Module composition patterns in Terraform - Mini Project: Build & Apply

Module Composition Patterns in Terraform

📖 Scenario: You are building a simple cloud infrastructure using Terraform. You want to organize your code by creating reusable modules. This project will guide you through composing modules to build a virtual private cloud (VPC) with subnets.

🎯 Goal: Build a Terraform configuration that uses module composition patterns to create a VPC and two subnets inside it. You will create the initial variables, configure module inputs, apply the main module calls, and complete the configuration with outputs.

📋 What You'll Learn

Create a variables file with exact variable names and values

Configure module input variables with correct values

Call modules with proper source and input arguments

Add outputs to expose the created resources

💡 Why This Matters

🌍 Real World

Organizing Terraform code into modules helps manage complex cloud infrastructure by reusing and composing components.

💼 Career

Understanding module composition is essential for cloud engineers and DevOps professionals to build scalable and maintainable infrastructure as code.

1

Create variables for VPC and subnet CIDR blocks

Create a Terraform variables file named variables.tf with two variables: vpc_cidr set to "10.0.0.0/16" and subnet_cidrs set to a list containing "10.0.1.0/24" and "10.0.2.0/24".

Terraform

# Your code here

Hint

Define variables using the variable block with default values.

2

Configure module input variables for VPC and subnet modules

In a new file named modules.tf, declare two module blocks: vpc with source ./modules/vpc and input variable cidr_block set to var.vpc_cidr, and subnets with source ./modules/subnet and input variable cidr_blocks set to var.subnet_cidrs.

Terraform

variable "vpc_cidr" {
  type    = string
  default = "10.0.0.0/16"
}

variable "subnet_cidrs" {
  type    = list(string)
  default = ["10.0.1.0/24", "10.0.2.0/24"]
}

# Your code here

Hint

Use module blocks with source and input variables referencing var. variables.

3

Use for_each to create multiple subnets from the subnet module

Modify the subnets module block to use for_each over var.subnet_cidrs. Pass each subnet CIDR block to the module input variable cidr_block using each.value. Rename the module block to subnet (singular).

Terraform

variable "vpc_cidr" {
  type    = string
  default = "10.0.0.0/16"
}

variable "subnet_cidrs" {
  type    = list(string)
  default = ["10.0.1.0/24", "10.0.2.0/24"]
}

module "vpc" {
  source     = "./modules/vpc"
  cidr_block = var.vpc_cidr
}

# Your code here

Hint

Use for_each = toset(var.subnet_cidrs) and pass cidr_block = each.value inside the module block.

4

Add outputs to expose VPC ID and subnet IDs

Create an outputs.tf file that defines two outputs: vpc_id which gets the id attribute from module.vpc, and subnet_ids which collects the id attributes from all module.subnet instances using values(module.subnet)[*].id.

Terraform

variable "vpc_cidr" {
  type    = string
  default = "10.0.0.0/16"
}

variable "subnet_cidrs" {
  type    = list(string)
  default = ["10.0.1.0/24", "10.0.2.0/24"]
}

module "vpc" {
  source     = "./modules/vpc"
  cidr_block = var.vpc_cidr
}

module "subnet" {
  source   = "./modules/subnet"
  for_each = toset(var.subnet_cidrs)
  cidr_block = each.value
}

# Your code here

Hint

Define outputs using output blocks and use a for expression to collect subnet IDs.

Practice

1. What is the main purpose of using module composition patterns in Terraform?

easy

A. To combine multiple modules by passing outputs as inputs for better organization

B. To write all resources in a single large file

C. To avoid using variables in Terraform configurations

D. To deploy resources without any dependencies

Solution

1. Step 1: Understand module composition

   Module composition means connecting modules by using outputs from one as inputs to another.

2. Step 2: Recognize benefits

   This approach keeps infrastructure code organized, reusable, and easier to manage.

3. Final Answer:

   To combine multiple modules by passing outputs as inputs for better organization -> Option A

4. Quick Check:

   Module composition = combining modules with outputs as inputs [OK]

Hint: Think of modules as building blocks connected by outputs [OK]

Common Mistakes:

• Thinking modules should be combined by copying code
• Believing variables are avoided in module composition
• Assuming modules work independently without connections

2. Which of the following is the correct syntax to pass an output from one module as an input to another in Terraform?

easy

A. output.moduleA.output_name

B. module.moduleA.output_name

C. var.moduleA.output_name

D. resource.moduleA.output_name

Solution

1. Step 1: Recall Terraform module output reference syntax

   Outputs from a module are accessed using module.<module_name>.<output_name>.

2. Step 2: Compare options

   Only module.moduleA.output_name matches the correct syntax for accessing module outputs.

3. Final Answer:

   module.moduleA.output_name -> Option B

4. Quick Check:

   Module output reference = module.module_name.output_name [OK]

Hint: Outputs are accessed via module.<name>.<output> [OK]

Common Mistakes:

• Using var or resource instead of module to access outputs
• Confusing output blocks with module references
• Using incorrect keywords like output.moduleA

3. Given the following Terraform code snippet, what will be the value of moduleB's input instance_id?

module "moduleA" {
  source = "./moduleA"
}

module "moduleB" {
  source      = "./moduleB"
  instance_id = module.moduleA.instance_id
}

medium

A. The value of the output named instance_id from moduleA

B. A syntax error because instance_id is not defined

C. The string literal 'module.moduleA.instance_id'

D. Null because moduleB does not have instance_id output

Solution

1. Step 1: Identify module output usage

   The input instance_id for moduleB is assigned from module.moduleA.instance_id, which means it uses the output named instance_id from moduleA.

2. Step 2: Understand value passing

   This passes the actual value of moduleA's output to moduleB's input, not a string or error.

3. Final Answer:

   The value of the output named instance_id from moduleA -> Option A

4. Quick Check:

   moduleB.instance_id = moduleA.instance_id output value [OK]

Hint: module.<name>.<output> passes actual output value, not string [OK]

Common Mistakes:

• Thinking it passes a string literal instead of value
• Assuming syntax error due to missing variable declaration
• Confusing inputs and outputs between modules

4. You have two modules: moduleA outputs vpc_id, and moduleB requires vpc_id as input. The following code causes an error:

module "moduleA" {
  source = "./moduleA"
}

module "moduleB" {
  source = "./moduleB"
  vpc_id = var.vpc_id
}

What is the most likely cause of the error?

medium

A. moduleB source path is incorrect

B. moduleA does not have an output named vpc_id

C. moduleB is using var.vpc_id instead of module.moduleA.vpc_id as input

D. Terraform does not support passing outputs between modules

Solution

1. Step 1: Check input assignment in moduleB

   moduleB expects vpc_id input, but it is assigned from var.vpc_id, which is a variable, not the output from moduleA.

2. Step 2: Identify correct way to pass output

   The correct way is to assign vpc_id = module.moduleA.vpc_id to pass the output from moduleA.

3. Final Answer:

   moduleB is using var.vpc_id instead of module.moduleA.vpc_id as input -> Option C

4. Quick Check:

   Pass outputs via module.moduleA.output, not var [OK]

Hint: Use module.<name>.<output> to pass outputs, not var.<name> [OK]

Common Mistakes:

• Assuming var.vpc_id automatically links to moduleA output
• Ignoring the need to explicitly connect modules
• Thinking Terraform cannot pass outputs between modules

5. You want to create a composed Terraform setup where moduleC depends on outputs from both moduleA and moduleB. Which pattern correctly composes these modules to pass outputs as inputs to moduleC?

// Option A
module "moduleC" {
  source = "./moduleC"
  inputA = module.moduleA.outputA
  inputB = module.moduleB.outputB
}

// Option B
module "moduleC" {
  source = "./moduleC"
  inputA = var.outputA
  inputB = var.outputB
}

// Option C
module "moduleC" {
  source = "./moduleC"
  inputA = module.moduleB.outputB
  inputB = module.moduleA.outputA
}

// Option D
module "moduleC" {
  source = "./moduleC"
  inputA = module.moduleA.outputB
  inputB = module.moduleB.outputA
}

hard

A. Mix outputs incorrectly between modules

B. Use variables instead of module outputs for inputs

C. Swap inputs but still use correct module outputs

D. Pass outputs correctly matching input names from moduleA and moduleB

Solution

1. Step 1: Match inputs to correct module outputs

   Pass outputs correctly matching input names from moduleA and moduleB passes inputA from moduleA.outputA and inputB from moduleB.outputB, matching names correctly.

2. Step 2: Evaluate other options

   Use variables instead of module outputs for inputs uses variables instead of outputs, which breaks composition. Swap inputs but still use correct module outputs swaps the assignments, mismatching inputA (expects moduleA.outputA) with moduleB.outputB and inputB with moduleA.outputA. Mix outputs incorrectly between modules mixes outputs incorrectly, likely causing errors.

3. Final Answer:

   Pass outputs correctly matching input names from moduleA and moduleB -> Option D

4. Quick Check:

   Correct output-to-input mapping = Pass outputs correctly matching input names from moduleA and moduleB [OK]

Hint: Match input names to correct module outputs carefully [OK]

Common Mistakes:

• Swapping outputs between modules without checking input expectations
• Using variables instead of module outputs
• Mixing output names incorrectly causing runtime errors