Modules

Modules are the primary way to organise, reuse, and share Terraform configuration. A module is simply a directory containing .tf files. This lesson covers creating modules, using module sources, the Terraform Registry, and module composition patterns.

---

What is a Module?

Every Terraform configuration is a module:

• Root module — the directory where you run terraform apply
• Child module — a module called from another module using a module block

project/
├── main.tf              ← root module
├── variables.tf
├── outputs.tf
└── modules/
    └── network/         ← child module
        ├── main.tf
        ├── variables.tf
        └── outputs.tf

---

Why Use Modules?

Benefit	Description
Reusability	Write once, use across multiple projects
Organisation	Group related resources into logical units
Encapsulation	Hide implementation details behind a clean interface
Consistency	Enforce standards across your organisation
Testability	Test modules independently

---

Creating a Module

Module Structure

A module has the same file structure as any Terraform configuration:

modules/network/
├── main.tf          # Resources
├── variables.tf     # Input variables (the module's interface)
├── outputs.tf       # Output values (what the module exposes)
└── README.md        # Documentation

Example: Network Module

modules/network/variables.tf:

variable "vpc_cidr" {
  description = "CIDR block for the VPC"
  type        = string
  default     = "10.0.0.0/16"
}

variable "environment" {
  description = "Environment name"
  type        = string
}

modules/network/main.tf:

resource "aws_vpc" "main" {
  cidr_block           = var.vpc_cidr
  enable_dns_hostnames = true
  tags                 = { Name = "${var.environment}-vpc" }
}

resource "aws_subnet" "public" {
  vpc_id                  = aws_vpc.main.id
  cidr_block              = cidrsubnet(var.vpc_cidr, 8, 1)
  map_public_ip_on_launch = true
  tags                    = { Name = "${var.environment}-public" }
}

modules/network/outputs.tf:

output "vpc_id" {
  description = "The ID of the VPC"
  value       = aws_vpc.main.id
}

output "public_subnet_id" {
  description = "The ID of the public subnet"
  value       = aws_subnet.public.id
}

---

Calling a Module

Use a module block to call a child module:

module "network" {
  source      = "./modules/network"
  vpc_cidr    = "10.0.0.0/16"
  environment = "production"
}

resource "aws_instance" "web" {
  ami           = data.aws_ami.ubuntu.id
  instance_type = "t3.micro"
  subnet_id     = module.network.public_subnet_id
}

After adding or changing a module source, run:

terraform init

---

Module Sources

Terraform supports many module sources: