📦 Modules

What is a Module?

A module is a container for multiple resources used together as a logical unit.

┌────────────────────────────────────────────────────┐
│  Module Structure                                  │
├────────────────────────────────────────────────────┤
│                                                    │
│  Root Module (your main directory)                │
│    ├── main.tf                                    │
│    ├── variables.tf                               │
│    ├── outputs.tf                                 │
│    └── modules/                                   │
│         └── vpc/  ← Child Module                  │
│              ├── main.tf                          │
│              ├── variables.tf                     │
│              └── outputs.tf                       │
│                                                    │
└────────────────────────────────────────────────────┘

---

Creating a Module

Module Directory Structure

modules/
└── vpc/
    ├── main.tf        # Resources
    ├── variables.tf   # Input variables
    ├── outputs.tf     # Output values
    └── README.md      # Documentation

modules/vpc/main.tf

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

  tags = merge(
    var.tags,
    {
      Name = var.vpc_name
    }
  )
}

resource "aws_subnet" "public" {
  count = length(var.public_subnet_cidrs)

  vpc_id                  = aws_vpc.main.id
  cidr_block              = var.public_subnet_cidrs[count.index]
  availability_zone       = var.availability_zones[count.index]
  map_public_ip_on_launch = true

  tags = merge(
    var.tags,
    {
      Name = "${var.vpc_name}-public-${count.index + 1}"
      Type = "public"
    }
  )
}

resource "aws_internet_gateway" "main" {
  vpc_id = aws_vpc.main.id

  tags = merge(
    var.tags,
    {
      Name = "${var.vpc_name}-igw"
    }
  )
}

modules/vpc/variables.tf

variable "vpc_name" {
  description = "Name of the VPC"
  type        = string
}

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

variable "public_subnet_cidrs" {
  description = "CIDR blocks for public subnets"
  type        = list(string)
  default     = ["10.0.1.0/24", "10.0.2.0/24"]
}

variable "availability_zones" {
  description = "Availability zones"
  type        = list(string)
}

variable "enable_dns_hostnames" {
  description = "Enable DNS hostnames"
  type        = bool
  default     = true
}

variable "enable_dns_support" {
  description = "Enable DNS support"
  type        = bool
  default     = true
}

variable "tags" {
  description = "Tags to apply to resources"
  type        = map(string)
  default     = {}
}

modules/vpc/outputs.tf

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

output "vpc_cidr" {
  description = "CIDR block of the VPC"
  value       = aws_vpc.main.cidr_block
}

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

output "internet_gateway_id" {
  description = "ID of the Internet Gateway"
  value       = aws_internet_gateway.main.id
}

---

Using a Module

main.tf

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

  vpc_name             = "production-vpc"
  vpc_cidr             = "10.0.0.0/16"
  public_subnet_cidrs  = ["10.0.1.0/24", "10.0.2.0/24", "10.0.3.0/24"]
  availability_zones   = ["us-east-1a", "us-east-1b", "us-east-1c"]

  tags = {
    Environment = "production"
    ManagedBy   = "Terraform"
  }
}

# Use module outputs
resource "aws_instance" "web" {
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t2.micro"
  subnet_id     = module.vpc.public_subnet_ids[0]

  tags = {
    Name = "web-server"
  }
}

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