Understanding Locals in Terraform: Best Practices and Usage Guide

In Terraform, locals serve as a way to define values that can be reused within your configuration. By computing values once and referencing them multiple times, locals help keep your code cleaner, more organized, and easier to maintain. They allow you to store intermediate results or constants that can be referenced throughout your configuration, reducing repetition and improving readability.

Key Takeaways About Locals:

1. What Are Locals?

   Locals are essentially values that exist only within the scope of the module where they’re defined. These are used for storing calculated values, constants, or temporary results that you may need to reuse at various points in your configuration. They’re not inputs or outputs but rather variables that are confined to the module.

2. How to Define Locals

   Locals are declared within the locals block. Here’s how to define them:

   locals {
     <local_variable_name> = <expression>
   }
               

   When Terraform processes the configuration, it calculates the expressions once and stores the result for future use.

3. Accessing Locals

   After you define a local variable, you can access it by referencing local.<local_variable_name>. This eliminates the need to repeat the same values or calculations multiple times, leading to cleaner, more concise code.

Example:

Here’s a sample locals.tf file that demonstrates how to use locals:

locals {
  instance_name = "web-server"
  tags = {
    Name        = local.instance_name
    Environment = "production"
  }
}

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

Breakdown:

• local.instance_name holds a reusable value, used in the tags for the aws_instance resource.
• local.tags is a map that contains the Name tag and the Environment tag, with Name dynamically set to local.instance_name.

Benefits of Using Locals:

• Reusability: Locals allow you to calculate a value once and use it in multiple places, reducing repetition and promoting the DRY (Don’t Repeat Yourself) principle.
• Improved Readability: By giving complex expressions clear, meaningful names, locals make your code easier to read and understand, even for others who might be working with it later.
• Simplified Logic: Instead of writing out the same complex expression repeatedly, you can calculate it once and reference it as needed, streamlining your configuration.

Real-World Example:

Let’s say you want to dynamically set the instance type for an AWS EC2 instance, but you want a fallback to a default value if the input variable isn’t provided. Locals are great for this:

locals {
  instance_type = var.instance_type != "" ? var.instance_type : "t2.micro"
}

resource "aws_instance" "example" {
  ami           = "ami-12345678"
  instance_type = local.instance_type
}
    

Explanation:

• local.instance_type will take the value of var.instance_type if provided. If not, it defaults to "t2.micro". This logic is calculated once, and then it’s reused when assigning the instance_type to the AWS instance resource.

When to Use Locals:

• To Eliminate Redundancy: If you have expressions that are used repeatedly, locals can help by calculating the result once and referencing it wherever needed.
• To Improve Code Readability: If an expression is complex or difficult to understand, locals can help simplify it by giving it a meaningful name, making your configuration easier to follow.
• For Storing Intermediate Results or Constants: Locals are perfect for holding values that you need throughout the configuration, such as computed values, constants, or default settings.

Conclusion:

Locals in Terraform are an incredibly useful tool for simplifying your infrastructure code. By storing values once and referring to them throughout your configuration, you reduce redundancy, improve code clarity, and make your Terraform scripts more maintainable—especially in large, complex projects. Whether you're managing complex calculations, default values, or reusable expressions, locals provide a clean, efficient way to structure your configuration and keep your infrastructure as code organized.