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App Service Deployment Slots

Zero-downtime deployments using staging slots.

Basic Staging Slot

resource stagingSlot 'Microsoft.Web/sites/slots@2022-09-01' = {
  parent: webApp
  name: 'staging'
  location: location
  properties: {
    serverFarmId: appServicePlan.id
  }
}

Slot Requirements — App Service

SKU Tier Slots Supported
Free/Shared 0
Basic 0
Standard 5
Premium 20

Slot Requirements — Azure Functions

⚠️ Slot support for Azure Functions varies by OS and hosting plan.

Hosting Plan OS Slots Supported
Flex Consumption (FC1) Linux ❌ 0
Consumption (Y1) Windows ✅ 1 staging slot
Consumption (Y1) Linux ❌ 0
Elastic Premium (EP1-EP3) Windows or Linux ✅ 20 slots
Dedicated (Standard+) Windows or Linux ✅ 5–20 slots

💡 For Azure Functions requiring deployment slots:

  • Windows Consumption (Y1) supports 1 staging slot — this is a supported platform capability. If you need it, use it. See the Bicep example below.
  • Recommendation for new projects: prefer Elastic Premium (EP1+) (no cold starts, VNet integration) or a Dedicated plan (Standard+). Y1 cold starts can affect slot swap warm-up reliability.
  • Linux Consumption and Flex Consumption do not support deployment slots.

Windows Consumption Function App with Staging Slot (Bicep)

resource functionAppPlan 'Microsoft.Web/serverfarms@2022-09-01' = {
  name: '${resourcePrefix}-funcplan-${uniqueHash}'
  location: location
  sku: { name: 'Y1', tier: 'Dynamic' }
  // No 'reserved: true' — Windows Consumption
}

resource functionApp 'Microsoft.Web/sites@2022-09-01' = {
  name: '${resourcePrefix}-${serviceName}-${uniqueHash}'
  location: location
  kind: 'functionapp'  // Windows (no 'linux' suffix)
  identity: { type: 'SystemAssigned' }
  properties: {
    serverFarmId: functionAppPlan.id
    httpsOnly: true
    siteConfig: {
      appSettings: [
        { name: 'WEBSITE_NODE_DEFAULT_VERSION', value: '~20' }
        { name: 'FUNCTIONS_EXTENSION_VERSION', value: '~4' }
        { name: 'FUNCTIONS_WORKER_RUNTIME', value: 'node' }
        { name: 'WEBSITE_CONTENTAZUREFILECONNECTIONSTRING', value: 'DefaultEndpointsProtocol=https;AccountName=${storageAccount.name};AccountKey=${storageAccount.listKeys().keys[0].value}' }
        { name: 'WEBSITE_CONTENTSHARE', value: '${toLower(serviceName)}-prod' }
        { name: 'AzureWebJobsStorage', value: 'DefaultEndpointsProtocol=https;AccountName=${storageAccount.name};AccountKey=${storageAccount.listKeys().keys[0].value}' }
        { name: 'APPLICATIONINSIGHTS_CONNECTION_STRING', value: applicationInsights.properties.ConnectionString }
      ]
    }
  }
}

// Staging slot — only 1 staging slot supported on Consumption
resource stagingSlot 'Microsoft.Web/sites/slots@2022-09-01' = {
  parent: functionApp
  name: 'staging'
  location: location
  kind: 'functionapp'
  properties: {
    serverFarmId: functionAppPlan.id
    siteConfig: {
      appSettings: [
        { name: 'WEBSITE_NODE_DEFAULT_VERSION', value: '~20' }
        { name: 'FUNCTIONS_EXTENSION_VERSION', value: '~4' }
        { name: 'FUNCTIONS_WORKER_RUNTIME', value: 'node' }
        { name: 'WEBSITE_CONTENTAZUREFILECONNECTIONSTRING', value: 'DefaultEndpointsProtocol=https;AccountName=${storageAccount.name};AccountKey=${storageAccount.listKeys().keys[0].value}' }
        { name: 'WEBSITE_CONTENTSHARE', value: '${toLower(serviceName)}-staging' }  // MUST differ from production
        { name: 'AzureWebJobsStorage', value: 'DefaultEndpointsProtocol=https;AccountName=${storageAccount.name};AccountKey=${storageAccount.listKeys().keys[0].value}' }
        { name: 'APPLICATIONINSIGHTS_CONNECTION_STRING', value: applicationInsights.properties.ConnectionString }
      ]
    }
  }
}

⚠️ WEBSITE_CONTENTSHARE must be unique per slot on Windows Consumption — each slot needs its own file share. Use slot-sticky settings (via slotConfigNames) for WEBSITE_CONTENTSHARE and WEBSITE_CONTENTAZUREFILECONNECTIONSTRING so these values do not swap with production.

Deployment Flow

  1. Deploy to staging slot
  2. Warm up and test staging
  3. Swap staging with production
  4. Rollback by swapping again if needed

Slot Settings

Configure settings that should not swap:

resource slotConfigNames 'Microsoft.Web/sites/config@2022-09-01' = {
  parent: webApp
  name: 'slotConfigNames'
  properties: {
    appSettingNames: [
      'APPLICATIONINSIGHTS_CONNECTION_STRING'
    ]
  }
}