Deconstructing JSON: Counting On


So here I am, in JSON-land, working on my deployment for VMs. But so far, I’ve only deployed one VM in my network. But one VM is boring. I want to be able to rack and stack more than one machine at a time! 🙂  Enter the copy object and the copyIndex function!  You can read the official documentation about it, but let see how this works within my existing template. Things to note:

  • You can only reference the copyIndex function in RESOURCES. Don’t try to build variables with them, you’ll just be sad.
  • You can’t use a copy loop in a nested resource (this blog series hasn’t encountered this yet).
  • You can’t use them to loop items that are defined as properties within a resource, like multiple data disks.

So in my example, let’s extend the template to deploy 3 VMs in the same subnet and put them all in the same NSG (network security group). First, you must declare in the parameters that you’d like count something:

   "count": { 
      "type": "int", 
      "defaultValue": 3 
    },

Now, whenever you want to reference the “current iteration” of the loop elsewhere in your template, you’ll use copyIndex(). Another thing to note – computers like to count starting at zero, humans like to count starting at 1. If we stick with copyIndex(), we’ll get VMs numbered 0, 1, 2. If you’d like them to be numbered 1, 2, 3 use copyIndex(1), which will offset the numbering by 1.

Since this template is going to create 3 VMs, we will need to make sure to account for other resources that VMs have a one to one relationship with and be sure to loop those as well. The only items that we are required to loop are the network interfaces and the public IP addresses (which have individual DNS names).  We don’t have to loop things like the storage account (which can be used by multiple VMs) or the network, since we only need one in this case.

In the resource for the publicIPAddresses, you’ll see the reference to the copyIndex() in the name, and in the DNS domain label. In each case, the incremented number will be tacked on at the end of each. Then finally, you’ll see the specific reference to looping this resource added, with the name of “pubipcopy”, pulling the count from the parameter you set early.

     {
      "type": "Microsoft.Network/publicIPAddresses",
      "apiVersion": "2015-06-15",
      "name": "[concat(parameters('publicIPAddressName'), parameters('dnsNameForPublicIP'),copyIndex())]", 
      "location": "[parameters('location')]",
      "properties": {
        "publicIPAllocationMethod": "[variables('publicIPAddressType')]",
        "idleTimeoutInMinutes": 4,
        "dnsSettings": {
          "domainNameLabel": "[concat(parameters('dnsNameForPublicIP'),copyIndex())]"
        }
      },
       "copy": {
        "name": "pubipcopy",
        "count": "[parameters('count')]"
      }
     },

One of the reasons you need to name each resource copy object is so they can be used in places where dependencies are required.

For the network interface, you’ll see I’ve made similar references to the copyIndex() in the name. Also a network interface is dependent on the existence of a public IP address, so you’ll see “pubipcopy” referenced under “dependsOn” as well as in the IP configuration properties.

   {
      "apiVersion": "2015-06-15",
      "type": "Microsoft.Network/networkInterfaces",
      "name": "[concat(parameters('nicName'), copyIndex())]",
      "location": "[parameters('location')]",
    
      "dependsOn": [
        "pubipcopy",
        "[concat('Microsoft.Network/virtualNetworks/', parameters('vnetName'))]",
        "[resourceId('Microsoft.Network/networkSecurityGroups', parameters('networkSecurityGroupName'))]"
      ],
      "properties": {
        "ipConfigurations": [
          {
            "name": "ipconfig1",
            "properties": {
             "privateIPAllocationMethod": "Dynamic",
             "publicIPAddress": {
                "id": "[resourceId('Microsoft.Network/publicIPAddresses', concat(parameters('publicIPAddressName'),parameters('dnsNameForPublicIP'),copyIndex()))]"
              },
              "subnet": {
                "id": "[variables('subnetRef')]"
              }
              }
          }
        ],
       "dnsSettings": {
         "dnsServers": []
       },
       "enableIPForwarding": false,
       "networkSecurityGroup": {
               "id": "[resourceId('Microsoft.Network/networkSecurityGroups', parameters('networkSecurityGroupName'))]"
            }
      },
       "copy": {
        "name": "niccopy",
        "count": "[parameters('count')]"
      }
    },

This all comes together when you get to the VM resource where it refers to the NIC in the network profile and using the copyIndex() when creating the VM name, computer name and OS disk.

    {
      "apiVersion": "2015-06-15",
      "type": "Microsoft.Compute/virtualMachines",
      "name": "[concat('NanoVM-', copyIndex())]", 
      "location": "[parameters('location')]",
      "copy": {
        "name": "vmcopy",
        "count": "[parameters('count')]"
        },
      "dependsOn": [
        
        "[concat('Microsoft.Storage/storageAccounts/', parameters('newStorageAccountName'))]",
        "niccopy"
      ],
      "properties": {
        "hardwareProfile": {
          "vmSize": "[parameters('vmSize')]"
        },
        "osProfile": {
          "computerName": "[concat('NanoVM-', copyIndex())]",
          "adminUsername": "[parameters('adminUsername')]",
          "adminPassword": "[parameters('adminPassword')]"
        },
        "storageProfile": {
          "imageReference": {
            "publisher": "[variables('imagePublisher')]",
            "offer": "[variables('imageOffer')]",
            "sku": "[parameters('windowsOSVersion')]",
            "version": "latest"
          },
          "osDisk": {
            "name": "[concat('osdisk_', concat('NanoVM-', copyIndex()))]" ,
            "vhd": {
              "uri": "[concat('http://',parameters('newStorageAccountName'),'.blob.core.windows.net/',variables('vmStorageAccountContainerName'),'/NanoVM-',copyIndex(),uniqueString(resourceGroup().id),'-osdisk.vhd')]"
            },
            "caching": "ReadWrite",
            "createOption": "FromImage"
          }
          },
            
          "networkProfile": {
          "networkInterfaces": [             
        {
              "id": "[resourceId('Microsoft.Network/networkInterfaces',concat(parameters('nicName'), copyIndex()))]"
            }
          ]
        },
          "diagnosticsProfile": {
          "bootDiagnostics": {
             "enabled": "true",
             "storageUri": "[concat('https://',parameters('newStorageAccountName'),'.blob.core.windows.net')]"
          }
        }
        }

      }

You can find my full template here: https://github.com/techbunny/Templates/tree/master/multiple_nano_server

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