Demo Creating Your Own Module From Scratch

In this tutorial, you’ll learn how to build a reusable Terraform module for an AWS S3 bucket and then consume it with Terragrunt. You’ll get hands-on experience with:

Directory structure
Writing module files (main.tf, variables.tf, outputs.tf)
Referencing the module in Terragrunt
Initializing, planning, and applying changes
Verifying the S3 bucket

1. Directory Structure

Organize your project by creating a dedicated module folder:

mkdir -p modules/s3
cd modules/s3
touch main.tf variables.tf outputs.tf

Your workspace should resemble:

.
├── modules
│   └── s3
│       ├── main.tf
│       ├── variables.tf
│       └── outputs.tf
└── terragrunt.hcl

File	Purpose	Description
main.tf	Resource definition	Defines the `aws_s3_bucket` resource
variables.tf	Input variables	Declares variables like `bucket_name`
outputs.tf	Outputs	Exposes attributes such as bucket name and ARN

2. Writing the S3 Module

main.tf

Create the S3 bucket using the bucket_name variable:

resource "aws_s3_bucket" "this" {
  bucket = var.bucket_name
}

variables.tf

Declare the bucket name as a required input:

variable "bucket_name" {
  type        = string
  description = "Unique name for the S3 bucket"
}

Warning

S3 bucket names must be globally unique and comply with AWS naming rules.
Avoid uppercase letters and underscores.

outputs.tf

Expose both the bucket name and ARN:

output "bucket_name" {
  description = "The name of the S3 bucket"
  value       = aws_s3_bucket.this.bucket
}

output "bucket_arn" {
  description = "The ARN of the S3 bucket"
  value       = aws_s3_bucket.this.arn
}

3. Consuming the Module with Terragrunt

In your root or environment-specific folder, configure Terragrunt to source the S3 module:

# terragrunt.hcl
terraform {
  source = "./modules/s3"
}

inputs = {
  bucket_name = "your-unique-s3-bucket-name-123"
}

Note

Terragrunt lets you keep your Terraform code DRY by abstracting common configurations.
Learn more in the Terragrunt documentation.

4. Initialize, Plan, and Apply

Run these commands from the directory containing terragrunt.hcl:

terragrunt init
terragrunt plan

Sample plan output:

Plan: 1 to add, 0 to change, 0 to destroy.

Changes to Outputs:
  + bucket_arn  = (known after apply)
  + bucket_name = "your-unique-s3-bucket-name-123"

Apply your changes:

terragrunt apply

Confirm with yes when prompted:

aws_s3_bucket.this: Creating...
aws_s3_bucket.this: Creation complete after 3s [id=your-unique-s3-bucket-name-123]

Apply complete! Resources: 1 added, 0 changed, 0 destroyed.

Outputs:
bucket_arn  = "arn:aws:s3:::your-unique-s3-bucket-name-123"
bucket_name = "your-unique-s3-bucket-name-123"

5. Verifying the S3 Bucket

Use the AWS CLI to confirm that your bucket exists:

aws s3 ls

Example:

2024-06-23 15:41:52 your-unique-s3-bucket-name-123

Next Steps

Store your module in a Git repository (e.g., GitHub or GitLab).
Pin module versions in terragrunt.hcl using a Git URL and tag.
Expand the module with features like versioning, lifecycle rules, or encryption.

Links and References

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