ARM template parameters and variables in Azure - Step-by-Step Execution

Process Flow - ARM template parameters and variables

Start ARM Template

↓

Define Parameters

↓

Define Variables

↓

Use Parameters & Variables in Resources

↓

Deploy Template with Parameter Values

↓

Resources Created Using Values

↓

End Deployment

The ARM template starts by defining parameters and variables, then uses them to configure resources during deployment.

Execution Sample

Azure

{
  "parameters": {
    "storageAccountName": { "type": "string" }
  },
  "variables": {
    "location": "eastus"
  }
}

Defines a parameter for storage account name and a variable for location.

Process Table

Step	Action	Parameters State	Variables State	Resource Property Value
1	Start template processing	storageAccountName: undefined	location: undefined	N/A
2	Define parameter 'storageAccountName'	storageAccountName: string (no value yet)	location: undefined	N/A
3	Define variable 'location'	storageAccountName: string (no value yet)	location: 'eastus'	N/A
4	Receive deployment parameter value	storageAccountName: 'mystorage123'	location: 'eastus'	N/A
5	Use parameter and variable in resource	storageAccountName: 'mystorage123'	location: 'eastus'	storageAccountName: 'mystorage123', location: 'eastus'
6	Deploy resource with these values	storageAccountName: 'mystorage123'	location: 'eastus'	Resource created with name 'mystorage123' in 'eastus'
7	End deployment	storageAccountName: 'mystorage123'	location: 'eastus'	Deployment complete

💡 Deployment ends after resources are created using parameter and variable values.

Status Tracker

Variable	Start	After Step 3	After Step 4	Final
storageAccountName	undefined	string (no value yet)	'mystorage123'	'mystorage123'
location	undefined	'eastus'	'eastus'	'eastus'

Key Moments - 3 Insights

Why is the parameter 'storageAccountName' undefined at first?

Parameters are placeholders until deployment time when actual values are provided, as shown in step 4 of the execution_table.

Can variables use parameter values in their definitions?

Yes, variables can reference parameters to compute values before deployment, but in this example, 'location' is a fixed string (step 3).

Why do resources use both parameters and variables?

Parameters allow input customization at deployment, variables simplify reuse and calculations, both combine to configure resources (step 5).

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'storageAccountName' at step 2?

A undefined

B string (no value yet)

C 'mystorage123'

D 'eastus'

💡 Hint

Check the 'Parameters State' column at step 2 in the execution_table.

At which step does the deployment receive the actual parameter value?

A Step 4

B Step 3

C Step 5

D Step 6

💡 Hint

Look for when 'storageAccountName' changes from no value to 'mystorage123' in the execution_table.

If the variable 'location' was changed to 'westus', what would change in the execution_table?

A The resource name would change to 'westus'

B The parameter value would change to 'westus'

C The 'Variables State' column would show 'westus' instead of 'eastus' from step 3 onward

D No change would occur

💡 Hint

Variables track their values in the 'Variables State' column in the execution_table.

Concept Snapshot

ARM templates use parameters to accept input values at deployment time.
Variables hold fixed or computed values inside the template.
Parameters are undefined until deployment; variables are defined in the template.
Resources use both to configure properties dynamically.
Parameters enable reuse; variables simplify complex expressions.

Full Transcript

This visual execution shows how ARM templates define parameters and variables. Initially, parameters like 'storageAccountName' are placeholders without values. Variables like 'location' are set inside the template. When deploying, the parameter receives a real value, here 'mystorage123'. Resources then use these values to create cloud resources, such as a storage account named 'mystorage123' in the 'eastus' location. Variables remain constant unless changed in the template. This flow helps customize deployments while keeping templates reusable and clear.

Practice

1.

What is the main purpose of parameters in an ARM template?

easy

A. To define the resources to deploy

B. To allow input values when deploying the template

C. To store fixed values inside the template

D. To specify deployment location only

Solution

1. Step 1: Understand parameters role

   Parameters let users provide input values at deployment time, making templates flexible.

2. Step 2: Differentiate from variables

   Variables store fixed or computed values inside the template, not input values.

3. Final Answer:

   To allow input values when deploying the template -> Option B

4. Quick Check:

   Parameters = input values [OK]

Hint: Parameters accept inputs during deployment, variables do not [OK]

Common Mistakes:

• Confusing parameters with variables
• Thinking parameters define resources
• Assuming parameters are fixed values

2.

Which of the following is the correct syntax to declare a variable named storageAccountName in an ARM template?

{
  "variables": {
    "storageAccountName": "myStorage"
  }
}

easy

A. "resources": { "storageAccountName": "myStorage" }

B. "parameters": { "storageAccountName": "myStorage" }

C. "variables": { "storageAccountName": "myStorage" }

D. "outputs": { "storageAccountName": "myStorage" }

Solution

1. Step 1: Identify variable declaration section

   Variables are declared inside the "variables" section as key-value pairs.

2. Step 2: Match syntax with example

   The example shows "variables": { "storageAccountName": "myStorage" }, which matches "variables": { "storageAccountName": "myStorage" }.

3. Final Answer:

   "variables": { "storageAccountName": "myStorage" } -> Option C

4. Quick Check:

   Variables declared under "variables" [OK]

Hint: Variables go inside "variables" block, not parameters or resources [OK]

Common Mistakes:

• Putting variables inside parameters
• Confusing variables with outputs
• Using wrong JSON section names

3.

Given this ARM template snippet, what will be the value of variables('fullName')?

{
  "parameters": {
    "firstName": { "type": "string", "defaultValue": "John" },
    "lastName": { "type": "string", "defaultValue": "Doe" }
  },
  "variables": {
    "fullName": "[concat(parameters('firstName'), ' ', parameters('lastName'))]"
  }
}

medium

A. parameters('firstName') parameters('lastName')

B. JohnDoe

C. [concat(parameters('firstName'), ' ', parameters('lastName'))]

D. John Doe

Solution

1. Step 1: Understand concat function in variables

   The variable "fullName" uses concat to join firstName, a space, and lastName.

2. Step 2: Substitute default parameter values

   firstName = "John", lastName = "Doe", so concat("John", " ", "Doe") = "John Doe".

3. Final Answer:

   John Doe -> Option D

4. Quick Check:

   concat with space joins names [OK]

Hint: concat joins strings; default parameters fill in values [OK]

Common Mistakes:

• Taking concat as literal string
• Ignoring space between names
• Confusing parameters with variables

4.

Identify the error in this ARM template snippet:

{
  "parameters": {
    "location": { "type": "string" }
  },
  "variables": {
    "region": "parameters('location')"
  }
}

medium

A. Missing square brackets around the parameter reference in variable

B. Variable names cannot be 'region'

C. Parameter type must be 'securestring' to use in variables

D. Variables cannot reference parameters

Solution

1. Step 1: Check variable referencing syntax

   Variables referencing parameters must use expression syntax with square brackets: "[parameters('location')]".

2. Step 2: Identify missing brackets

   The variable "region" is assigned "parameters('location')" as a string, not an expression.

3. Final Answer:

   Missing square brackets around the parameter reference in variable -> Option A

4. Quick Check:

   Parameter references need [ ] in variables [OK]

Hint: Use [parameters('name')] inside variables, not plain strings [OK]

Common Mistakes:

• Omitting square brackets for expressions
• Thinking variables can't use parameters
• Wrong parameter types for variables

5.

You want to create an ARM template that deploys a storage account with a name based on a parameter prefix and a variable uniqueString. Which is the best way to combine them in the template?

{
  "parameters": {
    "prefix": { "type": "string" }
  },
  "variables": {
    "uniqueString": "[uniqueString(resourceGroup().id)]"
  },
  "resources": [
    {
      "type": "Microsoft.Storage/storageAccounts",
      "name": "???",
      "apiVersion": "2022-09-01",
      "location": "[resourceGroup().location]"
    }
  ]
}

Choose the correct expression to use for the storage account name property.

hard

A. "[concat(parameters('prefix'), variables('uniqueString'))]"

B. "parameters('prefix') + variables('uniqueString')"

C. "[parameters('prefix') variables('uniqueString')]"

D. "concat(parameters.prefix, variables.uniqueString)"

Solution

1. Step 1: Understand ARM template expression syntax

   Expressions must be inside square brackets and use functions like concat() to join strings.

2. Step 2: Check correct usage of parameters and variables

   Use parameters('name') and variables('name') inside concat() within brackets.

3. Step 3: Evaluate options

   "[concat(parameters('prefix'), variables('uniqueString'))]" uses correct syntax: "[concat(parameters('prefix'), variables('uniqueString'))]".

4. Final Answer:

   "[concat(parameters('prefix'), variables('uniqueString'))]" -> Option A

5. Quick Check:

   Use [concat(parameters('x'), variables('y'))] for combined names [OK]

Hint: Use [concat(parameters('x'), variables('y'))] for combined strings [OK]

Common Mistakes:

• Using + instead of concat inside brackets
• Omitting square brackets for expressions
• Wrong syntax for parameters or variables