What are Modules

A module in OpenTofu (a Terraform fork) is any directory that contains configuration files. When you run OpenTofu commands inside that directory, it becomes the root module, orchestrating resources defined within.

Root Module Example

Suppose your workspace looks like this:

$ ls /root/opentofu-projects/aws-instance
main.tf  variables.tf

– main.tf

# /root/opentofu-projects/aws-instance/main.tf
resource "aws_instance" "webserver" {
  ami           = var.ami
  instance_type = var.instance_type
  key_name      = var.key
}

– variables.tf

# /root/opentofu-projects/aws-instance/variables.tf
variable "ami" {
  type        = string
  default     = "ami-0edab43b6fa892279"
  description = "Ubuntu AMI ID in the ca-central-1 region"
}

Running tofu init, tofu plan, or tofu apply inside aws-instance treats it as the root module.

Calling Child Modules

To avoid duplicating infrastructure code, package a directory as a child module and invoke it:

$ mkdir -p /root/opentofu-projects/development

Create a main.tf in development:

# /root/opentofu-projects/development/main.tf
module "dev-webserver" {
  source = "../aws-instance"
}

module "dev-webserver" assigns a logical name.
source = "../aws-instance" points to the child module’s path.

Now development is the root module, calling the ../aws-instance child module.

Building a Reusable Payroll App Module

FlexIT Consulting needs the same payroll stack in multiple regions. The architecture uses:

One EC2 instance (custom AMI)
One DynamoDB table
One S3 bucket

All resources live in the default VPC:

The image is a diagram of a simplified AWS architecture for FlexIT Consulting's payroll software, showing components like an AWS instance, S3 bucket, and DynamoDB table within a default VPC. It highlights aspects such as no IAM role considerations and default VPC and subnet usage.

Define the Module

Organize reusable code under modules/payroll-app:

$ mkdir -p /root/opentofu-projects/modules/payroll-app
$ ls /root/opentofu-projects/modules/payroll-app
app_server.tf  dynamodb_table.tf  s3_bucket.tf  variables.tf

app_server.tf

# modules/payroll-app/app_server.tf
resource "aws_instance" "app_server" {
  ami           = var.ami
  instance_type = "t2.medium"
  tags = {
    Name = "${var.app_region}-app-server"
  }
  depends_on = [
    aws_dynamodb_table.payroll_db,
    aws_s3_bucket.payroll_data
  ]
}

s3_bucket.tf

# modules/payroll-app/s3_bucket.tf
resource "aws_s3_bucket" "payroll_data" {
  bucket = "${var.app_region}-${var.bucket}"
}

dynamodb_table.tf

# modules/payroll-app/dynamodb_table.tf
resource "aws_dynamodb_table" "payroll_db" {
  name         = "user_data"
  billing_mode = "PAY_PER_REQUEST"
  hash_key     = "EmployeeID"

  attribute {
    name = "EmployeeID"
    type = "S"
  }
}

variables.tf

# modules/payroll-app/variables.tf
variable "app_region" {
  type = string
}

variable "bucket" {
  type    = string
  default = "flexit-payroll-alpha-22001c"
}

variable "ami" {
  type = string
}

Hardcoded: instance type, DynamoDB table name, and hash key.
Configurable: AMI, region, bucket via variables.

Deploy in US East (us-east-1)

Create a root module for the US deployment:

$ mkdir /root/opentofu-projects/us-payroll-app

– provider.tf

# /root/opentofu-projects/us-payroll-app/provider.tf
provider "aws" {
  region = "us-east-1"
}

– main.tf

# /root/opentofu-projects/us-payroll-app/main.tf
module "us_payroll" {
  source     = "../modules/payroll-app"
  app_region = "us-east-1"
  ami        = "ami-24e140119877avm"
}

Initialize and apply:

$ cd /root/opentofu-projects/us-payroll-app
$ tofu init

$ tofu apply

You’ll see:

module.us_payroll.aws_dynamodb_table.payroll_db will be created
module.us_payroll.aws_instance.app_server     will be created
module.us_payroll.aws_s3_bucket.payroll_data will be created

The S3 bucket name combines the region prefix with the default bucket variable.

Deploy in London (eu-west-2)

Repeat for the UK region:

$ mkdir /root/opentofu-projects/uk-payroll-app

– provider.tf

# /root/opentofu-projects/uk-payroll-app/provider.tf
provider "aws" {
  region = "eu-west-2"
}

– main.tf

# /root/opentofu-projects/uk-payroll-app/main.tf
module "uk_payroll" {
  source     = "../modules/payroll-app"
  app_region = "eu-west-2"
  ami        = "ami-35e140119877avm"
}

$ cd /root/opentofu-projects/uk-payroll-app
$ tofu init && tofu apply

Resources provisioned under:

module.uk_payroll.aws_instance.app_server
module.uk_payroll.aws_s3_bucket.payroll_data
module.uk_payroll.aws_dynamodb_table.payroll_db

OpenTofu can source community or verified modules from the registry, just like Terraform. For example, to provision a security group:

The image shows a search interface from the OpenTofu Registry, displaying results for "security-group" modules, including details about a Terraform module for creating EC2-VPC security groups on AWS.

module "security_group_ssh" {
  source              = "terraform-aws-modules/security-group/aws/modules/ssh"
  version             = "3.16.0"
  vpc_id              = "vpc-7d8d215"
  ingress_cidr_blocks = ["10.10.0.0/16"]
  name                = "ssh-access"
}

Always pin the version to prevent unexpected module changes. Use tofu get or tofu init to fetch registry modules.

The image is an infographic titled "OpenTofu Module" highlighting the benefits of using modules, including simpler configuration files, lower risk, and reusability.

Benefit	Description
Simpler configs	Keep root modules concise for easier maintenance
Reusability	Share the same module across multiple projects
Stability	Enforce default settings and reduce configuration drift
Reduced errors	Leverage tested modules from your team or the community registry

Course Introduction

Introduction to Infrastructure as Code

OpenTofu Basics

OpenTofu State

Working with OpenTofu

Remote State

OpenTofu Provisioners

OpenTofu Import, Tainting Resources and Deubugging

OpenTofu Modules

OpenTofu Functions and Conditional Expressions

OpenTofu: Beyond Basics

Course Conclusion

Getting Started with OpenTofu

Root Module Example

Calling Child Modules

Building a Reusable Payroll App Module

Define the Module

app_server.tf

s3_bucket.tf

dynamodb_table.tf

variables.tf

Deploy in US East (us-east-1)

Deploy in London (eu-west-2)

Links and References

Watch Video

​Root Module Example

​Calling Child Modules

​Building a Reusable Payroll App Module

​Define the Module

​app_server.tf

​s3_bucket.tf

​dynamodb_table.tf

​variables.tf

​Deploy in US East (us-east-1)

​Deploy in London (eu-west-2)

​Links and References

Watch Video

Root Module Example

Calling Child Modules

Building a Reusable Payroll App Module

Define the Module

app_server.tf

s3_bucket.tf

dynamodb_table.tf

variables.tf

Deploy in US East (us-east-1)

Deploy in London (eu-west-2)

Links and References