How to Reference a Resource Created by a Terraform Module

TLDR

To reference a resource created by a Terraform module, use the module's output values. Define outputs in the module and access them using the module.<module_name>.<output_name> syntax.

---

Terraform modules encapsulate resources and logic, making configurations reusable and modular. Referencing resources created by a module is essential for integrating modules into larger configurations. This guide explains how to achieve this with practical examples.

Step 1: Define Outputs in the Module

Start by defining output values in the module to expose the attributes of the resources you want to reference.

Example (Module Code)

resource "aws_instance" "example" {
  ami           = "ami-12345678"
  instance_type = "t2.micro"
  tags = {
    Name = "example-instance"
  }
}

output "instance_id" {
  value = aws_instance.example.id
}

Explanation

• resource "aws_instance" "example": Creates an EC2 instance.
• output "instance_id": Exposes the instance ID as an output value.

Step 2: Call the Module

Use the module in your root configuration and access its outputs.

Example (Root Configuration)

module "ec2_instance" {
  source = "./modules/ec2"
}

output "instance_id" {
  value = module.ec2_instance.instance_id
}

Explanation

• module "ec2_instance": Calls the module.
• module.ec2_instance.instance_id: References the instance_id output from the module.

Step 3: Use the Referenced Resource

Use the referenced resource in other parts of your configuration.

Example

resource "aws_security_group_rule" "allow_ssh" {
  type        = "ingress"
  from_port   = 22
  to_port     = 22
  protocol    = "tcp"
  cidr_blocks = ["0.0.0.0/0"]
  security_group_id = module.ec2_instance.instance_id
}

Explanation

• module.ec2_instance.instance_id: Provides the instance ID for the security group rule.

Best Practices

• Use Descriptive Output Names: Clearly indicate the purpose of each output.
• Document Outputs: Provide clear documentation for module outputs.
• Validate Outputs: Ensure outputs are correctly defined and used.

By following these steps, you can effectively reference resources created by Terraform modules, enabling seamless integration and modular configurations.