Terraform Modules: Build Reusable Infrastructure

Why Modules?

Without modules, Terraform projects become monolithic walls of configuration. Modules let you:

• Organize — group related resources logically
• Reuse — use the same pattern across projects
• Abstract — hide complexity behind simple interfaces
• Share — publish modules for your team or community

Module Structure

modules/
  vpc/
    main.tf
    variables.tf
    outputs.tf
  ec2/
    main.tf
    variables.tf
    outputs.tf

Creating a Module

modules/vpc/main.tf:

hcl
resource "aws_vpc" "main" {
  cidr_block           = var.cidr_block
  enable_dns_hostnames = true

  tags = merge(var.tags, {
    Name = "${var.project}-vpc"
  })
}

resource "aws_subnet" "public" {
  count             = length(var.public_subnets)
  vpc_id            = aws_vpc.main.id
  cidr_block        = var.public_subnets[count.index]
  availability_zone = var.azs[count.index]

  tags = merge(var.tags, {
    Name = "${var.project}-public-${count.index}"
  })
}

modules/vpc/variables.tf:

hcl
variable "cidr_block" {
  type = string
}

variable "public_subnets" {
  type = list(string)
}

variable "azs" {
  type = list(string)
}

variable "project" {
  type = string
}

variable "tags" {
  type    = map(string)
  default = {}
}

modules/vpc/outputs.tf:

hcl
output "vpc_id" {
  value = aws_vpc.main.id
}

output "subnet_ids" {
  value = aws_subnet.public[*].id
}

Using a Module

hcl
module "vpc" {
  source = "./modules/vpc"

  cidr_block     = "10.0.0.0/16"
  public_subnets = ["10.0.1.0/24", "10.0.2.0/24"]
  azs            = ["eu-west-1a", "eu-west-1b"]
  project        = "myapp"
  tags           = local.common_tags
}

# Reference module outputs
resource "aws_instance" "web" {
  subnet_id = module.vpc.subnet_ids[0]
}

Terraform Registry Modules

Use community modules from the Terraform Registry:

hcl
module "vpc" {
  source  = "terraform-aws-modules/vpc/aws"
  version = "5.0.0"

  name = "my-vpc"
  cidr = "10.0.0.0/16"
}

Best Practices

1. Keep modules focused — one concern per module
2. Version your modules — use Git tags or registry versions
3. Document inputs/outputs — add descriptions to all variables
4. Use sensible defaults — make modules easy to use out of the box
5. Don't hardcode — everything configurable via variables

Learn More

Build reusable Terraform modules in our Terraform for Beginners course.