Terraform Modules

What is a module?

A module is a collection of Terraform resources grouped for reuse. The folder you run terraform apply in is the root module; anything under modules/ is a child module.

Problem without modules

Project 1 → copy VPC code
Project 2 → copy VPC code
Project 3 → copy VPC code

Duplication grows with every new stack.

Solution

Create module once
        ↓
Reuse everywhere

Module architecture

Root Module
  ├── VPC Module
  ├── EC2 Module
  └── S3 Module

Recommended layout

terraform-project/
├── main.tf
├── variables.tf
├── outputs.tf
├── provider.tf
├── terraform.tfvars
└── modules/
    ├── ec2/
    │   ├── main.tf
    │   ├── variables.tf
    │   └── outputs.tf
    ├── s3/
    │   ├── main.tf
    │   ├── variables.tf
    │   └── outputs.tf
    └── vpc/
        ├── main.tf
        ├── variables.tf
        └── outputs.tf

Create an EC2 module

modules/ec2/main.tf

resource "aws_instance" "web" {
  ami           = var.ami
  instance_type = var.instance_type
}

modules/ec2/variables.tf

variable "ami" {}
 
variable "instance_type" {}

modules/ec2/outputs.tf

output "instance_id" {
  value = aws_instance.web.id
}

Call the module from the root

main.tf

module "ec2" {
  source = "./modules/ec2"
 
  ami           = "ami-xxxxxxxx"
  instance_type = "t3.micro"
}

Use a module output in the root:

output "ec2_id" {
  value = module.ec2.instance_id
}

After adding or changing a module source, run:

terraform init
terraform plan
terraform apply

Benefits