Azure App Service Deployment Slots Tips and Tricks

This post explains some of the not so well-known features and configurations settings of the Azure App Service deployment slots. These can be used to modify the swap logic as well as to improve the application availability during and after the swap. Here is what you can do with them:

Swap based on the status code

During the swap operation the site in the staging slot is warmed up by making an HTTP request to its root directory. More detailed explanation of that process is available at How to warm up Azure Web App during deployment slots swap. By default the swap will proceed as long as the site responds with any status code. However, if you prefer the swap to not proceed if the application fails to warm up then you can configure it by using these app settings:

  • WEBSITE_SWAP_WARMUP_PING_PATH: The path to make the warm up request to. Set this to a URL path that begins with a slash as the value. For example, “/warmup.php”. The default value is /.
  • WEBSITE_SWAP_WARMUP_PING_STATUSES:Expected HTTP response codes for the warm-up operation. Set this to a comma-separated list of HTTP status codes. For example: “200,202” . If the returned status code is not in the list, the swap operation will not complete. By default, all response codes are valid.

You can mark those two app setting as “Slot Settings” which would make them remain with the slot during the swap. Or you can have them as “non-sticky” settings meaning that they would move with the site as it gets swapped between slots.

Minimize random cold starts

In some cases after the swap the web app in the production slot may restart later without any action taken by the app owner. This usually happens when the underlying storage infrastructure of Azure App Service undergoes some changes. When that happens the application will restart on all VMs at the same time which may result in a cold start and a high latency of the HTTP requests. While you cannot control the underlying storage events you can minimize the effect they have on your app in the production slot. Set this app setting on every slot of the app:

  • WEBSITE_ADD_SITENAME_BINDINGS_IN_APPHOST_CONFIG: setting this to “1” will prevent web app’s worker process and app domain from recycling when the App Service’s storage infrastructure gets reconfigured.

The only side effect this setting has is that it may cause problems when used with some Windows Communication Foundation (WCF) application. If you app does not use WCF then there is no downside of using this setting.

Control SLOT-sticky configuration

Originally when deployment slots functionality was released it did not properly handle some of the common site configuration settings during swap. For example if you configured IP restrictions on the production slot but did not configure that on the staging slot and then performed the swap you would have had the production slot without any IP restrictions configuration, while the staging slot had the IP restrictions enabled. That did not make much sense so the product team has fixed that. Now the following settings always remain with the slot:

  • IP Restrictions
  • Always On
  • Protocol settings (Https Only, TLS version, client certificates)
  • Diagnostic Log settings
  • CORS

If however for any reason you need to revert to the old behavior of swapping these settings then you can add the app setting WEBSITE_OVERRIDE_PRESERVE_DEFAULT_STICKY_SLOT_SETTINGS to every slot of the app and set its value to “0” or “false”.

swap Diagnostics detector

If a swap operation did not complete successfully for any reason you can use the diagnostics detector to see what has happened during the swap operation and what caused it to fail. To get to it use the “Diagnose and solve problems” link in the portal:

How to find the Swap Diagnostics detector

From there click on “Check Swap Operations” which will open a page showing all the swaps performed on the webapp and their results. It will include possible root causes for the failures and recommendations on how to fix them.

Check Swap Operations page

12 thoughts on “Azure App Service Deployment Slots Tips and Tricks”

  1. Just to explain the specifics of what WEBSITE_ADD_SITENAME_BINDINGS_IN_APPHOST_CONFIG app setting does. By default we put the site’s hostnames into the site’s applicationHost.config file “bindings” section. Then when the swap happens the hostnames in the applicationHost.config get out of sync with what the actual site’s hostnames are. That does not affect the app in anyway while it is running, but as soon as some storage event occurs, e.g. storage volume fail over, that discrepancy causes the worker process app domain to recycle. If you use this app setting then instead of the hostnames we will put the sitename into the “bindings” section of the appHost.config file. The sitename does not change during the swap so there will be no such discrepancy after the swap and hence there should not be a restart.

    1. Thanks for the explanation. I would like to add that this setting is also fixing a cold restart of all instances the first time that you scale in/out after the swap. It took a while to us to identify that was the root cause…

  2. Hi,
    Could you explain more about ” Then when the swap happens the hostnames in the applicationHost.config get out of sync with what the actual site’s hostnames are.”

    It means applicationHost.config also is swapped and the applicationHost.config of stagging slot does not match with the production slot hostnames?

    Thank you.

    1. Correct, the applicationHost.config is also swapped. After the swap it has the staging slot’s hostnames in the “bindings” section even though the site is now in the production slot with production hostnames. That’s why it is recommended to use the above mentioned app setting so that no hostnames are placed in the “bindings” section at all.

  3. Hi,

    Could you tell more about

    “The only side effect this setting has is that it may cause problems when used with some Windows Communication Foundation (WCF) application. If you app does not use WCF then there is no downside of using this setting.”

    Why it affects?

    Thank you.

  4. So how does WEBSITE_SWAP_WARMUP_PING_PATH work together with a configured warmup in applicationInitialization? Will both warmups be fired and can it be any conflicts?

    We are using scaling for our app services so I guess we still need the applicationInitialization warmup.

  5. “The only side effect this setting has is that it may cause problems when used with some Windows Communication Foundation (WCF) application. If you app does not use WCF then there is no downside of using this setting.” – this isn’t quite true:
    If your app doesn’t host WCF services, then it is ok, Ie, web apps which consume a WCF can use this setting without issue, it is only an issue for web apps which host WCF services.

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