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

Need a 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

Need a 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

Need a 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

Need a hint?

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