ARM Template Functions Skill

When to Use This Skill

• Working with Azure ARM templates (azuredeploy.json files)
• Encountering template validation errors related to functions
• Implementing subscription-level deployments
• Using nested deployments with cross-scope references
• Accessing resource properties in outputs sections

Critical Rules for ARM Template Functions

reference() Function

Valid Usage Locations:

• ✅ Outputs section
• ✅ Properties object of resource definitions
• ❌ Top-level resource properties (type, name, location)
• ❌ Count property in copy loops

Scope Considerations:

// ✅ CORRECT: Simple reference in same scope
"outputs": {
  "myOutput": {
    "value": "[reference('myResourceName').someProperty]"
  }
}

// ❌ INCORRECT: Cannot nest reference() inside other functions in outputs
"outputs": {
  "myOutput": {
    "value": "[listKeys(resourceId('rg', 'type', reference('dep').outputs.name.value))]"
  }
}

Nested Deployment References:

• Use reference(deploymentName).outputs.propertyName.value for outputs
• Requires expressionEvaluationOptions.scope: inner in nested deployment
• Cannot use reference() inside resourceId() or listKeys() in outputs

Conditional Deployment Warning:

• reference() evaluates even if resource is conditionally not deployed
• This can cause deployment failures
• Always ensure referenced resources exist

listKeys() Function

Requirements:

• Can only be used in outputs section or resource properties
• Requires fully qualified resource ID
• Resource must exist before evaluation
• Requires correct API version

Valid Pattern:

"outputs": {
  "storageKey": {
    "value": "[listKeys(resourceId('Microsoft.Storage/storageAccounts', parameters('name')), '2023-01-01').keys[0].value]"
  }
}

Security Warning:

• Never expose sensitive list functions (listKeys, listSecrets) in outputs
• Output values are stored in deployment history
• Anyone with read access to deployment can see outputs

resourceId() Function Scope Variations

Resource Group Scope (default):

"[resourceId('Microsoft.Storage/storageAccounts', parameters('name'))]"
// Returns: /subscriptions/{sub}/resourceGroups/{rg}/providers/Microsoft.Storage/storageAccounts/{name}

Cross-Resource Group:

"[resourceId(parameters('resourceGroupName'), 'Microsoft.Storage/storageAccounts', parameters('name'))]"

Subscription Scope:

"[subscriptionResourceId('Microsoft.EventHub/namespaces', parameters('name'))]"
// Returns: /subscriptions/{sub}/providers/Microsoft.EventHub/namespaces/{name}

⚠️ CRITICAL: Cross-Scope from Subscription Template to Resource Group Resources

When in a subscription-level template referencing resources IN a resource group:

// ❌ WRONG - Will fail with "not valid subscription identifier"
"[resourceId(parameters('resourceGroupName'), 'Microsoft.Storage/storageAccounts', parameters('name'))]"

// ✅ CORRECT - Must include subscription ID
"[resourceId(subscription().subscriptionId, parameters('resourceGroupName'), 'Microsoft.Storage/storageAccounts', parameters('name'))]"

This applies to:

• listKeys() calls
• Any resourceId() in outputs or properties
• Diagnostic settings eventHubAuthorizationRuleId

Management Group/Tenant:

"[tenantResourceId('Microsoft.Authorization/policyDefinitions', parameters('name'))]"

⚠️ Critical: In subscription-level templates, use subscriptionResourceId() NOT resourceId() for subscription-scoped resources.

Common Errors and Solutions

Error: "The template function 'reference' is not expected at this location"

Cause: Using reference() inside another function in outputs section

Solution: Store the referenced value in variables, or restructure to avoid nested reference calls:

// ❌ INCORRECT
"outputs": {
  "connectionString": {
    "value": "[listKeys(resourceId('rg', 'type', reference('deployment').outputs.name.value))]"
  }
}

// ✅ CORRECT: Use parameters or variables for the name
"outputs": {
  "connectionString": {
    "value": "[listKeys(resourceId(parameters('resourceGroupName'), 'Microsoft.EventHub/namespaces/eventhubs/authorizationRules', parameters('namespace'), parameters('hub'), parameters('ruleName')), '2022-10-01-preview').primaryConnectionString]"
  }
}

// ✅ ALTERNATIVE: Use nested deployment outputs for simple values only
"outputs": {
  "simpleName": {
    "value": "[reference('deploymentName').outputs.nameOutput.value]"
  }
}

Error: "The content for this response was already consumed"

Cause: Azure CLI bug in versions 2.74.0 and earlier with subscription-level template validation

Solution:

# Update Azure CLI
brew upgrade azure-cli  # macOS
# or
apt-get update && apt-get upgrade azure-cli  # Linux

Error: Resource not found in outputs

Cause: Incorrect scope function or missing resource group parameter

Solution: Match the scope function to deployment level:

• Resource group deployment → resourceId()
• Subscription deployment → subscriptionResourceId() or resourceId(resourceGroupName, ...)
• Management group deployment → managementGroupResourceId()

Best Practices for Nested Deployments

Subscription-Level Template with Resource Group Nested Deployment

{
  "$schema": "https://schema.management.azure.com/schemas/2018-05-01/subscriptionDeploymentTemplate.json#",
  "resources": [
    {
      "type": "Microsoft.Resources/deployments",
      "name": "nestedDeployment",
      "resourceGroup": "[parameters('resourceGroupName')]",
      "properties": {
        "mode": "Incremental",
        "expressionEvaluationOptions": {
          "scope": "inner"  // Critical for reference() in nested outputs
        },
        "template": {
          // Inner template deploys to resource group
          "outputs": {
            "resourceName": {
              "value": "[parameters('name')]"  // Simple values work
            }
          }
        }
      }
    }
  ],
  "outputs": {
    // Access nested outputs
    "name": {
      "value": "[reference('nestedDeployment').outputs.resourceName.value]"
    },

    // Use outputs in other functions - pass as parameters, not nested reference
    "connectionString": {
      "value": "[listKeys(resourceId(parameters('resourceGroupName'), 'Microsoft.Storage/storageAccounts', parameters('storageName')), '2023-01-01').keys[0].value]"
    }
  }
}

Key Pattern: Avoid Nested Function Calls

The Golden Rule: In outputs, DO NOT nest reference() inside listKeys() or resourceId().

Instead:

1. Pass resource names as parameters
2. Store complex expressions in variables (where allowed)
3. Use nested deployment outputs only for simple string/number values
4. Build resource IDs with parameters, not with reference() results

Validation Commands

# Validate subscription-level template
az deployment sub validate \
  --location eastus \
  --template-file azuredeploy.json \
  --parameters @parameters.json

# Validate resource group template
az deployment group validate \
  --resource-group myResourceGroup \
  --template-file azuredeploy.json \
  --parameters @parameters.json

# What-if preview (subscription level)
az deployment sub what-if \
  --location eastus \
  --template-file azuredeploy.json \
  --parameters @parameters.json

Additional Resources

• ARM Template Functions Reference
• Resource Functions
• Deployment Scopes