Best Practices for Writing Azure ARM Templates and Sample VM Template

Azure Resource Manager (ARM) templates provide a declarative way to define resources in Azure. They are JSON files that specify the resources you need and configure them in a consistent manner. Writing effective ARM templates can make your deployments more robust and easier to manage. In this blog post, we’ll cover some best practices for writing ARM templates, and then provide a sample template for creating a virtual machine (VM).

Best Practices for Writing ARM Templates:

1. Start with the Visualizer: Before diving into coding, use the ARM template visualizer to understand how your resources relate to one another.
2. Parameterize Inputs: Parameters allow the template to be reused across different environments and scenarios. Instead of hardcoding values, make use of parameters for things like VM sizes, names, and storage configurations.
3. Use Variables for Repeated Values: Variables can be helpful to define values that are used more than once in a template.
4. Leverage Outputs: Outputs are useful for returning values from your deployed resources, like the public IP address of a newly created VM.
5. Modularize and Link Templates: Instead of one monolithic template, consider breaking it down into smaller linked templates. This makes your templates more readable and easier to manage.
6. Follow Naming Conventions: Consistency is key. Establish and stick to a naming convention for your resources.
7. Idempotency: Ensure your templates can be re-run without error. ARM templates should be idempotent, meaning you can run them multiple times and end up with the same resource configuration.
8. Comment Liberally: Use comments in your JSON to explain complex or non-intuitive portions of your template.
9. Validate Before Deployment: Before deploying, validate your template using the Azure CLI or PowerShell to catch errors early.
10. Stay Updated: Azure constantly evolves, and new properties or changes can be introduced. Regularly review the Azure template reference documentation.

Sample ARM Template to Create a VM:

Here’s a simple ARM template to create a virtual machine. This is a basic example and can be expanded based on your needs.

{
  "$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
  "contentVersion": "1.0.0.0",
  "parameters": {
    "vmName": {
      "type": "string",
      "defaultValue": "myVM",
      "metadata": {
        "description": "Name of the virtual machine."
      }
    },
    "adminUsername": {
      "type": "string",
      "metadata": {
        "description": "Admin username for the VM."
      }
    },
    "adminPassword": {
      "type": "securestring",
      "metadata": {
        "description": "Admin password for the VM."
      }
    }
  },
  "variables": {
    "location": "[resourceGroup().location]",
    "nsgName": "[concat(parameters('vmName'), 'NSG')]",
    "vnetName": "[concat(parameters('vmName'), 'VNet')]",
    "subnetName": "default",
    "ipName": "[concat(parameters('vmName'), 'IP')]",
    "nicName": "[concat(parameters('vmName'), 'NIC')]",
    "addressPrefix": "10.0.0.0/16",
    "subnetPrefix": "10.0.0.0/24"
  },
  "resources": [
    {
      "name": "[variables('vnetName')]",
      "type": "Microsoft.Network/virtualNetworks",
      "apiVersion": "2020-06-01",
      "location": "[variables('location')]",
      "properties": {
        "addressSpace": {
          "addressPrefixes": [
            "[variables('addressPrefix')]"
          ]
        },
        "subnets": [
          {
            "name": "[variables('subnetName')]",
            "properties": {
              "addressPrefix": "[variables('subnetPrefix')]"
            }
          }
        ]
      }
    },
    {
      "name": "[variables('ipName')]",
      "type": "Microsoft.Network/publicIPAddresses",
      "apiVersion": "2020-06-01",
      "location": "[variables('location')]",
      "properties": {
        "publicIPAllocationMethod": "Dynamic"
      }
    },
    {
      "name": "[variables('nsgName')]",
      "type": "Microsoft.Network/networkSecurityGroups",
      "apiVersion": "2020-06-01",
      "location": "[variables('location')]",
      "properties": {
        "securityRules": [
          {
            "name": "allowRDP",
            "properties": {
              "protocol": "Tcp",
              "sourcePortRange": "*",
              "destinationPortRange": "3389",
              "sourceAddressPrefix": "*",
              "destinationAddressPrefix": "*",
              "access": "Allow",
              "priority": 1000,
              "direction": "Inbound"
            }
          }
        ]
      }
    },
    {
      "name": "[variables('nicName')]",
      "type": "Microsoft.Network/networkInterfaces",
      "apiVersion": "2020-06-01",
      "location": "[variables('location')]",
      "dependsOn": [
        "[resourceId('Microsoft.Network/publicIPAddresses/', variables('ipName'))]",
        "[resourceId('Microsoft.Network/virtualNetworks/', variables('vnetName'))]",
        "[resourceId('Microsoft.Network/networkSecurityGroups', variables('nsgName'))]"
      ],
      "properties": {
        "ipConfigurations": [
          {
            "name": "ipconfig1",
            "properties": {
              "privateIPAllocationMethod": "Dynamic",
              "publicIPAddress": {
                "id": "[resourceId('Microsoft.Network/publicIPAddresses', variables('ipName'))]"
              },
              "subnet": {
                "id": "[resourceId('Microsoft.Network/virtualNetworks/subnets', variables('vnetName'), variables('subnetName'))]"
              }
            }
          }
        ],
        "networkSecurityGroup": {
          "id": "[resourceId('Microsoft.Network/networkSecurityGroups', variables('nsgName'))]"
        }
      }
    },
    {
      "name": "[parameters('vmName')]",
      "type": "Microsoft.Compute/virtualMachines",
      "apiVersion": "2020-06-01",
      "location": "[variables('location')]",
      "dependsOn": [
        "[resourceId('Microsoft.Network/networkInterfaces/', variables('nicName'))]"
      ],
      "properties": {
        "hardwareProfile": {
          "vmSize": "Standard_DS1_v2"
        },
        "storageProfile": {
          "imageReference": {
            "publisher": "MicrosoftWindowsServer",
            "offer": "WindowsServer",
            "sku": "2016-Datacenter",
            "version": "latest"
          },
          "osDisk": {
            "createOption": "FromImage"
          }
        },
        "osProfile": {
          "computerName": "[parameters('vmName')]",
          "adminUsername": "[parameters('adminUsername')]",
          "adminPassword": "[parameters('adminPassword')]"
        },
        "networkProfile": {
          "networkInterfaces": [
            {
              "id": "[resourceId('Microsoft.Network/networkInterfaces', variables('nicName'))]"
            }
          ]
        }
      }
    }
  ],
  "outputs": {
    "adminUsername": {
      "type": "string",
      "value": "[parameters('adminUsername')]"
    }
  }
}

Conclusion: Writing ARM templates can initially seem daunting, but with good practices in place and leveraging the resources Azure provides, you can simplify and standardize your deployments. Whether you’re working with VMs or other Azure resources, the power of Infrastructure as Code (IaC) can be a game-changer. Happy deploying!