Module inputs (variables) in Terraform - Step-by-Step Execution

Process Flow - Module inputs (variables)

Define variable in module

↓

Call module with input values

↓

Terraform reads input values

↓

Module uses inputs to create resources

↓

Outputs reflect resource state based on inputs

Terraform modules receive input values through variables defined inside the module and passed when calling the module. These inputs control resource creation.

Execution Sample

Terraform

variable "region" {
  type    = string
  default = "us-east-1"
}

module "example" {
  source = "./modules/example"
  region = "us-west-2"
}

Defines a variable 'region' with default, then calls a module passing a different region value.

Process Table

Step	Action	Variable	Value	Effect
1	Define variable 'region' with default	region	us-east-1	Variable ready with default
2	Call module 'example' passing region	region	us-west-2	Input overrides default
3	Terraform reads module input	region	us-west-2	Module uses passed value
4	Module creates resources using region	region	us-west-2	Resources created in us-west-2
5	Module outputs reflect resource state	region	us-west-2	Outputs show resources in us-west-2
6	Execution ends	-	-	All inputs processed, resources created

💡 Module inputs processed; resources created using passed variable values

Status Tracker

Variable	Defined	Passed to Module	Used in Module	Final
region	us-east-1 (default)	us-west-2	us-west-2	us-west-2

Key Moments - 2 Insights

Why does the module use 'us-west-2' instead of the default 'us-east-1'?

Because the module call passes 'region = "us-west-2"', which overrides the variable's default value as shown in execution_table step 2 and 3.

What happens if no value is passed to the module for 'region'?

Terraform uses the variable's default value 'us-east-1' inside the module, as defined in step 1.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what value does the module variable 'region' have?

A us-east-1

B undefined

C us-west-2

D null

💡 Hint

Check the 'Value' column at step 3 in the execution_table

At which step does Terraform read the input value passed to the module?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Look for the step where Terraform reads module input in the execution_table

If the module call did not pass any value for 'region', what would be the final value used inside the module?

A null

B us-east-1

C us-west-2

D empty string

💡 Hint

Refer to variable_tracker and key_moments about default values

Concept Snapshot

Terraform modules use variables to accept inputs.
Variables have defaults but can be overridden when calling the module.
Passed values replace defaults inside the module.
Modules use these inputs to create resources.
Outputs reflect the resource state based on inputs.

Full Transcript

In Terraform, modules can accept inputs through variables defined inside them. These variables can have default values. When you call a module, you can pass values to these variables, which override the defaults. Terraform reads these input values during execution and uses them to create resources inside the module. The outputs from the module then reflect the state of these resources based on the inputs provided. This process allows modules to be reusable and configurable by changing input values without modifying the module code.

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