Understanding ARM Template Parameters and Variables | Azure ARM

Azure Resource Manager (ARM) templates provide a powerful way to deploy and manage Azure resources. When working with ARM templates, it's essential to understand the concepts of parameters and variables. Parameters allow you to customize the deployment, while variables help you define reusable values and expressions. In this tutorial, we will explore how to use parameters and variables in ARM templates to create dynamic and flexible deployments.

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1. Introduction to ARM Template Parameters and Variables

ARM template parameters and variables enable you to make your templates more flexible and adaptable. Parameters act as placeholders for values that you can specify during deployment, while variables allow you to define and reuse expressions throughout the template.

2. Using Parameters in ARM Templates

Parameters allow you to externalize input values, making your templates more versatile. Here's an example of using a parameter in an ARM template to specify the location for a virtual machine:


    {
    "$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
    "contentVersion": "1.0.0.0",
    "parameters": {
    "vmName": {
    "type": "string",
    "defaultValue": "myVM"
    },
    "location": {
    "type": "string",
    "defaultValue": "East US"
    }
    },
    "resources": [
    {
    "type": "Microsoft.Compute/virtualMachines",
    "apiVersion": "2022-01-01",
    "name": "[parameters('vmName')]",
    "location": "[parameters('location')]",
    "properties": {
    "hardwareProfile": {
    "vmSize": "Standard_D2s_v3"
    },
    "storageProfile": {
    "imageReference": {
    "publisher": "MicrosoftWindowsServer",
    "offer": "WindowsServer",
    "sku": "2019-Datacenter",
    "version": "latest"
    },
    "osDisk": {
    "createOption": "FromImage"
    }
    },
    "networkProfile": {
    "networkInterfaces": [
    {
    "id": "[resourceId('Microsoft.Network/networkInterfaces', concat(parameters('vmName'), 'Nic'))]"
    }
    ]
    }
    }
    }
    ]
    }

Steps to Use Parameters:

Define parameters in the "parameters" section of the ARM template.
During deployment, provide parameter values using the Azure Portal, PowerShell, Azure CLI, or other deployment methods.
References to parameters in the template are denoted using [parameters('parameterName')] syntax.

3. Utilizing Variables in ARM Templates

Variables allow you to define reusable expressions within your ARM templates. You can use them to simplify complex calculations, concatenate strings, or define resource names. Here's an example of using a variable to set the storage account name for a virtual machine:


    {
    "$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
    "contentVersion": "1.0.0.0",
    "parameters": {
    "vmName": {
    "type": "string",
    "defaultValue": "myVM"
    },
    "location": {
    "type": "string",
    "defaultValue": "East US"
    }
    },
    "variables": {
    "storageAccountName": "[concat(uniquestring(resourceGroup().id), 'sa')]"
    },
    "resources": [
    {
    "type": "Microsoft.Storage/storageAccounts",
    "apiVersion": "2022-01-01",
    "name": "[variables('storageAccountName')]",
    "location": "[parameters('location')]",
    "sku": {
    "name": "Standard_LRS"
    },
    "kind": "StorageV2"
    },
    {
    "type": "Microsoft.Compute/virtualMachines",
    "apiVersion": "2022-01-01",
    "name": "[parameters('vmName')]",
    "location": "[parameters('location')]",
    "properties": {
    "hardwareProfile": {
    "vmSize": "Standard_D2s_v3"
    },
    "storageProfile": {
    "imageReference": {
    "publisher": "MicrosoftWindowsServer",
    "offer": "WindowsServer",
    "sku": "2019-Datacenter",
    "version": "latest"
    },
    "osDisk": {
    "createOption": "FromImage",
    "managedDisk": {
    "storageAccountType": "Standard_LRS"
    }
    },
    "dataDisks": []
    },
    "networkProfile": {
    "networkInterfaces": [
    {
    "id": "[resourceId('Microsoft.Network/networkInterfaces', concat(parameters('vmName'), 'Nic'))]"
    }
    ]
    }
    }
    ]
    }

Steps to Use Variables:

Define variables in the "variables" section of the ARM template.
Variables support expressions and functions to generate values dynamically.
References to variables in the template are denoted using [variables('variableName')] syntax.

Common Mistakes with ARM Template Parameters and Variables

Not defining default values for parameters, leading to deployment errors when not provided during deployment.
Incorrect usage of variables, resulting in unexpected behavior or resource naming collisions.
Using invalid syntax or expressions in parameters or variables.

FAQs about ARM Template Parameters and Variables

1. Can I change parameter values after deployment?

Yes, you can update parameter values after deployment using Azure Portal, PowerShell, Azure CLI, or REST API.

2. Are variables mutable during deployment?

No, variables are immutable and cannot be changed during deployment. You need to update them in the template and redeploy.

3. Can I use the same variable name in different ARM templates?

Yes, you can use the same variable name in different templates. Variables are scoped to the template they are defined in.

4. Can I use expressions in parameter default values?

Yes, you can use expressions in parameter default values, making them dynamic based on other parameters or variables.

5. Can I use parameters to conditionally deploy resources?

Yes, you can use parameters in conditional expressions to determine whether to include or exclude resources during deployment.

Summary

ARM template parameters and variables are essential components for creating dynamic and adaptable deployments in Azure. Parameters provide flexibility, allowing users to customize the deployment, while variables enable the reuse of expressions for increased maintainability. Understanding how to use parameters and variables correctly will enhance your experience with ARM templates and streamline your resource deployment process.