Building Reusable Terraform Modules: A Complete Guide

Terraform modules are the key to writing maintainable, reusable infrastructure code. In this guide, I'll walk you through creating effective Terraform modules.

What Are Terraform Modules?

A module is a container for multiple resources that are used together. Every Terraform configuration has at least one module, known as the root module.

Module Structure

A well-organized module follows this structure:

modules/
└── vpc/
    ├── main.tf
    ├── variables.tf
    ├── outputs.tf
    ├── versions.tf
    └── README.md

Creating a VPC Module

Let's create a reusable VPC module for AWS:

variables.tf

variable "name" {
  description = "Name prefix for resources"
  type        = string
}

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

variable "availability_zones" {
  description = "List of availability zones"
  type        = list(string)
}

variable "enable_nat_gateway" {
  description = "Enable NAT Gateway for private subnets"
  type        = bool
  default     = true
}

main.tf

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

  tags = {
    Name = "${var.name}-vpc"
  }
}

resource "aws_subnet" "public" {
  count             = length(var.availability_zones)
  vpc_id            = aws_vpc.main.id
  cidr_block        = cidrsubnet(var.cidr_block, 8, count.index)
  availability_zone = var.availability_zones[count.index]

  map_public_ip_on_launch = true

  tags = {
    Name = "${var.name}-public-${count.index + 1}"
    Type = "public"
  }
}

resource "aws_subnet" "private" {
  count             = length(var.availability_zones)
  vpc_id            = aws_vpc.main.id
  cidr_block        = cidrsubnet(var.cidr_block, 8, count.index + 10)
  availability_zone = var.availability_zones[count.index]

  tags = {
    Name = "${var.name}-private-${count.index + 1}"
    Type = "private"
  }
}

outputs.tf

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

output "public_subnet_ids" {
  description = "List of public subnet IDs"
  value       = aws_subnet.public[*].id
}

output "private_subnet_ids" {
  description = "List of private subnet IDs"
  value       = aws_subnet.private[*].id
}

Using the Module

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

  name               = "production"
  cidr_block         = "10.0.0.0/16"
  availability_zones = ["us-west-2a", "us-west-2b", "us-west-2c"]
  enable_nat_gateway = true
}

output "vpc_id" {
  value = module.vpc.vpc_id
}

Best Practices

1. Version your modules using semantic versioning
2. Document everything with clear README files
3. Use validation to catch errors early
4. Keep modules focused on a single responsibility
5. Test your modules with tools like Terratest

Conclusion

Well-designed modules save time, reduce errors, and make your infrastructure code a joy to work with. Start small and iterate!