Module inputs (variables) in Terraform - Mini Project: Build & Apply

Terraform Module Inputs with Variables

📖 Scenario: You are creating a reusable Terraform module to deploy a simple AWS EC2 instance. To make the module flexible, you will use variables as inputs.

🎯 Goal: Build a Terraform module that accepts input variables for instance_type and ami_id, and uses them to configure an AWS EC2 instance resource.

📋 What You'll Learn

Create variables for instance_type and ami_id with default values

Use the variables in the aws_instance resource configuration

Output the instance ID after deployment

💡 Why This Matters

🌍 Real World

Terraform modules often use variables to make infrastructure reusable and configurable for different environments or projects.

💼 Career

Understanding how to use module inputs is essential for cloud engineers and DevOps professionals to write flexible and maintainable infrastructure as code.

1

Create variables for instance type and AMI ID

Create two variables in Terraform: instance_type with default value t2.micro, and ami_id with default value ami-12345678. Use the variable block syntax.

Terraform

# Define variables for instance_type and ami_id
# Your code here

Hint

Use variable "name" {} blocks with default values.

2

Add provider configuration

Add the AWS provider configuration block with region set to us-east-1. Use the provider "aws" block.

Terraform

variable "instance_type" {
  type    = string
  default = "t2.micro"
}

variable "ami_id" {
  type    = string
  default = "ami-12345678"
}

# Add AWS provider configuration here
# Your code here

Hint

Use provider "aws" { region = "us-east-1" } to set the AWS region.

3

Create the AWS EC2 instance resource using variables

Create an aws_instance resource named example that uses the variables instance_type and ami_id for its instance_type and ami attributes respectively.

Terraform

variable "instance_type" {
  type    = string
  default = "t2.micro"
}

variable "ami_id" {
  type    = string
  default = "ami-12345678"
}

provider "aws" {
  region = "us-east-1"
}

# Create aws_instance resource using variables
# Your code here

Hint

Use var.instance_type and var.ami_id to reference variables inside the resource.

4

Add output for the instance ID

Add an output named instance_id that outputs the id attribute of the aws_instance.example resource.

Terraform

variable "instance_type" {
  type    = string
  default = "t2.micro"
}

variable "ami_id" {
  type    = string
  default = "ami-12345678"
}

provider "aws" {
  region = "us-east-1"
}

resource "aws_instance" "example" {
  instance_type = var.instance_type
  ami           = var.ami_id
}

# Add output for instance ID
# Your code here

Hint

Use output "name" { value = resource.attribute } syntax.

Practice

1. What is the main purpose of using variables in a Terraform module?

easy

A. To customize module behavior without changing its code

B. To store sensitive data only

C. To define output values from the module

D. To create resources automatically

Solution

1. Step 1: Understand module customization

   Variables allow users to provide input values to modules, changing how they behave without editing the module code itself.

2. Step 2: Differentiate variables from other features

   Variables are not for outputs or automatic resource creation; they are inputs to customize modules.

3. Final Answer:

   To customize module behavior without changing its code -> Option A

4. Quick Check:

   Module inputs = customization [OK]

Hint: Variables customize modules without code edits [OK]

Common Mistakes:

• Confusing variables with outputs
• Thinking variables create resources
• Assuming variables only store secrets

2. Which of the following is the correct syntax to declare a string variable named region with a default value of us-west-1?

easy

A. variable "region" = "us-west-1"

B. variable region { type = "string" default = us-west-1 }

C. variable "region" { default = us-west-1 type = string }

D. variable "region" { type = string default = "us-west-1" }

Solution

1. Step 1: Check variable block syntax

   Terraform variables use the syntax: variable "name" { type = string default = "value" } with quotes around strings.

2. Step 2: Validate variable "region" { type = string default = "us-west-1" }

   correctly uses quotes for the variable name and default string value, and sets type properly.

3. Final Answer:

   variable "region" { type = string default = "us-west-1" } -> Option D

4. Quick Check:

   Correct variable block syntax = variable "region" { type = string default = "us-west-1" } [OK]

Hint: Use quotes for names and string defaults in variable blocks [OK]

Common Mistakes:

• Missing quotes around strings
• Wrong order of attributes
• Assigning variable directly without block

3. Given this module variable declaration:

variable "instance_count" {
  type    = number
  default = 2
}

And this usage inside the module:

resource "aws_instance" "example" {
  count = var.instance_count
  ami           = "ami-123456"
  instance_type = "t2.micro"
}

What will happen if the module is called without specifying instance_count?

medium

A. Only one instance will be created

B. Two AWS instances will be created

C. An error occurs because instance_count is missing

D. No instances will be created

Solution

1. Step 1: Understand default variable behavior

   If a variable has a default value and is not overridden, Terraform uses the default.

2. Step 2: Apply default to resource count

   The count argument uses var.instance_count, which defaults to 2, so two instances are created.

3. Final Answer:

   Two AWS instances will be created -> Option B

4. Quick Check:

   Default variable value used = 2 instances [OK]

Hint: Default variable values apply if no input given [OK]

Common Mistakes:

• Assuming missing variable causes error
• Thinking default is ignored
• Confusing count with instance_type

4. You have this variable declaration inside a module:

variable "env" {
  type = string
}

When calling the module, you forget to provide a value for env. What will happen?

medium

A. Terraform will throw an error about missing required variable

B. Terraform will use an empty string as default

C. Terraform will assign null automatically

D. Terraform will use the string "env" as default

Solution

1. Step 1: Check variable declaration for default

   The variable env has no default value, so it is required.

2. Step 2: Understand Terraform behavior on missing required variables

   If a required variable is not provided, Terraform throws an error and stops.

3. Final Answer:

   Terraform will throw an error about missing required variable -> Option A

4. Quick Check:

   Required variable missing = error [OK]

Hint: No default means variable is required [OK]

Common Mistakes:

• Assuming empty string or null is automatic default
• Thinking Terraform guesses a value
• Confusing variable name with default

5. You want to create a reusable module that accepts a list of subnet IDs as input. Which variable declaration correctly enforces that the input must be a list of strings and has no default value?

hard

A. variable "subnet_ids" { type = string default = [] }

B. variable "subnet_ids" { type = list number default = ["subnet-123"] }

C. variable "subnet_ids" { type = list(string) }

D. variable "subnet_ids" { type = list(string) default = null }

Solution

1. Step 1: Identify correct type syntax for list of strings

   Terraform uses list(string) to specify a list where each item is a string.

2. Step 2: Check default value presence

   No default means the variable is required. variable "subnet_ids" { type = list(string) } has no default, so input must be provided.

3. Final Answer:

   variable "subnet_ids" { type = list(string) } -> Option C

4. Quick Check:

   List of strings required, no default = variable "subnet_ids" { type = list(string) } [OK]

Hint: Use type = list(string) and omit default to require input [OK]

Common Mistakes:

• Using wrong type like string instead of list(string)
• Providing default empty list when input should be required
• Using invalid syntax for type or default