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Terraform

Terraform Conditional Resource Creation with Variables

By Filip on 10/05/2024

Learn how to leverage Terraform's conditional logic to dynamically create resources based on variables defined in your .tfvars file, enabling flexible and configurable infrastructure deployments.

Terraform Conditional Resource Creation with Variables

Table of Contents

Introduction

In Terraform, you can conditionally create resources based on variables or expressions. This allows you to define infrastructure that adapts to different environments or configurations. This article explains how to use the count meta-argument with a ternary operator to conditionally create resources in Terraform. We'll illustrate this with an example and discuss its limitations, along with potential workarounds for more complex scenarios.

Step-by-Step Guide

To create resources in Terraform based on a condition, you can use the count meta-argument in combination with a ternary operator.

Let's say you have a variable create_resource in a .tfvars file that determines whether a resource should be created.

In your Terraform code, you would define your resource and set the count argument equal to a ternary expression. The expression would evaluate to 1 (creating one instance of the resource) if var.create_resource is true, and 0 (creating no instances) if it's false.

For example:

resource "example_resource" "example" {
 count = var.create_resource ? 1 : 0
  # ... other resource configuration ...
}

This approach is useful for simple conditional resource creation. However, Terraform doesn't directly support conditional loading of .tfvars files based on workspaces or environments.

For more complex scenarios, like conditionally creating resources within a for_each loop, you might need to explore alternative approaches such as using conditional logic within the resource configuration itself or restructuring your Terraform code.

Code Example

This code demonstrates conditional resource creation in Terraform by creating an AWS S3 bucket based on a variable. It defines a boolean variable "create_bucket" in variables.tf and sets its value to true in terraform.tfvars. The main.tf file uses this variable to conditionally create the S3 bucket using the count meta-argument. If "create_bucket" is true, one instance of the bucket is created; otherwise, none. The code also includes a random ID generation for bucket name uniqueness and a lifecycle rule to prevent accidental deletions.

This example demonstrates how to conditionally create an AWS S3 bucket based on the value of a variable defined in a .tfvars file.

1. Create a variables.tf file:

variable "create_bucket" {
  type = bool
  description = "Whether to create the S3 bucket"
}

2. Create a terraform.tfvars file:

create_bucket = true

3. Create a main.tf file:

terraform {
  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = "~> 4.0"
    }
  }
}

provider "aws" {
  region = "us-west-2"
}

resource "aws_s3_bucket" "example" {
  count = var.create_bucket ? 1 : 0
  bucket = "my-conditional-bucket-${random_id.bucket_suffix.hex}"

  # Default lifecycle rule to prevent accidental object deletion
  lifecycle_rule {
    enabled = true

    prefix = ""

    noncurrent_version_expiration {
      days = 30
    }
  }
}

resource "random_id" "bucket_suffix" {
  byte_length = 8
}

Explanation:

  • variables.tf: Defines a boolean variable create_bucket to control resource creation.
  • terraform.tfvars: Sets the create_bucket variable to true.
  • main.tf:
    • Defines an AWS provider.
    • Defines an aws_s3_bucket resource with the count meta-argument.
    • The count is set to 1 (create one instance) if var.create_bucket is true, and 0 (create no instances) if it's false.
    • A random ID is generated and appended to the bucket name to ensure uniqueness.
    • A default lifecycle rule is added to prevent accidental object deletion.

To apply the configuration:

  1. Run terraform init to initialize the working directory.
  2. Run terraform apply to create the resources.

To prevent the bucket from being created:

  1. Change the value of create_bucket to false in the terraform.tfvars file.
  2. Run terraform apply again. Terraform will detect the change and destroy the existing bucket.

This example demonstrates a simple use case for conditional resource creation in Terraform. For more complex scenarios, you might need to explore alternative approaches like conditional logic within resource configurations or restructuring your Terraform code.

Additional Notes

  • Alternatives to count: While count is suitable for simple conditions, more complex scenarios might benefit from:
    • Conditional logic within resources: Use for_each with dynamic expressions to iterate over maps or lists and create resources selectively.
    • Modules: Break down your infrastructure into reusable modules and conditionally deploy them using count or by passing boolean variables.
  • State management: Remember that changing the create_resource variable from true to false will trigger Terraform to destroy the resource. If you need to temporarily disable a resource without destroying it, consider using the lifecycle meta-argument with ignore_changes.
  • Limitations of .tfvars: Terraform doesn't natively support loading .tfvars files based on workspaces or environments. You might need to use tools or scripts to manage different variable sets for different environments.
  • Best practices:
    • Use clear and descriptive variable names to make your code easier to understand.
    • Add comments to explain the logic behind your conditional resource creation.
    • Test your code thoroughly to ensure that resources are created and destroyed as expected.
  • Beyond the basics: This approach can be extended to manage multiple resources based on the same condition or even use more complex expressions for decision-making.

Summary

Feature Description Notes
Conditional Resource Creation Create resources based on a condition. Useful for simple scenarios.
count meta-argument Determines the number of resource instances to create. Set to 1 to create one instance, 0 for none.
Ternary Operator condition ? value_if_true : value_if_false Evaluates the condition and returns the corresponding value.
Example count = var.create_resource ? 1 : 0 Creates one instance if var.create_resource is true, otherwise none.
Limitations No direct support for conditional .tfvars loading based on workspaces/environments.
Alternatives for Complex Scenarios
- Conditional Logic within Resource Use conditional expressions within the resource configuration.
- Code Restructuring Reorganize your Terraform code to achieve the desired conditional behavior.

Conclusion

In conclusion, Terraform provides powerful mechanisms for conditional resource creation, allowing you to tailor your infrastructure to different environments and configurations. Using the count meta-argument with a ternary operator offers a straightforward approach for simple conditions. However, as your infrastructure becomes more complex, exploring alternative methods like conditional logic within resources or leveraging modules becomes crucial. Understanding the limitations of .tfvars files and adhering to best practices for variable naming, commenting, and testing will further enhance the clarity, maintainability, and reliability of your Terraform code. By mastering these techniques, you can effectively manage the dynamic provisioning of your infrastructure and ensure its alignment with your evolving requirements.

References

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