High density hosting on Azure App Service

When using App Service, your application is decoupled from the capacity allocated to it by two concepts:

  • The Application: Represents the app and its runtime configuration. For example, it includes the version of .NET that the runtime should load, the app settings.
  • The App Service Plan: Defines the characteristics of the capacity, available feature set, and locality of the application. For example, characteristics might be large (four cores) machine, four instances, Premium features in East US.

An app is always linked to an App Service plan, but an App Service plan can provide capacity to one or more apps.

As a result, the platform provides the flexibility to isolate a single app or have multiple apps share resources by sharing an App Service plan.

However, when multiple apps share an App Service plan, an instance of that app runs on every instance of that App Service plan.

Per app scaling

Per app scaling is a feature that can be enabled at the App Service plan level and then used per application.

Per app scaling scales 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 Premium SKU App Service plans

Per app scaling using PowerShell

You can create a plan configured as a Per app scaling plan by passing in the -perSiteScaling $true attribute to the New-AzureRmAppServicePlan commandlet

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

If you want to update an existing App Service plan to use this feature:

  • get the target plan Get-AzureRmAppServicePlan
  • modifying the property locally $newASP.PerSiteScaling = $true
  • posting your changes back to azure Set-AzureRmAppServicePlan
# Get the new App Service Plan and modify the "PerSiteScaling" property.
$newASP = Get-AzureRmAppServicePlan -ResourceGroupName $ResourceGroup -Name $AppServicePlan
$newASP

#Modify the local copy to use "PerSiteScaling" property.
$newASP.PerSiteScaling = $true
$newASP

#Post updated app service plan back to azure
Set-AzureRmAppServicePlan $newASP

At the app level, we need to 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-AzureRmWebApp -ResourceGroupName $ResourceGroup -Name $webapp

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

# Post updated app back to azure
Set-AzureRmWebApp $newapp
Important

$newapp.SiteConfig.NumberOfWorkers is different form $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": "http://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 pools of capacity.

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

  1. Configure the App Service Environment and choose a worker pool that is dedicated to the high density hosting scenario.
  2. Create a single App Service plan, and scale it to use all the available capacity on the worker pool.
  3. Set the PerSiteScaling flag to true on the App Service plan.
  4. 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 on this worker pool.
  5. 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