Use App Service Always On

March 21, 2025 · View on GitHub

SYNOPSIS

Configure Always On for App Service apps.

Azure App Service apps are automatically unloaded when there's no traffic. Unloading apps reduces resource consumption when apps share a single App Services Plan. After an app have been unloaded, the next web request will trigger a cold start of the app. A cold start of the app can cause request timeouts.

Web apps using continuous WebJobs or WebJobs triggered with a CRON expression must use always on to start.

RECOMMENDATION

Consider enabling Always On for each App Services app.

EXAMPLES

Configure with Azure template

To deploy App Services that pass this rule:

Set the properties.siteConfig.alwaysOn property to true.

For example:

{
  "type": "Microsoft.Web/sites",
  "apiVersion": "2023-01-01",
  "name": "[parameters('name')]",
  "location": "[parameters('location')]",
  "identity": {
    "type": "SystemAssigned"
  },
  "kind": "web",
  "properties": {
    "serverFarmId": "[resourceId('Microsoft.Web/serverfarms', parameters('planName'))]",
    "httpsOnly": true,
    "siteConfig": {
      "alwaysOn": true,
      "minTlsVersion": "1.2",
      "ftpsState": "Disabled",
      "remoteDebuggingEnabled": false,
      "http20Enabled": true,
      "netFrameworkVersion": "v8.0",
      "healthCheckPath": "/healthz",
      "metadata": [
        {
          "name": "CURRENT_STACK",
          "value": "dotnet"
        }
      ]
    }
  },
  "dependsOn": [
    "[resourceId('Microsoft.Web/serverfarms', parameters('planName'))]"
  ]
}

Configure with Bicep

To deploy App Services that pass this rule:

Set the properties.siteConfig.alwaysOn property to true.

For example:

resource web 'Microsoft.Web/sites@2023-01-01' = {
  name: name
  location: location
  identity: {
    type: 'SystemAssigned'
  }
  kind: 'web'
  properties: {
    serverFarmId: plan.id
    httpsOnly: true
    siteConfig: {
      alwaysOn: true
      minTlsVersion: '1.2'
      ftpsState: 'Disabled'
      remoteDebuggingEnabled: false
      http20Enabled: true
      netFrameworkVersion: 'v8.0'
      healthCheckPath: '/healthz'
      metadata: [
        {
          name: 'CURRENT_STACK'
          value: 'dotnet'
        }
      ]
    }
  }
}

NOTES

The Always On feature of App Service is not applicable to Azure Functions and Standard Logic Apps under most circumstances. To reduce false positives, this rule ignores apps based on Azure Functions and Standard Logic Apps.

When running in a Consumption Plan or Premium Plan you should not enable Always On. On a Consumption plan the platform activates function apps automatically. On a Premium plan the platform keeps your desired number of pre-warmed instances always on automatically.