Bicep vs. ARM Templates: A Detailed Comparison⌗

When defining infrastructure in Microsoft Azure, using Infrastructure as Code (IaC) is critical for ensuring deployments are repeatable and version-controlled.

Historically, Azure Resource Manager (ARM) templates have been the go-to solution for defining Azure resources, but Microsoft’s introduction of Bicep offers a more user-friendly alternative.

This comparison highlights the key differences between ARM templates and Bicep, focusing on readability, ease of maintenance, and practical use cases.

What Are ARM Templates?⌗

ARM (Azure Resource Manager) templates use JSON to define and deploy Azure resources.

They allow users to specify resource types, properties, and dependencies in a declarative format. While offering granular control, ARM templates are often criticized for their verbosity and complexity, especially in larger deployments.

Key characteristics of ARM templates:

JSON-based: Though JSON is widely recognized, it isn’t particularly suited for handling complex logic or large-scale infrastructures.
Declarative syntax: You define the resources you want, not the specific steps to create them.
Cumbersome for large environments: Managing vast infrastructures can result in sprawling templates with thousands of lines of JSON.
Advanced features: ARM templates support loops, conditionals, and modularization, though implementing these features can make the structure even more complicated.

What is Bicep?⌗

Bicep is a domain-specific language (DSL) built for Azure resource deployment, designed to make infrastructure definition simpler without sacrificing any of ARM’s power.

Bicep files are transpiled into ARM templates before deployment, ensuring compatibility with all Azure services.

Key characteristics of Bicep:

Higher-level language: Bicep’s DSL is tailored to Azure, making it more concise and readable than ARM templates.
Familiar to developers: Its syntax is intuitive and cleaner, akin to modern programming languages.
Seamless conversion: Bicep compiles to ARM templates, meaning you can use both technologies together without compatibility issues.
Simplified loops and conditionals: Bicep simplifies the implementation of loops, conditionals, and modularization, reducing complexity in large deployments.

Readability⌗

ARM Templates: Written in JSON, ARM templates can quickly become hard to read, particularly in large configurations with nested objects and complex logic.

For instance:

{
  "$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
  "contentVersion": "1.0.0.0",
  "resources": [
    {
      "type": "Microsoft.Storage/storageAccounts",
      "apiVersion": "2021-01-01",
      "name": "mystorageaccount",
      "location": "[resourceGroup().location]",
      "sku": {
        "name": "Standard_LRS"
      },
      "kind": "StorageV2",
      "properties": {}
    },
    {
      "type": "Microsoft.Network/virtualNetworks",
      "apiVersion": "2021-05-01",
      "name": "myVnet",
      "location": "[resourceGroup().location]",
      "properties": {
        "addressSpace": {
          "addressPrefixes": [
            "10.0.0.0/16"
          ]
        },
        "subnets": [
          {
            "name": "default",
            "properties": {
              "addressPrefix": "10.0.0.0/24"
            }
          }
        ]
      }
    },
    {
      "type": "Microsoft.Compute/virtualMachines",
      "apiVersion": "2021-07-01",
      "name": "myVM",
      "location": "[resourceGroup().location]",
      "properties": {
        "hardwareProfile": {
          "vmSize": "Standard_DS1_v2"
        },
        "storageProfile": {
          "imageReference": {
            "publisher": "Canonical",
            "offer": "UbuntuServer",
            "sku": "18.04-LTS",
            "version": "latest"
          },
          "osDisk": {
            "createOption": "FromImage"
          }
        },
        "osProfile": {
          "computerName": "myVM",
          "adminUsername": "azureuser",
          "adminPassword": "P@ssw0rd123"
        },
        "networkProfile": {
          "networkInterfaces": [
            {
              "id": "[resourceId('Microsoft.Network/networkInterfaces', 'myNic')]"
            }
          ]
        }
      },
      "dependsOn": [
        "[resourceId('Microsoft.Network/networkInterfaces', 'myNic')]",
        "[resourceId('Microsoft.Storage/storageAccounts', 'mystorageaccount')]",
        "[resourceId('Microsoft.Network/virtualNetworks', 'myVnet')]"
      ]
    },
    {
      "type": "Microsoft.Network/networkInterfaces",
      "apiVersion": "2021-05-01",
      "name": "myNic",
      "location": "[resourceGroup().location]",
      "properties": {
        "ipConfigurations": [
          {
            "name": "ipconfig1",
            "properties": {
              "subnet": {
                "id": "[resourceId('Microsoft.Network/virtualNetworks/subnets', 'myVnet', 'default')]"
              },
              "privateIPAllocationMethod": "Dynamic"
            }
          }
        ]
      },
      "dependsOn": [
        "[resourceId('Microsoft.Network/virtualNetworks', 'myVnet')]"
      ]
    }
  ]
}

While this format provides detailed control, it can become overwhelming, especially in more complex configurations.

Bicep: Bicep’s syntax is much cleaner and easier to work with. The same configuration in Bicep looks significantly more concise:

resource storageAccount 'Microsoft.Storage/storageAccounts@2021-01-01' = {
  name: 'mystorageaccount'
  location: resourceGroup().location
  sku: {
    name: 'Standard_LRS'
  }
  kind: 'StorageV2'
}

resource virtualNetwork 'Microsoft.Network/virtualNetworks@2021-05-01' = {
  name: 'myVnet'
  location: resourceGroup().location
  properties: {
    addressSpace: {
      addressPrefixes: ['10.0.0.0/16']
    }
    subnets: [
      {
        name: 'default'
        properties: {
          addressPrefix: '10.0.0.0/24'
        }
      }
    ]
  }
}

resource networkInterface 'Microsoft.Network/networkInterfaces@2021-05-01' = {
  name: 'myNic'
  location: resourceGroup().location
  properties: {
    ipConfigurations: [
      {
        name: 'ipconfig1'
        properties: {
          subnet: {
            id: virtualNetwork.properties.subnets[0].id
          }
          privateIPAllocationMethod: 'Dynamic'
        }
      }
    ]
  }
}

resource virtualMachine 'Microsoft.Compute/virtualMachines@2021-07-01' = {
  name: 'myVM'
  location: resourceGroup().location
  properties: {
    hardwareProfile: {
      vmSize: 'Standard_DS1_v2'
    }
    storageProfile: {
      imageReference: {
        publisher: 'Canonical'
        offer: 'UbuntuServer'
        sku: '18.04-LTS'
        version: 'latest'
      }
      osDisk: {
        createOption: 'FromImage'
      }
    }
    osProfile: {
      computerName: 'myVM'
      adminUsername: 'azureuser'
      adminPassword: 'P@ssw0rd123'
    }
    networkProfile: {
      networkInterfaces: [
        {
          id: networkInterface.id
        }
      ]
    }
  }
}

The reduction in boilerplate code makes Bicep easier to write, read, and review, especially for larger projects.

Maintainability⌗

ARM Templates: As projects grow in size and complexity, maintaining ARM templates can be difficult. Nested structures and repetitive code often make debugging a challenge. ARM templates support modularity through linked templates, but managing these can be cumbersome and error-prone.

Bicep: Bicep enhances maintainability by simplifying the syntax and offering built-in module support. Its module system allows developers to break down large infrastructures into reusable components, which improves organization and reusability.

For example, using a module in Bicep to define reusable resources looks like this:

module myVMModule './modules/vm.bicep' = {
  name: 'vmModule'
  params: {
    name: 'myVM'
    location: resourceGroup().location
  }
}

Additionally, Bicep integrates with tools like Visual Studio Code, providing features like IntelliSense, error checking, and auto-completion, making it easier to develop and maintain infrastructure code.

Use Cases⌗

When to use ARM Templates:⌗

Legacy Deployments: For projects that are already heavily invested in ARM templates, there may be little reason to switch, especially if the templates are stable.
Complex Enterprise Deployments: In environments where highly complex infrastructures with extensive parameterization and linking are necessary, ARM templates remain a valid choice, particularly if you are familiar with them.

When to use Bicep:⌗

New Projects: Bicep should be the default for new Azure deployments due to its simplicity and maintainability.
Frequent Changes: If your infrastructure undergoes frequent updates, Bicep’s streamlined syntax makes it easier to manage.
Modularity and Reusability: Bicep’s module system makes it ideal for projects requiring reusable infrastructure components across various environments.

Tooling and Support⌗

Both ARM templates and Bicep are fully supported by Azure, but Bicep offers additional developer-friendly tooling. Bicep integrates seamlessly with Visual Studio Code, providing enhanced development features like syntax highlighting and error detection. ARM templates, while supported, don’t offer the same level of developer experience.

Microsoft also provides a decompiler (az bicep decompile) to help users convert ARM templates into Bicep, making it easier to transition without losing existing investments in ARM.

Conclusion⌗

Bicep represents a major improvement in defining and managing Azure infrastructure. Although ARM templates remain useful for legacy or highly complex environments, Bicep offers significant advantages in readability, maintainability, and modularity.

Readability: Bicep’s cleaner syntax is far easier to understand than ARM’s verbose JSON.
Maintainability: Bicep’s modular design and tooling make it more manageable for large-scale and evolving infrastructures.
Use cases: For new projects and those requiring frequent changes, Bicep is the superior choice. ARM templates may still be better suited to stable, legacy environments.

If you’re starting a new Azure project, Bicep should be your default. For those already using ARM templates, Bicep provides a smooth path forward while maintaining compatibility.

Bicep vs. ARM Templates: A Detailed Comparison

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