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Terraform

Terraform Nested Variables: A Comprehensive Guide

By Filip on 10/09/2024

Learn how to use Terraform variables within variables to create dynamic and reusable infrastructure configurations.

Terraform Nested Variables: A Comprehensive Guide

Table of Contents

Introduction

You might be tempted to nest Terraform variables for dynamic naming or configuration, but Terraform doesn't directly support this. Trying to use a variable's value within the default argument of another variable definition won't work due to Terraform's variable resolution order. Let's explore how to achieve this common use case correctly using Terraform's locals block.

Step-by-Step Guide

You can't directly nest Terraform variables within each other. Trying to use ${var.some_variable} inside the default argument of another variable definition won't work. Terraform evaluates variables in a specific order, and it can't resolve a variable that depends on another variable that hasn't been defined yet.

Here's how to achieve the desired outcome:

  1. Define Base Variables: Start by defining the fundamental variables that other values might depend on.

    variable "environment" {
  type = string
  default = "dev" 
}

  2. Use Locals for Derived Values: Terraform's locals block is designed for creating values derived from other variables. This is where you combine or transform your base variables.

    locals {
  resource_name_prefix = "${var.environment}-myapp" 
}

  3. Reference Locals in Resources: Now you can use the calculated values stored in locals within your resource definitions.

    resource "aws_instance" "example" {
  tags = {
    Name = local.resource_name_prefix 
  }
}

Key Points:

  • Variable Resolution Order: Terraform resolves variables in stages. It can't use a variable's value before it's fully defined.
  • Locals for Flexibility: locals provide a way to create reusable values within your Terraform configuration, making it more readable and maintainable.
  • Alternatives:
    • Modules: If you have complex logic or need to reuse variable combinations across different parts of your infrastructure, consider using Terraform modules. You can pass variables into modules to customize their behavior.
    • Data Sources: If you need to fetch values dynamically from external systems or APIs, use Terraform data sources.

Example:

Let's say you want to create resource names that combine an environment, a project code, and a resource type.

variable "environment" {
  type = string
  default = "prod"
}

variable "project_code" {
  type = string
  default = "myproject"
}

locals {
  db_instance_name = "${var.environment}-${var.project_code}-database"
  web_server_name = "${var.environment}-${var.project_code}-webserver"
}

resource "aws_db_instance" "example" {
  identifier = local.db_instance_name
  # ... other configurations
}

resource "aws_instance" "web" {
  tags = {
    Name = local.web_server_name
  }
  # ... other configurations
}

In this example, db_instance_name and web_server_name are dynamically generated based on the values of environment and project_code.

Code Example

The Terraform code defines base variables for environment and project code and uses a locals block to create derived values for resource naming. It then uses these local values to define an AWS database instance and an AWS instance, ensuring consistent naming conventions across resources.

# Define base variables
variable "environment" {
  type    = string
  default = "dev"
}

variable "project_code" {
  type    = string
  default = "myapp"
}

# Use locals for derived values
locals {
  resource_name_prefix = "${var.environment}-${var.project_code}"
  db_instance_name     = "${local.resource_name_prefix}-database"
  web_server_name     = "${local.resource_name_prefix}-webserver"
}

# Define resources using local values
resource "aws_db_instance" "example" {
  identifier = local.db_instance_name
  # ... other database configurations
}

resource "aws_instance" "web" {
  ami           = "ami-0c94855ba95c574c9" # Replace with your desired AMI
  instance_type = "t2.micro"

  tags = {
    Name = local.web_server_name
  }

  # ... other instance configurations
}

Explanation:

  1. Base Variables: We define environment and project_code as base variables with default values.
  2. Locals Block: Inside the locals block:
    • resource_name_prefix combines the environment and project code.
    • db_instance_name and web_server_name use the resource_name_prefix to create consistent naming.
  3. Resource Definitions: The aws_db_instance and aws_instance resources use the values from locals to set their identifiers and tags.

How it Works:

  • Terraform first resolves the environment and project_code variables.
  • It then evaluates the locals block, using the resolved variable values.
  • Finally, it creates the resources, using the calculated values from locals.

This approach keeps your code organized, improves readability, and makes it easy to manage naming conventions across your infrastructure.

Additional Notes

Why this matters:

  • Avoid Errors: Trying to nest variables directly will lead to errors during Terraform plan/apply cycles. Understanding the order of operations is crucial.
  • Maintainability: Using locals makes your code cleaner and easier to understand, especially as your infrastructure grows.
  • Reusability: The logic within your locals block can be reused throughout your configuration, reducing redundancy.

Best Practices:

  • Clear Naming: Use descriptive names for both your variables and your locals to make their purpose obvious.
  • Comments: Add comments to explain any complex logic within your locals block.
  • Modularity: For very complex scenarios or logic you want to reuse across projects, consider creating reusable Terraform modules.

Beyond the Basics:

  • Conditional Logic: You can use conditional expressions within your locals block to create values dynamically based on conditions.
  • Functions: Terraform offers a wide range of built-in functions that you can use within locals to manipulate strings, lists, maps, and more.

Troubleshooting:

  • Terraform Output: Use terraform output to inspect the values of your variables and locals during development to ensure they are being calculated as expected.
  • Debugging: Terraform's debugging mode can provide more verbose output to help you track down issues with variable resolution.

Remember: While it might seem convenient to nest variables directly, using locals is the correct and more maintainable approach in Terraform.

Summary

This article explains how to manage variable dependencies in Terraform using locals.

Problem: You cannot directly nest Terraform variables. Using ${var.some_variable} within the default argument of another variable definition won't work due to Terraform's variable resolution order.

Solution: Utilize the locals block to create derived values from base variables.

Steps:

  1. Define Base Variables: Declare fundamental variables like environment with default values.

    variable "environment" {
  type = string
  default = "dev" 
}

  2. Use Locals for Derived Values: Create reusable, calculated values within the locals block using base variables.

    locals {
  resource_name_prefix = "${var.environment}-myapp" 
}

  3. Reference Locals in Resources: Utilize the calculated values stored in locals within your resource definitions.

    resource "aws_instance" "example" {
  tags = {
    Name = local.resource_name_prefix 
  }
}

Key Takeaways:

  • Variable Resolution Order: Terraform resolves variables in stages, preventing the use of a variable before its definition.
  • Locals for Flexibility: locals enhance code readability and maintainability by providing reusable values.

Alternatives for Complex Scenarios:

  • Modules: Use for complex logic or reusable variable combinations across infrastructure parts.
  • Data Sources: Utilize to fetch values dynamically from external systems or APIs.

Example:

The article provides a practical example of generating dynamic resource names using environment and project_code variables combined within locals. This demonstrates how to create consistent naming conventions across resources based on predefined variables.

Conclusion

By understanding Terraform's variable resolution process and leveraging the power of locals, you can write cleaner, more maintainable, and error-free infrastructure code. Remember that while nested variables might seem appealing, using locals is the correct and more robust approach for managing complex configurations in Terraform.

References

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