Passing Variable Data Between a Root and Child Module in Terraform

Introduction

When working with Terraform, it’s common to break infrastructure code into reusable modules. Passing variable data from a root module to a child module is essential for modularity and configuration flexibility. This article explains how to pass variables between a root and child module in Terraform.

Child Module Setup

In the child module, variables must be declared to accept incoming data from the root module. This is done using variable blocks:

# modules/example/variables.tf
variable "instance_type" {
  description = "The type of EC2 instance"
  type        = string
}

variable "instance_count" {
  description = "The number of instances to create"
  type        = number
}

These variables will be used within the child module’s configuration files.

Using Variables in the Child Module

Within the child module, reference the defined variables wherever they are needed:

# modules/example/main.tf
resource "aws_instance" "example" {
  instance_type = var.instance_type
  count         = var.instance_count

  ami           = "ami-12345678"
}

Passing Variables from the Root Module

In the root module, use the module block to call the child module and pass values to the child module’s variables:

# root/main.tf
module "example" {
  source         = "./modules/example"
  instance_type  = "t3.micro"
  instance_count = 2
}

The source argument specifies the location of the child module, and the other arguments pass data to the child module’s variables.

Validating the Setup

To ensure the data is passed correctly:

1. Run terraform init to initialize the configuration.

2. Run terraform plan to review the resources that will be created.

3. Run terraform apply to deploy the infrastructure.

Conclusion

Passing variables between a root and child module in Terraform keeps the code modular, flexible, and easy to manage. By properly defining and using variables, infrastructure can be configured consistently and reused across multiple environments.