ARM Templates & Bicep: Infrastructure as Code

What is Infrastructure as Code?

ARM template structure

Every ARM template has this structure:

// ARM Template (JSON)
// "$schema": defines the template version
// "contentVersion": your versioning
// "parameters": inputs (like VM name, size)
// "variables": computed values
// "resources": what to create
// "outputs": values returned after deployment

The sections you must know:

Section	Purpose	Required?
$schema	Template format version	Yes
contentVersion	Your template version (e.g., “1.0.0.0”)	Yes
parameters	User inputs at deploy time (VM name, size, location)	No
variables	Computed values used in the template	No
resources	The Azure resources to create	Yes
outputs	Values returned after deployment (like IP addresses)	No

Bicep — the cleaner alternative

Bicep does the same thing as ARM JSON but with much cleaner syntax:

ARM Template Feature	ARM JSON	Bicep
Parameter	”parameters” section with type definitions	param vmName string
Variable	”variables” section	var subnetId = ...
Resource	Nested JSON object in “resources” array	resource vm 'Microsoft.Compute/virtualMachines@2023-03-01'
Reference another resource	[resourceId(...)] function	storageAccount.id (direct reference)
String concatenation	[concat('vm-', parameters('name'))]	String interpolation: 'vm-$dollarleftbracename}'

ARM JSON vs Bicep — Bicep compiles to ARM, so they produce identical deployments

Feature	ARM Template (JSON)	Bicep
Language	JSON — verbose, curly braces everywhere	Domain-specific — clean, concise syntax
Learning curve	Steep — deeply nested JSON	Gentle — reads almost like pseudocode
Tooling	VS Code extension, Azure portal	VS Code extension with intellisense, linting, formatting
Module support	Linked/nested templates (complex)	Native modules (simple imports)
Deployment engine	Azure Resource Manager	Compiles to ARM JSON — same engine
Which to learn?	Read existing templates	Write new infrastructure

💡 Exam tip: You need to READ templates, not write from scratch

The exam tests whether you can interpret and modify templates — not write them from memory. You’ll see a template snippet and be asked: “What does this deploy?” or “What parameter needs to change to use a different VM size?”

Focus on understanding the structure: find the resource type, find the parameters, understand what values are being used.

Deploying templates

Four ways to deploy ARM/Bicep templates:

Method	Command
Azure CLI	az deployment group create --resource-group myRG --template-file main.bicep
PowerShell	New-AzResourceGroupDeployment -ResourceGroupName myRG -TemplateFile main.bicep
Azure Portal	Upload template in “Deploy a custom template”
Cloud Shell	Same CLI/PowerShell commands in the browser

Deployment modes:

• Incremental (default) — adds resources defined in the template; leaves existing resources untouched
• Complete — adds resources AND deletes any resources in the resource group that aren’t in the template

💡 Exam tip: Complete mode is dangerous

Complete mode deletes resources not in the template. If you deploy a template with only a VM in Complete mode to a resource group that has a VM and a database, the database gets deleted. The exam tests whether you understand this distinction. Default is Incremental (safe). Always specify explicitly.

Exporting and converting

Export an existing deployment as ARM template:

• Azure Portal → Resource Group → Deployments → select deployment → Download template
• Azure Portal → Resource → Export template (sidebar)
• CLI: az group export --name myResourceGroup

Convert ARM JSON to Bicep:

az bicep decompile --file template.json

Convert Bicep to ARM JSON:

az bicep build --file main.bicep

Question

What are the two deployment modes for ARM templates?

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Answer

Incremental (default) — adds/updates resources in the template but leaves others untouched. Complete — adds/updates resources AND deletes any resources in the resource group that are NOT defined in the template. Complete mode is destructive.

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Question

What is the relationship between Bicep and ARM templates?

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Answer

Bicep is a domain-specific language that compiles to ARM template JSON. It provides cleaner syntax but produces identical deployments. Bicep is not a separate deployment engine — it transpiles to the same ARM JSON that Azure Resource Manager processes.

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Question

How do you convert an existing ARM JSON template to Bicep?

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Answer

Use the Azure CLI command: az bicep decompile --file template.json. This converts ARM JSON syntax to Bicep syntax. The reverse (Bicep to ARM) uses: az bicep build --file main.bicep.

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Knowledge check

Knowledge Check

Alex deploys an ARM template in Complete mode to a resource group that contains a VM, a storage account, and a SQL database. The template only defines the VM. What happens to the storage account and SQL database?

AThey remain unchanged — the template only adds what's definedBThey are deleted — Complete mode removes resources not in the templateCThey are moved to a different resource groupDThey are stopped but not deleted

Check Answer

Knowledge Check

A Bicep file contains: param location string = 'uksouth'. What does this line define?

AA variable that cannot be changedBA parameter with a default value of 'uksouth' that can be overridden at deployment timeCA resource deployed in the uksouth regionDAn output that returns 'uksouth'

Check Answer