Terraform Modules

Module Structure

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

# variables.tf
variable "vpc_cidr" {
  type        = string
  description = "CIDR block for the VPC"
  default     = "10.0.0.0/16"

  validation {
    condition     = can(cidrhost(var.vpc_cidr, 0))
    error_message = "Must be a valid CIDR block."
  }
}

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

variable "enable_nat_gateway" {
  type    = bool
  default = true
}

Module Sources

# Local path
module "vpc" {
  source = "./modules/vpc"
}

# Terraform Registry (public)
module "eks" {
  source  = "terraform-aws-modules/eks/aws"
  version = "~> 20.0"
}

# GitHub
module "vpc" {
  source = "github.com/my-org/terraform-modules//vpc?ref=v2.0.0"
}

# Git over HTTPS
module "vpc" {
  source = "git::https://github.com/my-org/modules.git//vpc?ref=v2.0.0"
}

# Git over SSH
module "vpc" {
  source = "git::ssh://git@github.com/my-org/modules.git//vpc?ref=main"
}

# Terraform Cloud / Enterprise private registry
module "vpc" {
  source  = "app.terraform.io/my-org/vpc/aws"
  version = "~> 2.0"
}

# S3 bucket
module "vpc" {
  source = "s3::https://s3-eu-west-1.amazonaws.com/my-modules/vpc.zip"
}

Calling a Module

# main.tf (root module)
module "production_vpc" {
  source = "./modules/vpc"

  vpc_cidr           = "10.0.0.0/16"
  public_subnets     = ["10.0.1.0/24", "10.0.2.0/24"]
  private_subnets    = ["10.0.10.0/24", "10.0.11.0/24"]
  enable_nat_gateway = true

  tags = {
    Environment = "production"
    Team        = "platform"
  }
}

# Use module outputs
resource "aws_security_group" "app" {
  vpc_id = module.production_vpc.vpc_id
}

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

# Pass provider to module
module "replica" {
  source    = "./modules/storage"
  providers = {
    aws = aws.us_west
  }
}

Outputs

# modules/vpc/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
  sensitive   = false
}

output "nat_gateway_ip" {
  description = "Elastic IP for NAT gateway"
  value       = try(aws_eip.nat[0].public_ip, null)
}

Module Composition & Count/For_each

# Create multiple instances of a module
variable "environments" {
  default = {
    dev     = { cidr = "10.0.0.0/16", az_count = 2 }
    staging = { cidr = "10.1.0.0/16", az_count = 2 }
    prod    = { cidr = "10.2.0.0/16", az_count = 3 }
  }
}

module "vpcs" {
  for_each = var.environments
  source   = "./modules/vpc"

  name     = each.key
  vpc_cidr = each.value.cidr
  az_count = each.value.az_count
}

# Access specific instance output
output "prod_vpc_id" {
  value = module.vpcs["prod"].vpc_id
}

# Combine outputs from all instances
output "all_vpc_ids" {
  value = { for k, v in module.vpcs : k => v.vpc_id }
}

Terraform Modules

Module Structure

Module Sources

Calling a Module

Outputs

Module Composition & Count/For_each

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