Terraform Modules

Chapter objectives

Understand modules
Create reusable modules
Use the Terraform Registry
Organize a project with modules

1 - What is a module?

Definition

A module is a container for several resources used together.

Advantages

Without modules	With modules
Duplicated code	Reusable code
Huge files	Isolated components
Hard to maintain	Easy to test
No versioning	Versionable

2 - Structure of a module

Standard tree

modules/
└── vpc/
    ├── main.tf           # Ressources principales
    ├── variables.tf      # Variables d'entrée
    ├── outputs.tf        # Valeurs de sortie
    ├── versions.tf       # Versions requises
    └── README.md         # Documentation

Example VPC module

# modules/vpc/variables.tf
variable "vpc_cidr" {
  description = "CIDR block for VPC"
  type        = string
}

variable "environment" {
  description = "Environment name"
  type        = string
}

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

# modules/vpc/main.tf
resource "aws_vpc" "main" {
  cidr_block           = var.vpc_cidr
  enable_dns_hostnames = true
  enable_dns_support   = true

  tags = {
    Name        = "${var.environment}-vpc"
    Environment = var.environment
  }
}

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

  vpc_id                  = aws_vpc.main.id
  cidr_block              = cidrsubnet(var.vpc_cidr, 8, count.index)
  availability_zone       = var.availability_zones[count.index]
  map_public_ip_on_launch = true

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

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

  tags = {
    Name = "${var.environment}-igw"
  }
}

resource "aws_route_table" "public" {
  vpc_id = aws_vpc.main.id

  route {
    cidr_block = "0.0.0.0/0"
    gateway_id = aws_internet_gateway.main.id
  }

  tags = {
    Name = "${var.environment}-public-rt"
  }
}

resource "aws_route_table_association" "public" {
  count = length(var.availability_zones)

  subnet_id      = aws_subnet.public[count.index].id
  route_table_id = aws_route_table.public.id
}

# modules/vpc/outputs.tf
output "vpc_id" {
  description = "ID of the VPC"
  value       = aws_vpc.main.id
}

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

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

3 - Use a module

Local module

# main.tf
module "vpc" {
  source = "./modules/vpc"

  vpc_cidr           = "10.0.0.0/16"
  environment        = "production"
  availability_zones = ["eu-west-1a", "eu-west-1b", "eu-west-1c"]
}

# Utiliser les outputs du module
resource "aws_instance" "web" {
  subnet_id = module.vpc.public_subnet_ids[0]
  # ...
}

# Re-exporter les outputs
output "vpc_id" {
  value = module.vpc.vpc_id
}

Module from the Registry

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

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

  azs             = ["eu-west-1a", "eu-west-1b", "eu-west-1c"]
  private_subnets = ["10.0.1.0/24", "10.0.2.0/24", "10.0.3.0/24"]
  public_subnets  = ["10.0.101.0/24", "10.0.102.0/24", "10.0.103.0/24"]

  enable_nat_gateway = true
  single_nat_gateway = true

  tags = {
    Environment = "production"
  }
}

Module from Git

# GitHub
module "vpc" {
  source = "github.com/myorg/terraform-modules//vpc?ref=v1.0.0"
}

# GitLab
module "vpc" {
  source = "git::https://gitlab.com/myorg/terraform-modules.git//vpc?ref=v1.0.0"
}

# SSH
module "vpc" {
  source = "[email protected]:myorg/terraform-modules.git//vpc?ref=v1.0.0"
}

4 - Module sources

Types of sources

Syntax

# Local
source = "./modules/vpc"
source = "../shared-modules/vpc"

# Terraform Registry
source  = "terraform-aws-modules/vpc/aws"
version = "5.0.0"

# GitHub
source = "github.com/hashicorp/example"
source = "github.com/hashicorp/example//subdir"
source = "github.com/hashicorp/example?ref=v1.2.0"

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

# HTTP
source = "https://example.com/modules/vpc.zip"

5 - Module versioning

Registry versions

module "vpc" {
  source  = "terraform-aws-modules/vpc/aws"
  version = "5.0.0"        # Exact
  version = "~> 5.0"       # >= 5.0.0, < 6.0.0
  version = ">= 5.0, < 6.0" # Range
}

Git refs

# Tag
source = "github.com/org/repo?ref=v1.0.0"

# Branch
source = "github.com/org/repo?ref=develop"

# Commit
source = "github.com/org/repo?ref=abc1234"

6 - Nested modules

Composition

Pass outputs between modules

# root/main.tf

module "vpc" {
  source = "./modules/vpc"
  
  vpc_cidr    = var.vpc_cidr
  environment = var.environment
}

module "ec2" {
  source = "./modules/ec2"
  
  vpc_id         = module.vpc.vpc_id
  subnet_ids     = module.vpc.public_subnet_ids
  instance_count = var.instance_count
}

module "rds" {
  source = "./modules/rds"
  
  vpc_id            = module.vpc.vpc_id
  subnet_ids        = module.vpc.private_subnet_ids
  allowed_sg_ids    = [module.ec2.security_group_id]
}

7 - Module count and for_each

count

module "web_cluster" {
  source = "./modules/ec2-cluster"
  count  = var.create_cluster ? 1 : 0

  instance_count = 3
  instance_type  = "t3.micro"
}

# Accès
output "cluster_ips" {
  value = var.create_cluster ? module.web_cluster[0].instance_ips : []
}

for_each

variable "environments" {
  default = {
    dev = {
      instance_type  = "t3.micro"
      instance_count = 1
    }
    staging = {
      instance_type  = "t3.small"
      instance_count = 2
    }
    production = {
      instance_type  = "t3.large"
      instance_count = 3
    }
  }
}

module "env" {
  source   = "./modules/environment"
  for_each = var.environments

  environment    = each.key
  instance_type  = each.value.instance_type
  instance_count = each.value.instance_count
}

# Accès
output "env_vpc_ids" {
  value = { for k, v in module.env : k => v.vpc_id }
}

8 - Terraform Registry

Popular public modules

Provider	Module	Description
AWS	`terraform-aws-modules/vpc/aws`	Complete VPC
AWS	`terraform-aws-modules/eks/aws`	EKS cluster
AWS	`terraform-aws-modules/rds/aws`	RDS database
Azure	`Azure/network/azurerm`	Virtual Network
GCP	`terraform-google-modules/network/google`	VPC Network

EKS example

module "eks" {
  source  = "terraform-aws-modules/eks/aws"
  version = "19.0.0"

  cluster_name    = "my-cluster"
  cluster_version = "1.28"

  vpc_id     = module.vpc.vpc_id
  subnet_ids = module.vpc.private_subnets

  eks_managed_node_groups = {
    default = {
      min_size     = 1
      max_size     = 3
      desired_size = 2

      instance_types = ["t3.medium"]
    }
  }

  tags = {
    Environment = "production"
  }
}

9 - Publishing a module

Structure for the Registry

terraform-aws-my-module/
├── main.tf
├── variables.tf
├── outputs.tf
├── versions.tf
├── README.md
├── LICENSE
├── examples/
│   ├── simple/
│   │   ├── main.tf
│   │   └── README.md
│   └── complete/
│       ├── main.tf
│       └── README.md
└── modules/
    └── submodule/
        ├── main.tf
        ├── variables.tf
        └── outputs.tf

Requirements

✅ Repo public sur GitHub
✅ Nommé terraform-<PROVIDER>-<NAME>
✅ README.md avec description
✅ LICENSE
✅ Tags Git pour versioning (v1.0.0)
✅ Structure standard

10 - Best practices

Structure

# ✅ Fichiers séparés
# main.tf - Ressources
# variables.tf - Variables d'entrée
# outputs.tf - Valeurs de sortie
# versions.tf - Contraintes de version

Variables

# ✅ Description et type pour toutes les variables
variable "instance_type" {
  description = "Type d'instance EC2"
  type        = string
  default     = "t2.micro"
}

# ✅ Validation
variable "environment" {
  type = string
  validation {
    condition     = contains(["dev", "staging", "prod"], var.environment)
    error_message = "Environment must be dev, staging, or prod."
  }
}

Documentation

# Module VPC

Creates a VPC with public and private subnets.

## Usage

```hcl
module "vpc" {
  source = "./modules/vpc"
  
  vpc_cidr    = "10.0.0.0/16"
  environment = "production"
}

Inputs

Name	Description	Type	Default
vpc_cidr	CIDR block for VPC	string	-
environment	Environment name	string	-

Outputs

Name	Description
vpc_id	ID of the VPC

---

## Summary

```mermaid
mindmap
  root((Modules))
    Sources
      Local
      Registry
      Git
    Structure
      main.tf
      variables.tf
      outputs.tf
    Reuse
      count
      for_each
    Registry
      Public
      Private

Key points

Modules encapsulate related resources
Use versions for stability
The Registry offers ready-to-use modules
Always document your modules
Test with examples

Practical exercises

Create a VPC module with public and private subnets
Use a module from the Registry (terraform-aws-modules/vpc/aws)
Create a module that calls other modules
Publish a module to a Git repo

← State Management | Best practices →

Chapter objectives​

1 - What is a module?​

Definition​

Advantages​

2 - Structure of a module​

Standard tree​

Example VPC module​

3 - Use a module​

Local module​

Module from the Registry​

Module from Git​

4 - Module sources​

Types of sources​

Syntax​

5 - Module versioning​

Registry versions​

Git refs​

6 - Nested modules​

Composition​

Pass outputs between modules​

7 - Module count and for_each​

count​

for_each​

8 - Terraform Registry​

Popular public modules​

EKS example​

9 - Publishing a module​

Structure for the Registry​

Requirements​

10 - Best practices​

Structure​

Variables​

Documentation​

Inputs​

Outputs​

Practical exercises​

Chapter objectives

1 - What is a module?

Definition

Advantages

2 - Structure of a module

Standard tree

Example VPC module

3 - Use a module

Local module

Module from the Registry

Module from Git

4 - Module sources

Types of sources

Syntax

5 - Module versioning

Registry versions

Git refs

6 - Nested modules

Composition

Pass outputs between modules

7 - Module count and for_each

count

for_each

8 - Terraform Registry

Popular public modules

EKS example

9 - Publishing a module

Structure for the Registry

Requirements

10 - Best practices

Structure

Variables

Documentation

Inputs

Outputs

Practical exercises