Terraform Modules

Example s3_bucket Module #

Folder Structure #

terraform-project/
├── main.tf
├── variables.tf
├── outputs.tf
└── modules/
    └── s3_bucket/
        ├── main.tf
        ├── variables.tf
        └── outputs.tf

📄 File: modules/s3_bucket/variables.tf

variable "bucket_name" {
  description = "Name of the S3 bucket"
  type        = string
}

variable "acl" {
  description = "Access control list for the bucket"
  type        = string
  default     = "private"
}

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

📄 File: modules/s3_bucket/main.tf

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

📄 File: modules/s3_bucket/outputs.tf

output "bucket_id" {
  description = "The ID of the bucket"
  value       = aws_s3_bucket.this.id
}

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

📄 File: Root Module: main.tf

module "my_bucket" {
  source      = "./modules/s3_bucket"
  bucket_name = "my-terraform-bucket"
  acl         = "private"
  tags = {
    Environment = "Dev"
    Owner       = "Rob"
  }
}

output "bucket_arn" {
  value = module.my_bucket.bucket_arn
}

When to Provide Default Values? #

1. To Make Inputs Optional

   • If you want a variable to be optional for the user of the module, you must provide a default value.
   • Without a default, the variable becomes required, and Terraform will throw an error if it’s not set.

variable "acl" {
  description = "Access control list for the bucket"
  type        = string
  default     = "private"  # Makes this optional
}

2. To Set Sensible Defaults

   • Use defaults to provide common or recommended values so users don’t have to specify them every time.
   • This improves usability and reduces boilerplate.

3. To Support Multiple Environments

   • Defaults can help standardize behavior across environments (e.g., default tags, naming conventions).

4. To Avoid Breaking Changes

   • When updating a module, adding a new variable without a default will break existing usage unless users update their code.
   • Providing a default ensures backward compatibility.

When Not to Provide Defaults #

• If the variable is critical and must be explicitly set (e.g., bucket_name), omit the default to force the user to provide it.
• If the value depends on external context (e.g., environment-specific settings), it’s better to require it.

Best Practice #

• Use defaults for convenience and safety.
• Avoid defaults for values that must be unique or environment-specific.
• Always document your variables with description to clarify intent.