Swap deployment slots

  • 3 minutes

You can swap deployment slots on your app's Deployment slots page and the Overview page. Before you swap an app from a deployment slot into production, make sure that production is your target slot and that all settings in the source slot are configured exactly as you want to have them in production.

Manually swapping deployment slots

To swap deployment slots:

  1. Go to your app's Deployment slots page and select Swap. The Swap dialog box shows settings in the selected source and target slots that will be changed.

  2. Select the desired Source and Target slots. Usually, the target is the production slot. Also, select the Source Changes and Target Changes tabs and verify that the configuration changes are expected. When you're finished, you can swap the slots immediately by selecting Swap.

    To see how your target slot would run with the new settings before the swap actually happens, don't select Swap, but follow the instructions in Swap with preview below.

  3. When you're finished, close the dialog box by selecting Close.

Swap with preview (multi-phase swap)

Before you swap into production as the target slot, validate that the app runs with the swapped settings. The source slot is also warmed up before the swap completion, which is desirable for mission-critical applications.

When you perform a swap with preview, App Service performs the same swap operation but pauses after the first step. You can then verify the result on the staging slot before completing the swap.

If you cancel the swap, App Service reapplies configuration elements to the source slot.

To swap with preview:

  1. Follow the steps above in Swap deployment slots but select Perform swap with preview. The dialog box shows you how the configuration in the source slot changes in phase 1, and how the source and target slot change in phase 2.

  2. When you're ready to start the swap, select Start Swap.

    When phase 1 finishes, you're notified in the dialog box. Preview the swap in the source slot by going to https://<app_name>-<source-slot-name>.azurewebsites.net.

  3. When you're ready to complete the pending swap, select Complete Swap in Swap action and select Complete Swap.

    To cancel a pending swap, select Cancel Swap instead.

  4. When you're finished, close the dialog box by selecting Close.

Configure auto swap

Auto swap streamlines Azure DevOps Services scenarios where you want to deploy your app continuously with zero cold starts and zero downtime for customers of the app. When auto swap is enabled from a slot into production, every time you push your code changes to that slot, App Service automatically swaps the app into production after it's warmed up in the source slot.

Note

Auto swap isn't currently supported in web apps on Linux and Web App for Containers.

To configure auto swap:

  1. Go to your app's resource page and select the deployment slot you want to configure to auto swap. The setting is on the Configuration > General settings page.

  2. Set Auto swap enabled to On. Then select the desired target slot for Auto swap deployment slot, and select Save on the command bar.

  3. Execute a code push to the source slot. Auto swap happens after a short time, and the update is reflected at your target slot's URL.

Specify custom warm-up

Some apps might require custom warm-up actions before the swap. The applicationInitialization configuration element in web.config lets you specify custom initialization actions. The swap operation waits for this custom warm-up to finish before swapping with the target slot. Here's a sample web.config fragment.

<system.webServer>
    <applicationInitialization>
        <add initializationPage="/" hostName="[app hostname]" />
        <add initializationPage="/Home/About" hostName="[app hostname]" />
    </applicationInitialization>
</system.webServer>

For more information on customizing the applicationInitialization element, see Most common deployment slot swap failures and how to fix them.

You can also customize the warm-up behavior with one or both of the following app settings:

  • WEBSITE_SWAP_WARMUP_PING_PATH: The path to ping to warm up your site. Add this app setting by specifying a custom path that begins with a slash as the value. An example is /statuscheck. The default value is /.
  • WEBSITE_SWAP_WARMUP_PING_STATUSES: Valid HTTP response codes for the warm-up operation. Add this app setting with a comma-separated list of HTTP codes. An example is 200,202 . If the returned status code isn't in the list, the warmup and swap operations are stopped. By default, all response codes are valid.
  • WEBSITE_WARMUP_PATH: A relative path on the site that should be pinged whenever the site restarts (not only during slot swaps). Example values include /statuscheck or the root path, /.

Roll back and monitor a swap

If any errors occur in the target slot (for example, the production slot) after a slot swap, restore the slots to their pre-swap states by swapping the same two slots immediately.

If the swap operation takes a long time to complete, you can get information on the swap operation in the activity log.

On your app's resource page in the portal, in the left pane, select Activity log.

A swap operation appears in the log query as Swap Web App Slots. You can expand it and select one of the suboperations or errors to see the details.