High-density hosting on Azure App Service using per-app scaling

Note

This article has been updated to use the new Azure PowerShell Az module. You can still use the AzureRM module, which will continue to receive bug fixes until at least December 2020. To learn more about the new Az module and AzureRM compatibility, see Introducing the new Azure PowerShell Az module. For Az module installation instructions, see Install Azure PowerShell.

When using App Service, you can scale your apps by scaling the App Service plan they run on. When multiple apps are run in the same App Service plan, each scaled-out instance runs all the apps in the plan.

Per-app scaling can be enabled at the App Service plan level to allow for scaling an app independently from the App Service plan that hosts it. This way, an App Service plan can be scaled to 10 instances, but an app can be set to use only five.

Note

Per-app scaling is available only for Standard, Premium, Premium V2 and Isolated pricing tiers.

Apps are allocated to available App Service plan using a best effort approach for an even distribution across instances. While an even distribution is not guaranteed, the platform will make sure that two instances of the same app will not be hosted on the same App Service plan instance.

The platform does not rely on metrics to decide on worker allocation. Applications are rebalanced only when instances are added or removed from the App Service plan.

Per app scaling using PowerShell

Create a plan with per-app scaling by passing in the -PerSiteScaling $true parameter to the New-AzAppServicePlan cmdlet.

New-AzAppServicePlan -ResourceGroupName $ResourceGroup -Name $AppServicePlan `
                            -Location $Location `
                            -Tier Premium -WorkerSize Small `
                            -NumberofWorkers 5 -PerSiteScaling $true

Enable per-app scaling with an existing App Service Plan by passing in the -PerSiteScaling $true parameter to the Set-AzAppServicePlan cmdlet.

# Enable per-app scaling for the App Service Plan using the "PerSiteScaling" parameter.
Set-AzAppServicePlan -ResourceGroupName $ResourceGroup `
   -Name $AppServicePlan -PerSiteScaling $true

At the app level, configure the number of instances the app can use in the App Service plan.

In the example below, the app is limited to two instances regardless of how many instances the underlying app service plan scales out to.

# Get the app we want to configure to use "PerSiteScaling"
$newapp = Get-AzWebApp -ResourceGroupName $ResourceGroup -Name $webapp

# Modify the NumberOfWorkers setting to the desired value.
$newapp.SiteConfig.NumberOfWorkers = 2

# Post updated app back to azure
Set-AzWebApp $newapp

Important

$newapp.SiteConfig.NumberOfWorkers is different from $newapp.MaxNumberOfWorkers. Per-app scaling uses $newapp.SiteConfig.NumberOfWorkers to determine the scale characteristics of the app.

Per-app scaling using Azure Resource Manager

The following Azure Resource Manager template creates:

  • An App Service plan that's scaled out to 10 instances
  • an app that's configured to scale to a max of five instances.

The App Service plan is setting the PerSiteScaling property to true "perSiteScaling": true. The app is setting the number of workers to use to 5 "properties": { "numberOfWorkers": "5" }.

{
    "$schema": "https://schema.management.azure.com/schemas/2015-01-01/deploymentTemplate.json#",
    "contentVersion": "1.0.0.0",
    "parameters":{
        "appServicePlanName": { "type": "string" },
        "appName": { "type": "string" }
        },
    "resources": [
    {
        "comments": "App Service Plan with per site perSiteScaling = true",
        "type": "Microsoft.Web/serverFarms",
        "sku": {
            "name": "P1",
            "tier": "Premium",
            "size": "P1",
            "family": "P",
            "capacity": 10
            },
        "name": "[parameters('appServicePlanName')]",
        "apiVersion": "2015-08-01",
        "location": "West US",
        "properties": {
            "name": "[parameters('appServicePlanName')]",
            "perSiteScaling": true
        }
    },
    {
        "type": "Microsoft.Web/sites",
        "name": "[parameters('appName')]",
        "apiVersion": "2015-08-01-preview",
        "location": "West US",
        "dependsOn": [ "[resourceId('Microsoft.Web/serverFarms', parameters('appServicePlanName'))]" ],
        "properties": { "serverFarmId": "[resourceId('Microsoft.Web/serverFarms', parameters('appServicePlanName'))]" },
        "resources": [ {
                "comments": "",
                "type": "config",
                "name": "web",
                "apiVersion": "2015-08-01",
                "location": "West US",
                "dependsOn": [ "[resourceId('Microsoft.Web/Sites', parameters('appName'))]" ],
                "properties": { "numberOfWorkers": "5" }
            } ]
        }]
}

Per app scaling is a feature that is enabled in both global Azure regions and App Service Environments. However, the recommended strategy is to use App Service Environments to take advantage of their advanced features and the larger App Service plan capacity.

Follow these steps to configure high-density hosting for your apps:

  1. Designate an App Service plan as the high-density plan and scale it out to the desired capacity.
  2. Set the PerSiteScaling flag to true on the App Service plan.
  3. New apps are created and assigned to that App Service plan with the numberOfWorkers property set to 1.
    • Using this configuration yields the highest density possible.
  4. The number of workers can be configured independently per app to grant additional resources as needed. For example:
    • A high-use app can set numberOfWorkers to 3 to have more processing capacity for that app.
    • Low-use apps would set numberOfWorkers to 1.

Next steps