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Implement a property transformer and collector in an Azure Resource Manager template

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In article Use objects as parameters in a copy loop in an Azure Resource Manager template you can see how to store resource property values in an object and how to apply them to a resource during deployment. This is a very useful way to manage your parameters, but it requires that you map the properties of the object to resource properties each time you use the object in your template.

To work around this, you can implement a property transform and collector template that iterates your object array and transforms it into the JSON schema for the resource.

Important

This approach requires that you have a deep understanding of Resource Manager templates and functions.

Let's look at an example that implements a property collector and transformer to deploy a network security group. The diagram below shows how our templates are related to the resources in those templates:

property collector and transformer architecture

Our calling template includes two resources:

  • A template link that invokes our collector template
  • The network security group resource to deploy

Our collector template includes two resources:

  • An anchor resource
  • A template link that invokes the transform template in a copy loop

Our transform template includes a single resource: an empty template with a variable that transforms our source JSON to the JSON schema that's expected by our network security group resource in the main template.

Parameter object

We use our securityRules parameter object from Use objects as parameters in a copy loop in an Azure Resource Manager template. Our transform template transforms each object in the securityRules array into the JSON schema that's expected by the network security group resource in our calling template.

{
    "$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentParameters.json#",
    "contentVersion": "1.0.0.0",
    "parameters": {
        "networkSecurityGroupsSettings": {
            "value": {
                "securityRules": [
                    {
                        "name": "RDPAllow",
                        "description": "allow RDP connections",
                        "direction": "Inbound",
                        "priority": 100,
                        "sourceAddressPrefix": "*",
                        "destinationAddressPrefix": "10.0.0.0/24",
                        "sourcePortRange": "*",
                        "destinationPortRange": "3389",
                        "access": "Allow",
                        "protocol": "Tcp"
                    },
                    {
                        "name": "HTTPAllow",
                        "description": "allow HTTP connections",
                        "direction": "Inbound",
                        "priority": 200,
                        "sourceAddressPrefix": "*",
                        "destinationAddressPrefix": "10.0.1.0/24",
                        "sourcePortRange": "*",
                        "destinationPortRange": "80",
                        "access": "Allow",
                        "protocol": "Tcp"
                    }
                ]
            }
        }
    }
}

Let's look at our transform template first.

Transform template

Our transform template includes two parameters that are passed from the collector template:

  • source is an object that receives one of the property value objects from the property array. In our example, each object from the securityRules array is passed one at a time.
  • state is an array that receives the concatenated results of all the previous transforms. This is the collection of transformed JSON.

Our parameters look like this:

{
    "$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
    "contentVersion": "1.0.0.0",
    "parameters": {
        "source": {
            "type": "object"
        },
        "state": {
            "type": "array",
            "defaultValue": []
        }
    },

Our template also defines a variable named instance that transforms our source object into the required JSON schema:

"variables": {
    "instance": [
        {
            "name": "[parameters('source').name]",
            "properties": {
                "description": "[parameters('source').description]",
                "protocol": "[parameters('source').protocol]",
                "sourcePortRange": "[parameters('source').sourcePortRange]",
                "destinationPortRange": "[parameters('source').destinationPortRange]",
                "sourceAddressPrefix": "[parameters('source').sourceAddressPrefix]",
                "destinationAddressPrefix": "[parameters('source').destinationAddressPrefix]",
                "access": "[parameters('source').access]",
                "priority": "[parameters('source').priority]",
                "direction": "[parameters('source').direction]"
            }
        }
    ]
}

Finally, the output of our template concatenates the collected transforms of our state parameter with the current transform that's performed by our instance variable:

"resources": [],
"outputs": {
    "collection": {
        "type": "array",
        "value": "[concat(parameters('state'), variables('instance'))]"
    }
}

Next, let's take a look at our collector template to see how it passes our parameter values.

Collector template

Our collector template includes three parameters:

  • source is our complete parameter object array. It's passed by the calling template. It has the same name as the source parameter in our transform template, but there's one key difference: although it's the complete array, we only pass one array element at a time to the transform template.
  • transformTemplateUri is the URI of our transform template. We define it as a parameter for template reusability.
  • state is an initially empty array that we pass to our transform template. It stores the collection of transformed parameter objects after the copy loop finishes.

Our parameters look like this:

"parameters": {
    "source": {
        "type": "array"
    },
    "transformTemplateUri": {
        "type": "string"
    },
    "state": {
        "type": "array",
        "defaultValue": []
    }
}

Next, we define a variable named count. Its value is the length of the source parameter object array:

"variables": {
    "count": "[length(parameters('source'))]"
}

We use it for the number of iterations in our copy loop.

Now let's take a look at our resources. We define two resources:

  • loop-0 is the zero-based resource for our copy loop.
  • loop- is concatenated with the result of the copyIndex(1) function to generate a unique iteration-based name for our resource, starting with 1.

Our resources look like this:

"resources": [
    {
        "type": "Microsoft.Resources/deployments",
        "apiVersion": "2015-01-01",
        "name": "loop-0",
        "properties": {
            "mode": "Incremental",
            "parameters": { },
            "template": {
                "$schema": "https://schema.management.azure.com/schemas/2015-01-01/deploymentTemplate.json#",
                "contentVersion": "1.0.0.0",
                "parameters": { },
                "variables": { },
                "resources": [ ],
                "outputs": {
                    "collection": {
                        "type": "array",
                        "value": "[parameters('state')]"
                    }
                }
            }
        }
    },
    {
        "type": "Microsoft.Resources/deployments",
        "apiVersion": "2015-01-01",
        "name": "[concat('loop-', copyindex(1))]",
        "copy": {
            "name": "iterator",
            "count": "[variables('count')]",
            "mode": "serial"
        },
        "dependsOn": [
            "loop-0"
        ],
        "properties": {
            "mode": "Incremental",
            "templateLink": { "uri": "[parameters('transformTemplateUri')]" },
            "parameters": {
                "source": { "value": "[parameters('source')[copyindex()]]" },
                "state": { "value": "[reference(concat('loop-', copyindex())).outputs.collection.value]" }
            }
        }
    }
]

Let's take a closer look at the parameters that we pass to our transform template in the nested template. Recall from earlier that our source parameter passes the current object in the source parameter object array. The state parameter is where the collection happens, because it takes the output of the previous iteration of our copy loop and passes it to the current iteration. Notice that the reference() function uses the copyIndex() function with no parameter to reference the name of our previous linked template object.

Finally, the output of our template returns the output of the last iteration of our transform template:

"outputs": {
    "result": {
        "type": "array",
        "value": "[reference(concat('loop-', variables('count'))).outputs.collection.value]"
    }
}

It may seem counterintuitive to return the output of the last iteration of our transform template to our calling template, because it appears that we stored it in our source parameter. However, it's the last iteration of our transform template that holds the complete array of transformed property objects, and that's what we want to return.

Finally, let's take a look at how to call the collector template from our calling template.

Calling template

Our calling template defines a single parameter named networkSecurityGroupsSettings:

...
"parameters": {
    "networkSecurityGroupsSettings": {
        "type": "object"
    }
}

Next, our template defines a single variable named collectorTemplateUri:

"variables": {
    "collectorTemplateUri": "[uri(deployment().properties.templateLink.uri, 'collector.template.json')]"
}

This is the URI for the collector template that's used by our linked template resource:

{
    "apiVersion": "2020-06-01",
    "name": "collector",
    "type": "Microsoft.Resources/deployments",
    "properties": {
        "mode": "Incremental",
        "templateLink": {
            "uri": "[variables('collectorTemplateUri')]",
            "contentVersion": "1.0.0.0"
        },
        "parameters": {
            "source": {
                "value": "[parameters('networkSecurityGroupsSettings').securityRules]"
            },
            "transformTemplateUri": {
                "value": "[uri(deployment().properties.templateLink.uri, 'transform.json')]"
            }
        }
    }
}

We pass two parameters to the collector template:

  • source is our property object array. In our example, it's our networkSecurityGroupsSettings parameter.
  • transformTemplateUri is the variable that we just defined with the URI of our collector template.

Finally, our Microsoft.Network/networkSecurityGroups resource directly assigns the output of the collector linked template resource to its securityRules property:

"resources": [
    {
        "apiVersion": "2020-05-01",
        "type": "Microsoft.Network/networkSecurityGroups",
        "name": "networkSecurityGroup1",
        "location": "[resourceGroup().location]",
        "properties": {
            "securityRules": "[reference('collector').outputs.result.value]"
        }
    }
],
"outputs": {
    "instance": {
        "type": "array",
        "value": "[reference('collector').outputs.result.value]"
    }
}

Try the template

An example template is available on GitHub. To deploy the template, clone the repo and run the following Azure CLI commands:

git clone https://github.com/mspnp/template-examples.git
cd template-examples/example4-collector
az group create --location <location> --name <resource-group-name>
az deployment group create -g <resource-group-name> \
    --template-uri https://raw.githubusercontent.com/mspnp/template-examples/master/example4-collector/deploy.json \
    --parameters deploy.parameters.json

Next steps