:
az webapp deployment slot create --name --resource-group --slot
Alternatively, Azure PowerShell uses the following cmdlet:
New-AzWebAppSlot -ResourceGroupName -Name -Slot -AppServicePlan
To protect production environments, organizations should never enable continuous deployment directly on the production slot. Instead, developers configure continuous deployment pipelines to deliver main branches to a staging slot for testing, smoke tests, and stakeholder reviews. For container-based applications, teams push explicitly tagged images to a container registry and then update the staging slot's image tag to initiate a controlled rollout.
Managing how configuration settings change during a swap is critical to maintaining application stability. When a swap occurs, some settings transition with the application code, while sticky settings remain bound to the physical slot. To make an app setting or connection string sticky, administrators check the Deployment slot setting box in the Azure portal.
Certain settings are inherently slot-specific and never move during a swap operation. These sticky configurations must remain tied to their environment to prevent errors. Examples of these slot-specific elements include:
Conversely, non-slot-specific settings, such as general application code configurations, travel with the deployment package during the swap.
Azure App Service allows administrators to direct a specific percentage of production traffic to a staging slot to perform testing. When users are routed to a specific slot for canary or A/B testing, they are pinned to that slot with a cookie to maintain their session. For manual routing, internal teams can bypass automated routing rules by appending the x-ms-routing-name query parameter to the URL.
Swaps can be automated using Azure Pipelines, GitHub Actions, or the Azure CLI with the az webapp deployment slot swap command. To streamline deployments, teams can enable auto swap, which automatically pushes a staging slot into production immediately after a successful deployment. During any swap, App Service prepares the source slot, restarts instances, and executes custom warm-up scripts before routing traffic, automatically rolling back the operation if any instance fails to initialize.
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