Module inputs (variables) in Terraform - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Terraform module variable defaults

Given a Terraform module with a variable defined as variable "region" { default = "us-west-1" }, what will be the value of var.region inside the module if the caller does NOT provide a value?

A "us-west-1"

B null

C An error occurs because the variable is not set

D "us-east-1"

💡 Hint

Think about what happens when a variable has a default value and no override is given.

If a variable has a default value and the caller does not provide a value, Terraform uses the default inside the module.

❓ Configuration

intermediate

Passing variables to a Terraform module

You have a Terraform module that expects a variable instance_count. Which of the following is the correct way to pass the value 3 to this variable when calling the module?

Terraform

module "webserver" {
  source = "./modules/webserver"
  // fill in here
}

A set instance_count 3

B instance_count = 3

C var.instance_count = 3

D variable "instance_count" = 3

💡 Hint

Remember how to assign values to module inputs in the module block.

Inside the module block, you assign values to variables by using variable_name = value syntax.

❓ Architecture

advanced

Variable type constraints in Terraform modules

A module variable is defined as variable "tags" { type = map(string) }. Which of the following values passed to this variable will cause a Terraform plan error?

A {}

B { "Environment" = "prod", "Owner" = "team-a" }

C { Environment = "prod", Owner = "team-a" }

D ["prod", "team-a"]

💡 Hint

Consider the expected type and what each option represents.

The variable expects a map of strings. Option D is a list, not a map, so it causes a type error.

❓ security

advanced

Sensitive variables in Terraform modules

You want to pass a password to a Terraform module variable without showing it in logs or state files. Which variable attribute should you use inside the module to mark it as sensitive?

A sensitive = true

B hidden = true

C private = true

D secure = true

💡 Hint

Terraform has a specific attribute to mark variables as sensitive.

The 'sensitive = true' attribute marks variables so their values are not shown in logs or UI.

❓ service_behavior

expert

Effect of missing required module variables

A Terraform module defines a variable project_id without a default value. The caller does NOT provide a value for this variable. What happens when you run terraform apply?

A Terraform fails with an error about the missing variable

B Terraform applies using a null value for the variable

C Terraform prompts for the missing variable value before applying

D Terraform applies with an empty string as the variable value

💡 Hint

Think about how Terraform handles required variables without defaults and no input.

Terraform will prompt the user to enter a value for required variables without defaults before applying.

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