Dependency inversion with modules in Terraform - Mini Project: Build & Apply

Dependency inversion with modules

📖 Scenario: You are building a Terraform configuration to deploy cloud infrastructure. You want to organize your code using modules so that your main configuration depends on module outputs, not on internal details. This approach helps keep your code clean and flexible.

🎯 Goal: Create a Terraform project with a root module and a child module. The root module will use the child module's output to configure a resource, demonstrating dependency inversion by relying on module outputs instead of internal variables.

📋 What You'll Learn

Create a child module with a variable and an output

Create a root module that calls the child module

Use the child module's output in the root module to configure a resource

Follow Terraform best practices for module usage and dependency inversion

💡 Why This Matters

🌍 Real World

Organizing Terraform code into modules with clear inputs and outputs is a common practice in real-world cloud infrastructure projects. It helps teams manage complexity and reuse code.

💼 Career

Understanding dependency inversion with Terraform modules is important for cloud engineers and DevOps professionals to build scalable, maintainable infrastructure as code.

1

Create the child module with a variable and output

Create a Terraform module in a folder called child_module. Inside it, create a file variables.tf with a variable called instance_name of type string. Then create a file outputs.tf with an output called instance_name_output that outputs the value of the variable instance_name.

Terraform

# Create variable 'instance_name' of type string in variables.tf
# Create output 'instance_name_output' that outputs variable 'instance_name' in outputs.tf

Need a hint?

Define a variable block with the exact name instance_name and type string. Then define an output block named instance_name_output that returns var.instance_name.

2

Create the root module and call the child module

In the root module, create a main.tf file. Add a module block named child that uses the source ./child_module. Pass the string "my-instance" to the module variable instance_name.

Terraform

variable "instance_name" {
  type = string
}

output "instance_name_output" {
  value = var.instance_name
}

# Add module block named 'child' with source './child_module' and pass instance_name = "my-instance"

Need a hint?

Use a module block with the exact name child. Set source to ./child_module and pass instance_name = "my-instance".

3

Use the child module output in the root module resource

In the root module main.tf, add a resource block of type null_resource named example. Set the resource's triggers argument to a map with a key instance_name and value from the child module output module.child.instance_name_output.

Terraform

variable "instance_name" {
  type = string
}

output "instance_name_output" {
  value = var.instance_name
}

module "child" {
  source        = "./child_module"
  instance_name = "my-instance"
}

# Add null_resource 'example' with triggers using module.child.instance_name_output

Need a hint?

Create a null_resource named example. Inside, set triggers to a map with key instance_name and value module.child.instance_name_output.

4

Add output in root module to expose child module output

In the root module main.tf, add an output block named root_instance_name that outputs the value of module.child.instance_name_output.

Terraform

variable "instance_name" {
  type = string
}

output "instance_name_output" {
  value = var.instance_name
}

module "child" {
  source        = "./child_module"
  instance_name = "my-instance"
}

resource "null_resource" "example" {
  triggers = {
    instance_name = module.child.instance_name_output
  }
}

# Add output 'root_instance_name' that outputs module.child.instance_name_output

Need a hint?

Define an output block named root_instance_name that returns module.child.instance_name_output.