Copy-pasting the same 50 lines of Terraform for every environment is a trap. Modules let you package infrastructure into reusable components. Combined with a disciplined workflow, they’re how professional teams manage large systems safely.

🎯 Learning Objectives

By the end of this lesson you will:

• Understand what a Terraform module is
• Write and call your own module
• Use modules from the public registry
• Follow the professional Terraform workflow

What is a Module?

A module is just a folder of .tf files that can be reused with different inputs. In fact, your root configuration is already a module — modules simply let you call one from another.

modules/
webserver/
  main.tf         # the resources
  variables.tf    # inputs
  outputs.tf      # outputs
environments/
prod/
  main.tf         # calls the webserver module

Writing a Module

The module (modules/webserver/main.tf) declares inputs and resources:

variable "name"          { type = string }
variable "instance_type" { type = string, default = "t3.micro" }

resource "aws_instance" "this" {
ami           = "ami-0abc123"
instance_type = var.instance_type
tags = { Name = var.name }
}

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

Calling a Module

From your environment config, call the module and pass inputs:

module "web" {
source        = "../../modules/webserver"
name          = "prod-web"
instance_type = "t3.large"
}

module "api" {
source = "../../modules/webserver"
name   = "prod-api"
}

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

One module, called twice, builds two servers with different settings — no duplication.

Using Registry Modules

You don’t have to write everything. The Terraform Registry has battle-tested modules for common infrastructure:

module "vpc" {
source  = "terraform-aws-modules/vpc/aws"
version = "5.5.0"

name = "prod-vpc"
cidr = "10.0.0.0/16"
azs  = ["us-east-1a", "us-east-1b"]
}

The Professional Workflow

Real teams follow a consistent cycle, usually automated in CI/CD:

terraform fmt        # format code consistently
terraform validate   # check syntax and basic correctness
terraform plan -out=tfplan   # produce a reviewable plan
# → open a pull request, review the plan
terraform apply tfplan       # apply exactly what was reviewed

🧪 Hands-on Lab

# root main.tf
module "note_a" {
source  = "./modules/note"
name    = "a.txt"
content = "First"
}
module "note_b" {
source  = "./modules/note"
name    = "b.txt"
content = "Second"
}

Success! The configuration is valid.
Apply complete! Resources: 2 added, 0 changed, 0 destroyed.
a.txt  b.txt

🧠 Knowledge Check

💼 Interview Preparation

Summary

You’ve completed the Terraform track: Infrastructure as Code, configuration and variables, state and remote backends, and now modules and the professional workflow. You can build, share, and safely operate infrastructure as code. Next, we automate builds and deployments with Jenkins.