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When you deploy your web app, web app on Linux, mobile back end, or API app to Azure App Service, you can use a separate deployment slot instead of the default production slot when you're running in the Standard, Premium, or Isolated App Service plan tier. Deployment slots are live apps with their own host names. App content and configurations elements can be swapped between two deployment slots, including the production slot.

Deploying your application to a non-production slot has the following benefits:

You can validate app changes in a staging deployment slot before swapping it with the production slot.
Deploying an app to a slot first and swapping it into production makes sure that all instances of the slot are warmed up before being swapped into production. This eliminates downtime when you deploy your app. The traffic redirection is seamless, and no requests are dropped because of swap operations. You can automate this entire workflow by configuring auto swap when pre-swap validation isn't needed.
After a swap, the slot with previously staged app now has the previous production app. If the changes swapped into the production slot aren't as you expect, you can perform the same swap immediately to get your 'last known good site' back.

Each App Service plan tier supports a different number of deployment slots. There's no additional charge for using deployment slots. To find out the number of slots your app's tier supports, see App Service limits.

Navigate to your Azure App Service that you created in your environment. Click on ' Deployment Slots ' in the left panel and click ' Add Slot ' to create a new slot. Give it a name like ' staging ' so that you can identify it easily. Deploying to Staging Slot - If a cloud service has a deployment in either slot, then the entire cloud service is pinned to a specific cluster. This means that if a deployment already exists in the production slot, then a new staging deployment can only be allocated in the same cluster as the production slot. Deployment slots are in fact resources of the App Service itself and can be deployed as such. This means you can specify them within the resources array of your web application / App Service. ARM code for deployment slot looks like below: ARM Code for Azure Web App Slot. Here, slot name is defined inside a property named stagingSlotName. 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.

To scale your app to a different tier, make sure that the target tier supports the number of slots your app already uses. For example, if your app has more than five slots, you can't scale it down to the Standard tier, because the Standard tier supports only five deployment slots.

Add a slot

The app must be running in the Standard, Premium, or Isolated tier in order for you to enable multiple deployment slots.

in the Azure portal, search for and select App Services and select your app.
In the left pane, select Deployment slots > Add Slot.
Note
If the app isn't already in the Standard, Premium, or Isolated tier, you receive a message that indicates the supported tiers for enabling staged publishing. At this point, you have the option to select Upgrade and go to the Scale tab of your app before continuing.
In the Add a slot dialog box, give the slot a name, and select whether to clone an app configuration from another deployment slot. Select Add to continue.
You can clone a configuration from any existing slot. Settings that can be cloned include app settings, connection strings, language framework versions, web sockets, HTTP version, and platform bitness.
After the slot is added, select Close to close the dialog box. The new slot is now shown on the Deployment slots page. By default, Traffic % is set to 0 for the new slot, with all customer traffic routed to the production slot.
Select the new deployment slot to open that slot's resource page.
The staging slot has a management page just like any other App Service app. You can change the slot's configuration. To remind you that you're viewing the deployment slot, the app name is shown as <app-name>/<slot-name>, and the app type is App Service (Slot). You can also see the slot as a separate app in your resource group, with the same designations.
Select the app URL on the slot's resource page. The deployment slot has its own host name and is also a live app. To limit public access to the deployment slot, see Azure App Service IP restrictions.

The new deployment slot has no content, even if you clone the settings from a different slot. For example, you can publish to this slot with Git. You can deploy to the slot from a different repository branch or a different repository.

What happens during a swap

Swap operation steps

When you swap two slots (usually from a staging slot into the production slot), App Service does the following to ensure that the target slot doesn't experience downtime:

Apply the following settings from the target slot (for example, the production slot) to all instances of the source slot:
- Slot-specific app settings and connection strings, if applicable.
- Continuous deployment settings, if enabled.
- App Service authentication settings, if enabled.
Any of these cases trigger all instances in the source slot to restart. During swap with preview, this marks the end of the first phase. The swap operation is paused, and you can validate that the source slot works correctly with the target slot's settings.
Wait for every instance in the source slot to complete its restart. If any instance fails to restart, the swap operation reverts all changes to the source slot and stops the operation.
If local cache is enabled, trigger local cache initialization by making an HTTP request to the application root ('/') on each instance of the source slot. Wait until each instance returns any HTTP response. Local cache initialization causes another restart on each instance.
If auto swap is enabled with custom warm-up, trigger Application Initiation by making an HTTP request to the application root ('/') on each instance of the source slot.
If applicationInitialization isn't specified, trigger an HTTP request to the application root of the source slot on each instance.
If an instance returns any HTTP response, it's considered to be warmed up.
If all instances on the source slot are warmed up successfully, swap the two slots by switching the routing rules for the two slots. After this step, the target slot (for example, the production slot) has the app that's previously warmed up in the source slot.
Now that the source slot has the pre-swap app previously in the target slot, perform the same operation by applying all settings and restarting the instances.

At any point of the swap operation, all work of initializing the swapped apps happens on the source slot. The target slot remains online while the source slot is being prepared and warmed up, regardless of where 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 produ

Azure Cloud Service Deployment Slots