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Azure Functions deployment slots
Learn to create and use deployment slots with Azure Functions
conceptual
03/02/2022

Azure Functions deployment slots

Azure Functions deployment slots allow your function app to run different instances called "slots". Slots are different environments exposed via a publicly available endpoint. One app instance is always mapped to the production slot, and you can swap instances assigned to a slot on demand. Function apps running under the Apps Service plan may have multiple slots, while under the Consumption plan only one slot is allowed.

The following reflect how functions are affected by swapping slots:

  • Traffic redirection is seamless; no requests are dropped because of a swap. This seamless behavior is a result of the next function triggers being routed to the swapped slot.
  • Currently executing function are terminated during the swap. Please review Improve the performance and reliability of Azure Functions to learn how to write stateless and defensive functions.

Why use slots?

There are a number of advantages to using deployment slots. The following scenarios describe common uses for slots:

  • Different environments for different purposes: Using different slots gives you the opportunity to differentiate app instances before swapping to production or a staging slot.
  • Prewarming: Deploying to a slot instead of directly to production allows the app to warm up before going live. Additionally, using slots reduces latency for HTTP-triggered workloads. Instances are warmed up before deployment, which reduces the cold start for newly deployed functions.
  • Easy fallbacks: After a swap with production, the slot with a previously staged app now has the previous production app. If the changes swapped into the production slot aren't as you expect, you can immediately reverse the swap to get your "last known good instance" back.
  • Minimize restarts: Changing app settings in a production slot requires a restart of the running app. You can instead change settings in a staging slot and swap the settings change into production with a prewarmed instance. This is the recommended way to upgrade between Functions runtime versions while maintaining the highest availability. To learn more, see Minimum downtime upgrade.

Swap operations

During a swap, one slot is considered the source and the other the target. The source slot has the instance of the application that is applied to the target slot. The following steps ensure the target slot doesn't experience downtime during a swap:

  1. Apply settings: Settings from the target slot are applied to all instances of the source slot. For example, the production settings are applied to the staging instance. The applied settings include the following categories:

  2. Wait for restarts and availability: The swap waits for every instance in the source slot to complete its restart and to be available for requests. If any instance fails to restart, the swap operation reverts all changes to the source slot and stops the operation.

  3. Update routing: If all instances on the source slot are warmed up successfully, the two slots complete the swap by switching routing rules. After this step, the target slot (for example, the production slot) has the app that's previously warmed up in the source slot.

  4. Repeat operation: Now that the source slot has the pre-swap app previously in the target slot, complete the same operation by applying all settings and restarting the instances for the source slot.

Keep in mind the following points:

  • At any point of the swap operation, initialization of the swapped apps happens on the source slot. The target slot remains online while the source slot is prepared, whether the swap succeeds or fails.

  • To swap a staging slot with the production slot, make sure that the production slot is always the target slot. This way, the swap operation doesn't affect your production app.

  • Settings related to event sources and bindings must be configured as deployment slot settings before you start a swap. Marking them as "sticky" ahead of time ensures events and outputs are directed to the proper instance.

Manage settings

Some configuration settings are slot-specific. The following lists detail which settings change when you swap slots, and which remain the same.

Slot-specific settings:

  • Publishing endpoints
  • Custom domain names
  • Non-public certificates and TLS/SSL settings
  • Scale settings
  • IP restrictions
  • Always On
  • Diagnostic settings
  • Cross-origin resource sharing (CORS)
  • Private endpoints

Non slot-specific settings:

  • General settings, such as framework version, 32/64-bit, web sockets
  • App settings (can be configured to stick to a slot)
  • Connection strings (can be configured to stick to a slot)
  • Handler mappings
  • Public certificates
  • Hybrid connections *
  • Virtual network integration *
  • Service endpoints *
  • Azure Content Delivery Network *

Features marked with an asterisk (*) are planned to be unswapped.

Note

Certain app settings that apply to unswapped settings are also not swapped. For example, since diagnostic settings are not swapped, related app settings like WEBSITE_HTTPLOGGING_RETENTION_DAYS and DIAGNOSTICS_AZUREBLOBRETENTIONDAYS are also not swapped, even if they don't show up as slot settings.

Create a deployment setting

You can mark settings as a deployment setting, which makes it "sticky". A sticky setting doesn't swap with the app instance.

If you create a deployment setting in one slot, make sure to create the same setting with a unique value in any other slot that is involved in a swap. This way, while a setting's value doesn't change, the setting names remain consistent among slots. This name consistency ensures your code doesn't try to access a setting that is defined in one slot but not another.

Use the following steps to create a deployment setting:

  1. Navigate to Deployment slots in the function app, and then select the slot name.

    :::image type="content" source="./media/functions-deployment-slots/functions-navigate-slots.png" alt-text="Find slots in the Azure portal." border="true":::

  2. Select Configuration, and then select the setting name you want to stick with the current slot.

    :::image type="content" source="./media/functions-deployment-slots/functions-configure-deployment-slot.png" alt-text="Configure the application setting for a slot in the Azure portal." border="true":::

  3. Select Deployment slot setting, and then select OK.

    :::image type="content" source="./media/functions-deployment-slots/functions-deployment-slot-setting.png" alt-text="Configure the deployment slot setting." border="true":::

  4. Once setting section disappears, select Save to keep the changes

    :::image type="content" source="./media/functions-deployment-slots/functions-save-deployment-slot-setting.png" alt-text="Save the deployment slot setting." border="true":::

Deployment

Slots are empty when you create a slot. You can use any of the supported deployment technologies to deploy your application to a slot.

Scaling

All slots scale to the same number of workers as the production slot.

  • For Consumption plans, the slot scales as the function app scales.
  • For App Service plans, the app scales to a fixed number of workers. Slots run on the same number of workers as the app plan.

Add a slot

You can add a slot via the CLI or through the portal. The following steps demonstrate how to create a new slot in the portal:

  1. Navigate to your function app.

  2. Select Deployment slots, and then select + Add Slot.

    :::image type="content" source="./media/functions-deployment-slots/functions-deployment-slots-add.png" alt-text="Add Azure Functions deployment slot." border="true":::

  3. Type the name of the slot and select Add.

    :::image type="content" source="./media/functions-deployment-slots/functions-deployment-slots-add-name.png" alt-text="Name the Azure Functions deployment slot." border="true":::

Swap slots

You can swap slots via the CLI or through the portal. The following steps demonstrate how to swap slots in the portal:

  1. Navigate to the function app.

  2. Select Deployment slots, and then select Swap.

    :::image type="content" source="./media/functions-deployment-slots/functions-swap-deployment-slot.png" alt-text="Screenshot that shows the 'Deployment slot' page with the 'Add Slot' action selected." border="true":::

  3. Verify the configuration settings for your swap and select Swap

    :::image type="content" source="./media/functions-deployment-slots/azure-functions-deployment-slots-swap-config.png" alt-text="Swap the deployment slot." border="true":::

The operation may take a moment while the swap operation is executing.

Roll back a swap

If a swap results in an error or you simply want to "undo" a swap, you can roll back to the initial state. To return to the pre-swapped state, do another swap to reverse the swap.

Remove a slot

You can remove a slot via the CLI or through the portal. The following steps demonstrate how to remove a slot in the portal:

  1. Navigate to Deployment slots in the function app, and then select the slot name.

    :::image type="content" source="./media/functions-deployment-slots/functions-navigate-slots.png" alt-text="Find slots in the Azure portal." border="true":::

  2. Select Delete.

    :::image type="content" source="./media/functions-deployment-slots/functions-delete-deployment-slot.png" alt-text="Screenshot that shows the 'Overview' page with the 'Delete' action selected." border="true":::

  3. Type the name of the deployment slot you want to delete, and then select Delete.

    :::image type="content" source="./media/functions-deployment-slots/functions-delete-deployment-slot-details.png" alt-text="Delete the deployment slot in the Azure portal." border="true":::

  4. Close the delete confirmation pane.

    :::image type="content" source="./media/functions-deployment-slots/functions-deployment-slot-deleted.png" alt-text="Deployment slot delete confirmation." border="true":::

Automate slot management

Using the Azure CLI, you can automate the following actions for a slot:

Change App Service plan

With a function app that is running under an App Service plan, you can change the underlying App Service plan for a slot.

Note

You can't change a slot's App Service plan under the Consumption plan.

Use the following steps to change a slot's App Service plan:

  1. Navigate to Deployment slots in the function app, and then select the slot name.

    :::image type="content" source="./media/functions-deployment-slots/functions-navigate-slots.png" alt-text="Find slots in the Azure portal." border="true":::

  2. Under App Service plan, select Change App Service plan.

  3. Select the plan you want to upgrade to, or create a new plan.

    :::image type="content" source="./media/functions-deployment-slots/azure-functions-deployment-slots-change-app-service-apply.png" alt-text="Change the App Service plan in the Azure portal." border="true":::

  4. Select OK.

Considerations

Azure Functions deployment slots have the following considerations:

  • The number of slots available to an app depends on the plan. The Consumption plan is only allowed one deployment slot. Additional slots are available for apps running under other plans. For details, see Service limits.
  • Swapping a slot resets keys for apps that have an AzureWebJobsSecretStorageType app setting equal to files.
  • When slots are enabled, your function app is set to read-only mode in the portal.
  • Use function app names shorter than 32 characters. Names longer than 32 characters are at risk of causing host ID collisons.

Next steps