Using Variables and Expressions in ARM Templates | Azure ARM Tutorial

Welcome to the tutorial on using variables and expressions in ARM templates for Azure Resource Manager. ARM templates provide a declarative way to define and deploy infrastructure and applications in Azure. Variables and expressions allow you to store and manipulate values, making your templates more dynamic and reusable.

1. Defining Variables

Variables in ARM templates are used to store and reference values throughout the template. They can be used for various purposes such as storing resource names, connection strings, or configuration settings.


"variables": {
  "storageAccountName": "mystorageaccount",
  "location": "[resourceGroup().location]"
}

2. Using Expressions

Expressions in ARM templates allow you to dynamically evaluate and calculate values based on the context of the deployment. Expressions are enclosed within square brackets and can reference variables, functions, and other template properties.


"resources": [
  {
    "type": "Microsoft.Storage/storageAccounts",
    "name": "[variables('storageAccountName')]",
    "apiVersion": "2021-04-01",
    "location": "[variables('location')]",
    ...
  }
]

3. Steps to Use Variables and Expressions

Follow these steps to utilize variables and expressions in your ARM templates:

Step 1: Define Variables

Start by defining the variables section within your ARM template. Specify each variable and its corresponding value.


"variables": {
  "myVariable": "myValue",
  ...
}

Step 2: Reference Variables

To reference a variable, enclose its name within single quotes and use the syntax variables('variableName').


"resources": [
  {
    "type": "Microsoft.Compute/virtualMachines",
    "name": "myVM",
    "apiVersion": "2021-03-01",
    "location": "[resourceGroup().location]",
    ...
    "tags": {
      "myTag": "[variables('myVariable')]"
    },
    ...
  }
]

Step 3: Use Expressions

When using expressions, enclose them within square brackets []. Expressions can reference variables, functions, and template properties.


"resources": [
  {
    "type": "Microsoft.Network/virtualNetworks",
    "name": "[concat('myVnet-', uniqueString(resourceGroup().id))]",
    "apiVersion": "2021-02-01",
    "location": "[variables('location')]",
    ...
  }
]

Mistakes to Avoid

Forgetting to enclose variables within variables('variableName') when referencing them.
Using invalid syntax for expressions, such as mismatched brackets or incorrect function names.
Not properly scoping variables within the template.

Frequently Asked Questions (FAQs)

Q: Can I modify the value of a variable during deployment?
A: No, variables are meant to be read-only during deployment. If you need to modify values, consider using parameters instead.
Q: Can I use expressions to conditionally create resources?
A: Yes, expressions can be used to conditionally create resources based on certain criteria. You can use functions like if() or equals() to define conditional logic within your templates.
Q: Can I use variables in output statements?
A: Yes, variables can be referenced in output statements to display or pass values from the deployed resources.
Q: Are there any limitations on using expressions in ARM templates?
A: Yes, there are certain limitations on the usage of expressions in ARM templates, such as limited nesting levels and available functions. It's recommended to refer to the official Azure documentation for detailed information.
Q: Can I use expressions to perform mathematical calculations?
A: Yes, expressions support basic mathematical operations like addition, subtraction, multiplication, and division. You can also use functions like add(), sub(), mul(), and div() for more complex calculations.

Summary

In this tutorial, you learned how to use variables and expressions in ARM templates for Azure Resource Manager. Variables allow you to store values for reuse, while expressions enable dynamic evaluation and calculation of values during deployment. By leveraging variables and expressions effectively, you can create more flexible and reusable ARM templates for your Azure deployments.