ARM Template – reference existing resources

Despite being a rather organized person in my professional life, I begin learning new topics and technologies in quite the opposite manner.

As I begin to dive into Azure automation, today I wanted to understand the proper syntax to deploy small resources with an ARM template while referencing existing resources, rather than what is declared and built within the JSON file itself.

This Azure Quickstart template was very valuable as I spent some time exploring this.

The basic idea:

Virtual Network and Subnet already exist
Add Network Interface through ARM template

Once I wrapped my head around the proper way to input the names of existing resource group, virtual network, and subnet, it all kind of clicked for me.

Resource Group is defined during the PowerShell command that calls the JSON file (I’ll show this at the bottom of this post). Parameters are built to accept simple text strings of the virtual network name and subnet name that I’m targeting. Then I used these parameters to build a variable for the subnet, and used that variable in the resource declaration for the network interface.

Here’s the JSON:

{
    "$schema": "https://schema.management.azure.com/schemas/2015-01-01/deploymentTemplate.json#",
    "contentVersion": "1.0.0.0",
    "parameters": {
        "virtualNetworkName": {
            "type": "string",
            "metadata": {
                "description": "Type existing virtual network name"
            }
        }
        ,"subnetName": {
            "type": "string",
            "metadata": {
                "description": "Type existing Subnet name"
            }
        }
    },
    "variables": {
        ,"subnetRef": "[resourceID('Microsoft.Network/virtualNetworks/subnets', parameters('virtualNetworkName'), parameters('subnetName'))]"
    },
    "resources": [
        {
            "apiVersion": "2015-06-15",
            "type": "Microsoft.Network/networkInterfaces",
            "name": "WindowsVM1-NetworkInterface",
            "location": "[resourceGroup().location]",
            "tags": {
                "displayName": "WindowsVM1 Network Interface"
            },
            "properties": {
                "ipConfigurations": [
                    {
                        "name": "ipconfig1",
                        "properties": {
                            "privateIPAllocationMethod": "Dynamic",
                            "subnet": {
                                "id": "[variables('subnetRef')]"
                            }
                        }
                    }
                ]
            }
        }
 
    ],
    "outputs": {}
}

To deploy this, I make a connection to Azure PowerShell, and ran:

New-AzureRmResourceGroupDeployment -ResourceGroupName "Group1" -TemplateFile "C:\Azure\Templates\NewSubnet.json"

This prompts me for my two parameters, which I could set defaults for in the JSON or provide a parameter file and use that in the deployment command as well.

Next up – deploy a full VM with managed disks attaching to existing resources, and then working into Desired State Config.

ARM Template – reference existing resources

Jeff Miles

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