Exploring ARM Template components

Exploring ARM Template components

An Azure Resource Manager (ARM) template is a JSON document used to deploy, configure, and manage Azure resources in a consistent and repeatable manner. ARM templates define the infrastructure and configuration for Azure resources and their dependencies in a declarative format. Below is a detailed exploration of the key components of an ARM template: JSON document, Parameters, Variables, Functions, Resources, and Outputs.

1. JSON Document

An ARM template is a JSON document that defines the infrastructure and configuration for the resources to be deployed in Azure. The structure of an ARM template follows a strict schema that allows it to be parsed and validated by Azure.

The typical structure of an ARM template includes the following key sections:

1. $schema: A reference to the ARM template schema used for validation.

2. contentVersion: Version of the template.

3. resources: Defines the resources to be deployed.

4. parameters: Defines the parameters that can be provided during template deployment.

5. variables: Defines values that can be reused within the template.

6. functions: Defines custom functions that can be used in the template.

2. Parameters

Parameters are placeholders that allow the user to provide input values at the time of deployment. They allow you to create reusable templates because the values can change with each deployment.

Example:

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"parameters": {
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  "adminUsername": {
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    "type": "string",
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    "metadata": {
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      "description": "Admin username for the virtual machine"
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    }
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  },
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  "adminPassword": {
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    "type": "securestring",
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    "metadata": {
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      "description": "Admin password for the virtual machine"
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    }
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  }
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}

In this example:

1. adminUsername and adminPassword are parameters.

2. The type of the parameters is specified (string for adminUsername and securestring for adminPassword).

3. metadata provides a description of the parameter.

4. Parameter types can include:

   • string, int, bool, array, object, securestring, etc.

You can also define default values, allowed values, and constraints (e.g., minLength, maxLength).

3. Variables

Variables in ARM templates are used to store values that are reused throughout the template. They are typically used for values that might require calculation or that are used in multiple places, helping avoid repetition.

Example:

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"variables": {
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  "location": "East US",
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  "vmName": "MyVM"
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}

Here:

1. location and vmName are variables.

2. The variables can be referenced within the template, reducing redundancy.

3. Variables can be used in expressions or assigned dynamic values, and they can reference parameters, functions, or other variables.

4. Functions

Functions in ARM templates are used to compute values dynamically during the deployment process. Functions can perform various operations, such as string manipulation, resource identification, mathematical operations, or date-time functions.

Common Functions:

1. concat() – Concatenates two or more strings.

2. reference() – Retrieves the properties of a deployed resource.

3. resourceId() – Generates a resource ID for a resource.

4. format() – Formats a string using placeholders.

5. if() – Conditional evaluation.

6. length() – Returns the length of an array or string.

7. uniqueString() – Generates a unique string based on input values.

Example of a Function:

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"variables": {
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  "uniqueString": "[uniqueString(parameters('adminUsername'))]"
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}

In this example

The uniqueString() function generates a unique string based on the adminUsername parameter value.

5. Resources

The resources section is the core of an ARM template. It defines the Azure resources that are to be deployed, along with their properties, configurations, and dependencies. Each resource typically corresponds to an Azure service (e.g., Virtual Machine, Storage Account, Network Interface).

Example:

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"resources": [
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  {
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    "type": "Microsoft.Compute/virtualMachines",
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    "apiVersion": "2023-03-01",
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    "location": "[variables('location')]",
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    "name": "[variables('vmName')]",
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    "properties": {
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      "hardwareProfile": {
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        "vmSize": "Standard_DS1_v2"
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      },
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      "storageProfile": {
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        "imageReference": {
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          "publisher": "MicrosoftWindowsServer",
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          "offer": "WindowsServer",
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          "sku": "2019-Datacenter",
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          "version": "latest"
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        },
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        "osDisk": {
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          "createOption": "FromImage",
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          "name": "myVMOSDisk"
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        }
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      },
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      "osProfile": {
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        "computerName": "[variables('vmName')]",
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        "adminUsername": "[parameters('adminUsername')]",
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        "adminPassword": "[parameters('adminPassword')]"
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      }
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    }
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  }
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]

In this example:

1. The type indicates that a Virtual Machine resource is being created (Microsoft.Compute/virtualMachines).

2. The location and name are dynamically set using variables.

3. The properties section configures the VM’s settings, such as size, OS image, and admin credentials.

4. Each resource in the resources array has the following structure:

   • type: The resource type (e.g., Microsoft.Compute/virtualMachines).

   • apiVersion: The version of the API to be used.

   • location: The Azure region where the resource will be deployed.

   • name: The name of the resource.

   • properties: The properties specific to the resource type.

   • dependsOn: (optional) Defines resource dependencies.

6. Outputs

The outputs section is used to return values from the ARM template after deployment. These values can be useful for other processes or for sharing information after a deployment completes, such as IP addresses, URLs, or resource IDs.

Example:

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"outputs": {
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  "adminUsername": {
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    "type": "string",
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    "value": "[parameters('adminUsername')]"
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  },
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  "vmId": {
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    "type": "string",
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    "value": "[resourceId('Microsoft.Compute/virtualMachines', variables('vmName'))]"
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  }
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}

Here:

1. adminUsername and vmId are output values.

2. The adminUsername is directly taken from the parameter input, and vmId is dynamically generated using the resourceId() function.

These outputs can be accessed after the deployment to use in subsequent operations or to report back to users.

Summary of ARM Template Components:

1. JSON Document: Defines the structure of the template.

2. Parameters: Allow external input values to be passed at deployment time.

3. Variables: Store values for reuse throughout the template.

4. Functions: Perform calculations or transformations of data in the template.

5. Resources: Define the Azure resources to be deployed.

6. Outputs: Return values from the deployment for use after the deployment completes.

These components together allow ARM templates to define and manage Azure resources in a flexible, reusable, and scalable way, ensuring consistent infrastructure deployments.