Terraform Modules

As your infrastructure grows, a single Terraform configuration file becomes hard to manage. Modules allow you to group resources together and reuse them across projects. A module is a container for multiple resources that are used together.

What Is a Module?

A module is a collection of .tf files in a directory. You can call a module from your root configuration to include its resources. Modules can be local (filesystem) or remote (Terraform Registry, GitHub, S3).

Modules are the primary way to package and reuse Terraform configurations.

Creating a Simple Module

Create a directory modules/aws-instance with a file main.tf:

variable "ami" { type = string }
variable "instance_type" { default = "t2.micro" }

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

output "id" {
  value = aws_instance.this.id
}

Using the Module in Root

In your root main.tf:

module "web" {
  source = "./modules/aws-instance"
  ami    = "ami-0c55b159cbfafe1f0"
}

output "web_id" {
  value = module.web.id
}

Run terraform init to install local modules.

Using Modules from Terraform Registry

The public Terraform Registry hosts thousands of modules. Example: AWS VPC module.

module "vpc" {
  source  = "terraform-aws-modules/vpc/aws"
  version = "5.0.0"
  name    = "my-vpc"
  cidr    = "10.0.0.0/16"
}

Module Outputs

Modules can expose outputs, which you can access with module.<name>.<output>.

Module Versioning

For remote modules, specify a version to avoid breaking changes. Use version constraints like >= 1.0.0.

Two Minute Drill

Modules are reusable collections of Terraform resources.
Define module in its own directory with variables and outputs.
Call module with module "name" { source = "..." }.
Use Terraform Registry for community modules.
Always version remote modules.

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