Variables, Outputs, and Locals

# CHAPTER 6

Variables, Outputs, and Locals #

1. Introduction #

2. Learning Objectives #

• Define and use Input Variables (variable block) to parameterize configurations.

• Understand variable typing and validation.

• Extract useful data from Terraform using Output Values (output block).

• Utilize Local Values (locals block) to calculate and reuse internal logic.

• Pass variables securely via .tfvars files or command-line arguments.

3. Beginner-Friendly Explanation #

• The Input Variable: The buttons on the front. Do you want "Small" or "Large"? Do you want "Dark Roast" or "Light Roast"? The machine's internal mechanics don't change, but the *inputs* change the final result.

• The Output Value: The digital screen that says "Your coffee is ready at Spout 2." It gives you the information you need *after* the machine finishes its job.

• The Local Value: The machine's internal memory. It calculates "Small + Dark Roast = 30 seconds of brewing." You never see this calculation; it just helps the machine stay organized internally so it doesn't have to recalculate the math every time.

4. Input Variables (variable block) #

# Defining the variable
variable "server_size" {
  description = "The EC2 instance type"
  type        = string
  default     = "t2.micro" # Used if no value is provided
}

Now, in your main.tf, you replace the hardcoded string with the variable reference using the var. prefix:

resource "aws_instance" "web" {
  ami           = "ami-12345"
  instance_type = var.server_size # Dynamic!
}

5. Output Values (output block) #

output "website_url" {
  description = "The public IP address of the new server"
  value       = aws_instance.web.public_ip
}

When you run terraform apply, the very last thing the terminal will print is: Outputs: website_url = "192.168.1.50"

6. Mini Project: Build Reusable Infrastructure Template #

Step-by-Step Architecture Concept:

# 1. Input Variables
variable "env" {
  type = string
}
variable "project" {
  type = string
}

# 2. Local Values (Internal calculations)
locals {
  # We combine the inputs to create a standard prefix: e.g., "prod-billing-"
  name_prefix = "${var.env}-${var.project}-"
}

# 3. The Resource (Using the Local)
resource "aws_instance" "app" {
  ami           = "ami-xyz"
  instance_type = "t2.micro"
  
  tags = {
    # The server will be named: "prod-billing-web-server"
    Name = "${local.name_prefix}web-server" 
  }
}

7. How to Pass Variable Values #

env = "production"
project = "billing"
server_size = "t3.large"

Terraform automatically loads any file named terraform.tfvars and uses those values to run the code.

8. Real-World Scenarios #

9. Best Practices #

• Variable Validation: You can force Terraform to reject bad inputs. In your variable block, add a validation block to ensure serversize is only allowed to be "t2.micro" or "t3.large". If a developer tries to deploy an expensive "p3.16xlarge" server, Terraform will reject the code and halt the deployment immediately.

13. Interview Questions #

• Q: Differentiate the operational use cases for Input Variables (variable), Local Values (locals), and Output Values (output) within a Terraform module.

• Q: You have a variable that accepts an API token. How do you configure the HCL to ensure this token is never printed in plain text to the console during a terraform plan execution?

14. Summary #

15. Next Chapter Recommendation #