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Terraform

Terraform Maps: Using Variables as Keys

By Filip on 10/09/2024

This article explores the use of variable keys in Terraform maps, enhancing flexibility and dynamic resource provisioning in your infrastructure code.

Terraform Maps: Using Variables as Keys

Table of Contents

Introduction

In Terraform, while you can't directly use variables as keys within a map definition, there's a powerful workaround: dynamically constructing maps. This article will guide you through the process of using variables as keys in Terraform maps, enabling greater flexibility in your infrastructure as code.

Step-by-Step Guide

Let's break down how to work with variable keys in Terraform maps.

Understanding the Basics

  • Maps: In Terraform, a map is a data structure that stores values associated with specific keys. Think of it like a dictionary.
  • Variable Keys: You can't directly use a variable as a key within a map definition. Terraform expects literal strings for keys when you define the map structure.

The Solution: Dynamically Constructing Maps

The way to achieve the desired outcome is to build your map dynamically using Terraform functions and expressions. Here's a breakdown:

  1. Define Your Variables:

    variable "environment_name" {
  type = string
}

variable "tags_data" {
  type = map(string)
  default = {
    CostCentre = "C1234"
    Project    = "TerraformTest"
    Department = "Systems"
  }
}

  2. Create the Map with a local Block:

    locals {
  tags = {
    Environment = var.environment_name
    for k, v in var.tags_data : k => v
  }
}

Explanation

  • variable "environment_name": This defines a variable to hold your environment name.
  • variable "tags_data": This variable stores the rest of your tag data.
  • locals {}: The locals block is where you perform calculations and create values for use within your Terraform module.
  • tags = {}: Inside locals, you define a new map called tags.
  • Environment = var.environment_name: You directly assign the value of var.environment_name to the key "Environment".
  • for k, v in var.tags_data : k => v: This loop iterates through the tags_data map. For each key-value pair (k, v) in tags_data, it adds an entry to the tags map using the key (k) and value (v) from tags_data.

Using the Dynamic Map

Now you can use the local.tags map in your Terraform resources:

resource "aws_instance" "example" {
  # ... other resource configurations

  tags = local.tags 
}

Key Points

  • Flexibility: This approach gives you the flexibility to dynamically populate map keys based on variables.
  • Clarity: Using locals keeps your code organized and makes it clear how the final map is constructed.

Let me know if you'd like to explore more complex scenarios or have any other Terraform questions!

Code Example

The Terraform code defines variables for environment name and default tags. It uses a local block to dynamically create a tags map by combining the environment name and default tags. This map is then used to assign tags to an AWS instance resource, demonstrating how to dynamically generate tags in Terraform.

# Define variables for environment name and tag data
variable "environment_name" {
  type = string
  default = "prod" # Example default value
}

variable "tags_data" {
  type = map(string)
  default = {
    CostCentre = "C1234"
    Project    = "TerraformTest"
    Department = "Systems"
  }
}

# Create a local block to dynamically construct the tags map
locals {
  tags = {
    Environment = var.environment_name # Add environment name as a key

    # Iterate through the tags_data map and add each key-value pair to the tags map
    for k, v in var.tags_data : k => v 
  }
}

# Example resource using the dynamically generated tags
resource "aws_instance" "example" {
  # ... other instance configurations ...

  tags = local.tags # Assign the dynamically created tags to the instance
}

Explanation:

  1. Variables:

    • environment_name: Stores the environment name (e.g., "prod", "dev").
    • tags_data: Contains a map of default tags.

  2. locals Block:

    • tags = {}: A new map called tags is created within the locals block.
    • Environment = var.environment_name: The value of the environment_name variable is directly assigned to the key "Environment" in the tags map.
    • for k, v in var.tags_data : k => v: This loop iterates over each key-value pair (k, v) in the tags_data map and adds them to the tags map.

  3. Resource Usage:

    • The local.tags map is used to set the tags argument of the aws_instance resource.

How it Works:

  • The for loop is the key to dynamically building the map. It allows you to iterate through the tags_data map and include its contents in the final tags map.
  • By using locals, you create a reusable map (local.tags) that incorporates both the dynamic environment name and the default tag data.

To run this example:

  1. Save the code: Save the code as a .tf file (e.g., main.tf).
  2. Initialize Terraform: Run terraform init in your terminal.
  3. Apply the configuration: Run terraform apply.

This will create an AWS instance with the tags defined in the tags map, including the dynamically generated "Environment" tag.

Additional Notes

Beyond the Basics:

  • Nested Structures: You can use this technique to create maps with nested maps or lists, allowing for more complex data structures.
  • Conditional Logic: Combine this with Terraform's if statements or ternary operators to dynamically add or modify keys and values based on conditions.
  • Input Validation: While powerful, dynamic map construction can lead to errors. Consider adding input validation to your variables (e.g., using validation blocks) to ensure data consistency.

Alternative Approaches (Less Common):

  • Data Sources: In some cases, you might be able to fetch data from external sources (like a configuration file) that already has the desired key-value structure.
  • Modules: If you need to reuse this pattern frequently, consider encapsulating the logic within a Terraform module for better organization and reusability.

Best Practices:

  • Readability: While this approach offers flexibility, prioritize code readability. If a map becomes overly complex, consider breaking it down or using comments for clarity.
  • Testing: Thoroughly test your Terraform code, especially when using dynamic map construction, to catch potential issues early.

Real-World Examples:

  • Dynamically tagging resources based on environment, application, or team.
  • Creating flexible configuration maps for applications or services.
  • Building dynamic inventory structures for automation tools.

Summary

This article explains how to use variables as keys when defining maps in Terraform.

Problem: Terraform requires literal strings for map keys during definition, making it impossible to directly use variables.

Solution: Dynamically construct the map using a locals block and Terraform functions.

Steps:

  1. Define Variables:

    • Create variables for both the dynamic key (e.g., environment_name) and the remaining map data (e.g., tags_data).

  2. Construct the Map in locals:

    • Use a for loop to iterate through the existing map data (tags_data).
    • Assign the dynamic key (environment_name) directly.
    • Populate the new map (tags) with both the dynamic key-value pair and the iterated data.

Example:

variable "environment_name" { ... }
variable "tags_data" { ... }

locals {
  tags = {
    Environment = var.environment_name
    for k, v in var.tags_data : k => v
  }
}

Usage:

Reference the dynamically constructed map (local.tags) in your resources.

Benefits:

  • Flexibility: Dynamically populate map keys based on variables.
  • Clarity: Improved code organization and readability.

Conclusion

By dynamically constructing maps in a locals block, Terraform empowers you to use variables as keys, unlocking greater flexibility in managing your infrastructure as code. This approach, while requiring a bit more code, offers significant advantages in terms of adaptability and maintainability, especially for configurations involving dynamic tagging, environment-specific settings, or complex data structures. Remember to prioritize code readability and thorough testing when implementing this technique to ensure clarity and prevent potential issues.

References

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