Module 8, Modules

What is a Module

A module is a directory of Terraform files used together. Every Terraform configuration IS a module (the root module). When you call another module, it becomes a child module.

finpay-infrastructure/
├── main.tf           ← root module
├── variables.tf
├── outputs.tf
└── modules/
    ├── networking/   ← child module
    │   ├── main.tf
    │   ├── variables.tf
    │   └── outputs.tf
    └── database/     ← child module
        ├── main.tf
        ├── variables.tf
        └── outputs.tf

Creating a Module

# modules/s3-bucket/main.tf
resource "aws_s3_bucket" "this" {
  bucket = var.bucket_name
  tags   = var.tags
}

resource "aws_s3_bucket_versioning" "this" {
  bucket = aws_s3_bucket.this.id
  versioning_configuration {
    status = var.versioning_enabled ? "Enabled" : "Disabled"
  }
}

# modules/s3-bucket/variables.tf
variable "bucket_name"        { type = string }
variable "versioning_enabled" { type = bool; default = true }
variable "tags"               { type = map(string); default = {} }

# modules/s3-bucket/outputs.tf
output "bucket_arn"  { value = aws_s3_bucket.this.arn }
output "bucket_name" { value = aws_s3_bucket.this.id }

Calling a Module

# root main.tf
module "artifacts_bucket" {
  source = "./modules/s3-bucket"

  bucket_name        = "finpay-artifacts-${var.environment}"
  versioning_enabled = true
  tags = {
    Environment = var.environment
    Purpose     = "pipeline-artifacts"
  }
}

module "logs_bucket" {
  source             = "./modules/s3-bucket"
  bucket_name        = "finpay-logs-${var.environment}"
  versioning_enabled = false
  tags               = { Environment = var.environment }
}

# Access module outputs
output "artifacts_arn" {
  value = module.artifacts_bucket.bucket_arn
}

tip

Run terraform init after adding any new module source, Terraform needs to download or resolve it.

Public Registry Modules

# Use a community-maintained VPC module from the Terraform Registry
module "vpc" {
  source  = "terraform-aws-modules/vpc/aws"
  version = "~> 5.0"

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

  azs             = ["us-east-1a", "us-east-1b", "us-east-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
}

Knowledge Check

1. What command must be run after adding a new module source?
2. How do you reference an output from a child module in the root config?
3. What is the difference between source = "./modules/networking" and source = "terraform-aws-modules/vpc/aws"?
4. True or False: A module can contain other modules.
5. What is the purpose of module outputs?
6. Why would you wrap resources in a module even for a solo project?

Exam Mapping

• HashiCorp Associate Obj 4: Use Terraform outside of core workflow
• HashiCorp Associate Obj 5: Interact with Terraform modules (10-15% of exam)