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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.

Time slot definition: 1. A time when something can happen or is planned to happen, especially when it is one of several. ‘When placed into a slot that fits the screw's groove and shape, this allows for rotary motion to be converted into forward or backward motion.’ ‘It's easy to find a bit that fits the screw slot properly.’ ‘Of course, the screw slot is pretty fine, so without a properly fitting screwdriver, there is a real risk of marring things.’.

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.

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.

  1. in the Azure portal, search for and select App Services and select your app.

  2. 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.

  3. 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.

  4. 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.

  5. 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.

  6. 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:

  1. 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.

  2. 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.

  3. 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.

  4. 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.

  5. 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.

  6. 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 production app.

Which settings are swapped?

When you clone configuration from another deployment slot, the cloned configuration is editable. Some configuration elements follow the content across a swap (not slot specific), whereas other configuration elements stay in the same slot after a swap (slot specific). The following lists show the settings that change when you swap slots.

Settings that are swapped:

  • 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
  • WebJobs content
  • Hybrid connections *
  • Service endpoints *
  • Azure Content Delivery Network *

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

Settings that aren't swapped:

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

Note

To make these settings swappable, add the app setting WEBSITE_OVERRIDE_PRESERVE_DEFAULT_STICKY_SLOT_SETTINGS in every slot of the app and set its value to 0 or false. These settings are either all swappable or not at all. You can’t make just some settings swappable and not the others.

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.

To configure an app setting or connection string to stick to a specific slot (not swapped), go to the Configuration page for that slot. Add or edit a setting, and then select deployment slot setting. Selecting this check box tells App Service that the setting is not swappable.

Swap two slots

You can swap deployment slots on your app's Deployment slots page and the Overview page. For technical details on the slot swap, see What happens during swap.

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Important

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.

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.

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

If you have any problems, see Troubleshoot swaps.

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 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.

If you have any problems, see Troubleshoot swaps.

To automate a multi-phase swap, see Automate with PowerShell.

Roll back 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.

Configure auto swap

Note

Auto swap isn't supported in web apps on Linux.

Auto swap streamlines Azure DevOps 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

Before you configure auto swap for the production slot, consider testing auto swap on a non-production target slot.

To configure auto swap:

  1. Go to your app's resource page. Select Deployment slots > <desired source slot> > Configuration > General settings.

  2. For Auto swap enabled, select 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.

If you have any problems, see Troubleshoot swaps.

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.

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.

Note

The <applicationInitialization> configuration element is part of each app start-up, whereas the two warm-up behavior app settings apply only to slot swaps.

If you have any problems, see Troubleshoot swaps.

Monitor a swap

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.

Route traffic

By default, all client requests to the app's production URL (http://<app_name>.azurewebsites.net) are routed to the production slot. You can route a portion of the traffic to another slot. This feature is useful if you need user feedback for a new update, but you're not ready to release it to production.

Route production traffic automatically

To route production traffic automatically:

  1. Go to your app's resource page and select Deployment slots.

  2. In the Traffic % column of the slot you want to route to, specify a percentage (between 0 and 100) to represent the amount of total traffic you want to route. Select Save.

After the setting is saved, the specified percentage of clients is randomly routed to the non-production slot.

After a client is automatically routed to a specific slot, it's 'pinned' to that slot for the life of that client session. On the client browser, you can see which slot your session is pinned to by looking at the x-ms-routing-name cookie in your HTTP headers. A request that's routed to the 'staging' slot has the cookie x-ms-routing-name=staging. A request that's routed to the production slot has the cookie x-ms-routing-name=self.

Note

Next to the Azure portal, you can also use the az webapp traffic-routing set command in the Azure CLI to set the routing percentages from CI/CD tools like DevOps pipelines or other automation systems.

Route production traffic manually

In addition to automatic traffic routing, App Service can route requests to a specific slot. This is useful when you want your users to be able to opt in to or opt out of your beta app. To route production traffic manually, you use the x-ms-routing-name query parameter.

To let users opt out of your beta app, for example, you can put this link on your webpage:

The string x-ms-routing-name=self specifies the production slot. After the client browser accesses the link, it's redirected to the production slot. Every subsequent request has the x-ms-routing-name=self cookie that pins the session to the production slot.

To let users opt in to your beta app, set the same query parameter to the name of the non-production slot. Here's an example:

By default, new slots are given a routing rule of 0%, shown in grey. When you explicitly set this value to 0% (shown in black text), your users can access the staging slot manually by using the x-ms-routing-name query parameter. But they won't be routed to the slot automatically because the routing percentage is set to 0. This is an advanced scenario where you can 'hide' your staging slot from the public while allowing internal teams to test changes on the slot.

Delete a slot

Search for and select your app. Select Deployment slots > <slot to delete> > Overview. The app type is shown as App Service (Slot) to remind you that you're viewing a deployment slot. Select Delete on the command bar.

Automate with PowerShell

Note

This article has been updated to use the Azure Az PowerShell module. The Az PowerShell module isthe recommended PowerShell module for interacting with Azure. To get started with the AzPowerShell module, see Install Azure PowerShell. To learn howto migrate to the Az PowerShell module, seeMigrate Azure PowerShell from AzureRM to Az.

Azure PowerShell is a module that provides cmdlets to manage Azure through Windows PowerShell, including support for managing deployment slots in Azure App Service.

For information on installing and configuring Azure PowerShell, and on authenticating Azure PowerShell with your Azure subscription, see How to install and configure Microsoft Azure PowerShell.

Create a web app

Create a slot

Initiate a swap with a preview (multi-phase swap), and apply destination slot configuration to the source slot

Cancel a pending swap (swap with review) and restore the source slot configuration

Swap deployment slots

Monitor swap events in the activity log

Delete a slot

Automate with Resource Manager templates

Azure Resource Manager templates are declarative JSON files used to automate the deployment and configuration of Azure resources. To swap slots by using Resource Manager templates, you will set two properties on the Microsoft.Web/sites/slots and Microsoft.Web/sites resources:

  • buildVersion: this is a string property which represents the current version of the app deployed in the slot. For example: 'v1', '1.0.0.1', or '2019-09-20T11:53:25.2887393-07:00'.
  • targetBuildVersion: this is a string property that specifies what buildVersion the slot should have. If the targetBuildVersion does not equal the current buildVersion, then this will trigger the swap operation by finding the slot which has the specified buildVersion.

Example Resource Manager template

The following Resource Manager template will update the buildVersion of the staging slot and set the targetBuildVersion on the production slot. This will swap the two slots. The template assumes you already have a webapp created with a slot named 'staging'.

This Resource Manager template is idempotent, meaning that it can be executed repeatedly and produce the same state of the slots. After the first execution, targetBuildVersion will match the current buildVersion, so a swap will not be triggered.

Automate with the CLI

For Azure CLI commands for deployment slots, see az webapp deployment slot.

Troubleshoot swaps

If any error occurs during a slot swap, it's logged in D:homeLogFileseventlog.xml. It's also logged in the application-specific error log.

Here are some common swap errors:

  • An HTTP request to the application root is timed. The swap operation waits for 90 seconds for each HTTP request, and retries up to 5 times. If all retries are timed out, the swap operation is stopped.

  • Local cache initialization might fail when the app content exceeds the local disk quota specified for the local cache. For more information, see Local cache overview.

  • During custom warm-up, the HTTP requests are made internally (without going through the external URL). They can fail with certain URL rewrite rules in Web.config. For example, rules for redirecting domain names or enforcing HTTPS can prevent warm-up requests from reaching the app code. To work around this issue, modify your rewrite rules by adding the following two conditions:

  • Without a custom warm-up, the URL rewrite rules can still block HTTP requests. To work around this issue, modify your rewrite rules by adding the following condition:

  • After slot swaps, the app may experience unexpected restarts. This is because after a swap, the hostname binding configuration goes out of sync, which by itself doesn't cause restarts. However, certain underlying storage events (such as storage volume failovers) may detect these discrepancies and force all worker processes to restart. To minimize these types of restarts, set the WEBSITE_ADD_SITENAME_BINDINGS_IN_APPHOST_CONFIG=1 app setting on all slots. However, this app setting does not work with Windows Communication Foundation (WCF) apps.

Next steps

You’re browsing the documentation for v2.x and earlier. For v3.x, click here.

This page assumes you’ve already read the Components Basics. Read that first if you are new to components.

In 2.6.0, we introduced a new unified syntax (the v-slot directive) for named and scoped slots. It replaces the slot and slot-scope attributes, which are now deprecated, but have not been removed and are still documented here. The rationale for introducing the new syntax is described in this RFC.

Slot Content

Vue implements a content distribution API inspired by the Web Components spec draft, using the <slot> element to serve as distribution outlets for content.

This allows you to compose components like this:

Then in the template for <navigation-link>, you might have:

When the component renders, <slot></slot> will be replaced by “Your Profile”. Slots can contain any template code, including HTML:

Or even other components:

If <navigation-link>‘s template did not contain a <slot> element, any content provided between its opening and closing tag would be discarded.

Compilation Scope

When you want to use data inside a slot, such as in:

That slot has access to the same instance properties (i.e. the same “scope”) as the rest of the template. The slot does not have access to <navigation-link>‘s scope. For example, trying to access url would not work:

As a rule, remember that:

Everything in the parent template is compiled in parent scope; everything in the child template is compiled in the child scope.

Fallback Content

There are cases when it’s useful to specify fallback (i.e. default) content for a slot, to be rendered only when no content is provided. For example, in a <submit-button> component:

We might want the text “Submit” to be rendered inside the <button> most of the time. To make “Submit” the fallback content, we can place it in between the <slot> tags:

Now when we use <submit-button> in a parent component, providing no content for the slot:

will render the fallback content, “Submit”:

But if we provide content:

Then the provided content will be rendered instead:

Named Slots

Updated in 2.6.0+. See here for the deprecated syntax using the slot attribute.

There are times when it’s useful to have multiple slots. For example, in a <base-layout> component with the following template:

For these cases, the <slot> element has a special attribute, name, which can be used to define additional slots:

A <slot> outlet without name implicitly has the name “default”.

To provide content to named slots, we can use the v-slot directive on a <template>, providing the name of the slot as v-slot‘s argument:

Now everything inside the <template> elements will be passed to the corresponding slots. Any content not wrapped in a <template> using v-slot is assumed to be for the default slot.

However, you can still wrap default slot content in a <template> if you wish to be explicit:

Either way, the rendered HTML will be:

Note that v-slot can only be added to a <template> (with one exception), unlike the deprecated slot attribute.

Scoped Slots

Updated in 2.6.0+. See here for the deprecated syntax using the slot-scope attribute.

Sometimes, it’s useful for slot content to have access to data only available in the child component. For example, imagine a <current-user> component with the following template:

We might want to replace this fallback content to display the user’s first name, instead of last, like this:

That won’t work, however, because only the <current-user> component has access to the user and the content we’re providing is rendered in the parent.

To make user available to the slot content in the parent, we can bind user as an attribute to the <slot> element:

Attributes bound to a <slot> element are called slot props. Now, in the parent scope, we can use v-slot with a value to define a name for the slot props we’ve been provided:

In this example, we’ve chosen to name the object containing all our slot props slotProps, but you can use any name you like.

Abbreviated Syntax for Lone Default Slots

In cases like above, when only the default slot is provided content, the component’s tags can be used as the slot’s template. This allows us to use v-slot directly on the component:

This can be shortened even further. Just as non-specified content is assumed to be for the default slot, v-slot without an argument is assumed to refer to the default slot:

Note that the abbreviated syntax for default slot cannot be mixed with named slots, as it would lead to scope ambiguity:

Whenever there are multiple slots, use the full <template> based syntax for all slots:

Destructuring Slot Props

Internally, scoped slots work by wrapping your slot content in a function passed a single argument:

That means the value of v-slot can actually accept any valid JavaScript expression that can appear in the argument position of a function definition. So in supported environments (single-file components or modern browsers), you can also use ES2015 destructuring to pull out specific slot props, like so:

This can make the template much cleaner, especially when the slot provides many props. It also opens other possibilities, such as renaming props, e.g. user to person:

You can even define fallbacks, to be used in case a slot prop is undefined:

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Dynamic Slot Names

New in 2.6.0+

Dynamic directive arguments also work on v-slot, allowing the definition of dynamic slot names:

Named Slots Shorthand

New in 2.6.0+

Similar to v-on and v-bind, v-slot also has a shorthand, replacing everything before the argument (v-slot:) with the special symbol #. For example, v-slot:header can be rewritten as #header:

However, just as with other directives, the shorthand is only available when an argument is provided. That means the following syntax is invalid:

Instead, you must always specify the name of the slot if you wish to use the shorthand:

Other Examples

Slot props allow us to turn slots into reusable templates that can render different content based on input props. This is most useful when you are designing a reusable component that encapsulates data logic while allowing the consuming parent component to customize part of its layout.

For example, we are implementing a <todo-list> component that contains the layout and filtering logic for a list:

Instead of hard-coding the content for each todo, we can let the parent component take control by making every todo a slot, then binding todo as a slot prop:

Now when we use the <todo-list> component, we can optionally define an alternative <template> for todo items, but with access to data from the child:

However, even this barely scratches the surface of what scoped slots are capable of. For real-life, powerful examples of scoped slot usage, we recommend browsing libraries such as Vue Virtual Scroller, Vue Promised, and Portal Vue.

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Deprecated Syntax

The v-slot directive was introduced in Vue 2.6.0, offering an improved, alternative API to the still-supported slot and slot-scope attributes. The full rationale for introducing v-slot is described in this RFC. The slot and slot-scope attributes will continue to be supported in all future 2.x releases, but are officially deprecated and will eventually be removed in Vue 3.

Named Slots with the slot Attribute

Deprecated in 2.6.0+. See here for the new, recommended syntax.

To pass content to named slots from the parent, use the special slot attribute on <template> (using the <base-layout> component described here as example):

Or, the slot attribute can also be used directly on a normal element:

There can still be one unnamed slot, which is the default slot that serves as a catch-all for any unmatched content. In both examples above, the rendered HTML would be:

Scoped Slots with the slot-scope Attribute

Deprecated in 2.6.0+. See here for the new, recommended syntax.

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To receive props passed to a slot, the parent component can use <template> with the slot-scope attribute (using the <slot-example> described here as example):

Here, slot-scope declares the received props object as the slotProps variable, and makes it available inside the <template> scope. You can name slotProps anything you like similar to naming function arguments in JavaScript.

Here slot='default' can be omitted as it is implied:

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The slot-scope attribute can also be used directly on a non-<template> element (including components):

The value of slot-scope can accept any valid JavaScript expression that can appear in the argument position of a function definition. This means in supported environments (single-file components or modern browsers) you can also use ES2015 destructuring in the expression, like so:

Using the <todo-list> described here as an example, here’s the equivalent usage using slot-scope:

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