Variables, tfvars, and Modular Patterns

Last updated on 2025-11-30 | Edit this page

Overview

Questions

  • How do variables improve Terraform configuration?
  • What is the role of terraform.tfvars?
  • Why do modules make infrastructure clearer and easier to manage?

Objectives

  • Define Terraform variables and describe their purpose.
  • Use terraform.tfvars to store environment-specific values.
  • Understand how modules group related infrastructure.
  • Interpret a Terraform plan that references module outputs.

Why Variables Matter

Terraform variables help you avoid hard-coding values.
Instead of embedding values directly in a resource, you define them once in a separate file and reference them everywhere.

Example variable definition (do NOT include backticks when pasting into Workbench):

variable “instance_type” { description = “EC2 instance size” type = string default = “t3.large” }

Referenced in a resource:

instance_type = var.instance_type

This keeps your configuration clean, reusable, and portable.

Using terraform.tfvars

terraform.tfvars lets you override defaults without editing your Terraform source files.

Example contents:

instance_type = “t3.xlarge” ssh_cidr = “91.196.220.29/32”

Terraform automatically loads these values when you run:

terraform plan

Why Use Modules?

As your infrastructure grows, a single main.tf becomes cluttered.
Modules organize related resources into reusable folders.

Example structure:

modules/ dataverse_ec2/ main.tf variables.tf

Your root configuration stays clean:

module “dataverse_ec2” { source = “./modules/dataverse_ec2” instance_ami = var.instance_ami instance_type = var.instance_type key_pair_name = var.key_pair_name ssh_cidr = var.ssh_cidr instance_name = var.instance_name root_volume_size = var.root_volume_size }

Terraform passes all variables into the module at plan time.

Putting It All Together

When terraform plan runs, Terraform:

  1. Loads variables from variables.tf
  2. Loads overrides from terraform.tfvars
  3. Injects them into the module
  4. Builds a dependency graph
  5. Produces a plan showing expected changes

This is the core of reproducible infrastructure.

Challenge

Try It

  1. Change your EC2 instance type in terraform.tfvars
  2. Run terraform plan
  3. Observe what Terraform proposes
  4. Revert your change when done

Terraform should show an in-place update if the instance type supports resizing.
If not, Terraform will propose a destroy-and-recreate operation.

Key Points
  • Variables avoid hard-coded values and keep Terraform flexible.
  • terraform.tfvars stores environment-specific settings.
  • Modules organize infrastructure into logical components.
  • Terraform wires everything together at plan time.