Using Deployment Slots

1. Start Locally

Firstly, we need to launch Locally which we can do from a terminal by running:

Once Locally has started, the Locally Dashboard will open automatically:

2. Create a Resource Group

Next we can create the Resource Group:

There's two things to note here:

1. The Azure CLI supports Automatic Configuration, meaning that it can automatically be configured to work against Locally just by prefixing commands with locally run.
2. Locally intentionally uses a different set of locations to Azure as a safety precaution, so that you can be confident you're deploying against Locally rather than regular Azure. You can also configure Locally to use the Azure locations too, but you'll want to be extra sure that you're prefixing commands with locally run when you do.

3. Create the App Service Plan and Web App

Slots need a Standard tier plan or above, so we'll use S1:

{
  "note": "some fields skipped for brevity",

  "kind": "linux",
  "location": "berlin",
  "name": "sample-slots-plan",
  "provisioningState": "Succeeded",
  "resourceGroup": "sample-deployment-slots",
  "sku": {
    "capacity": 1,
    "name": "S1",
    "size": "S1",
    "tier": "STANDARD"
  },
  "status": "Ready",
  "type": "Microsoft.Web/serverFarms"
}

Then the Web App on that plan:

{
  "note": "some fields skipped for brevity",

  "defaultHostName": "sample-slots-app2.furnace.locally:5663",
  "kind": "app,linux",
  "location": "berlin",
  "name": "sample-slots-app2",
  "resourceGroup": "sample-deployment-slots",
  "state": "Running",
  "type": "Microsoft.Web/sites"
}

The app itself is the production slot. For more on Web Apps in general, see Provisioning a Web App.

4. Add a staging slot

Now let's add a slot called staging:

{
  "note": "some fields skipped for brevity",

  "defaultHostName": "sample-slots-app2-staging.furnace.locally:5663",
  "kind": "app,linux",
  "location": "berlin",
  "name": "staging",
  "resourceGroup": "sample-deployment-slots",
  "state": "Running",
  "type": "Microsoft.Web/sites/slots"
}

The slot gets its own host name, with the slot name added to the app's, served by Locally's own DNS server.

5. Build two versions of the app

We need two builds we can tell apart. A tiny Node server that says which version it is will do. First a package.json, which tells App Service how to start it:

Then the server, listening on the port App Service passes it in PORT:

require("http")
  .createServer((req, res) => res.end("Hello from v1"))
  .listen(process.env.PORT);

Zip that up as version 1:

Then change the greeting and zip it again as version 2:

6. Deploy a version to each slot

Version 1 goes to production:

{
  "note": "some fields skipped for brevity",

  "name": "8c341cbb-21c6-4443-9504-85976b096d1b",
  "properties": {
    "errors": [],
    "numberOfInstancesFailed": 0,
    "numberOfInstancesSuccessful": 1,
    "status": "RuntimeSuccessful"
  },
  "resourceGroup": "sample-deployment-slots",
  "type": "Microsoft.Web/sites/deploymentStatus"
}

And version 2 to staging. The only difference is --slot:

Now we can request production:

Hello from v1

And staging:

Hello from v2

If you get web app is warming up; please retry shortly, the slot's container is still starting. Give it a few seconds and try again.

We can list the app's slots too:

Name     State    Host
-------  -------  ----------------------------------------------
staging  Running  sample-slots-app2-staging.furnace.locally:5663

7. Swap staging into production

Happy with version 2? Let's swap it into production:

Both slots restart, so give them up to a minute to warm up. Then production serves version 2:

Hello from v2

And staging now has version 1, ready to swap back if version 2 turns out to be a mistake:

Hello from v1

8. Tidy up

Finally, we can tidy up. To remove the Resource Group and everything within it:

That stops both slots' containers too, so nothing is left running on your machine.

Doing this with other tooling

Whilst this guide used the Azure CLI, slots work the same way through any of the tooling that Locally supports - a Microsoft.Web/sites/slots resource in HashiCorp Terraform or OpenTofu, Pulumi, Bicep or an ARM Template all provision against Locally in the same way, with only the location changed.

Next steps

The Web App Emulator shows each slot's deployments and streams its console output. For the basics of App Service on Locally, see Provisioning a Web App.

Should you encounter any issues, please take a look at the troubleshooting section.

Resource types this guide uses

These are the Azure resource types this guide creates in Locally:

• Microsoft.Web/sites/slots
• Microsoft.Web/sites
• Microsoft.Web/serverFarms