Terraform's ignore_changes: A Comprehensive Guide

Terraform, an Infrastructure as Code (IaC) tool by HashiCorp, offers a multitude of features to manage cloud infrastructure seamlessly. Among its arsenal of capabilities is the ignore_changes attribute, a tool that can significantly enhance the stability and manageability of your infrastructure deployments. In this guide, we will delve into the intricacies of ignore_changes, explore its pros and cons, and provide best practices to help you use it effectively.

What is `ignore_changes` in Terraform?

In the realm of Terraform, managing resources involves comparing the desired state, defined in Terraform configurations, with the actual state in the cloud platform. Based on this comparison, Terraform performs actions like creating, updating, or deleting resources to converge the two states. However, not all changes are meaningful or desired. Enter ignore_changes.

Understanding `ignore_changes`

The ignore_changes attribute allows you to specify which attributes of a resource Terraform should ignore during its state comparisons. By doing so, you can prevent certain attributes from triggering updates, thereby maintaining stability and reducing unnecessary noise in your infrastructure deployments.

Practical Examples

Let’s dive into some practical examples to illustrate the use of ignore_changes.

Example 1: Ignoring Changes to Tags

Consider a scenario where you have an AWS S3 bucket defined in your Terraform configuration with tags that you want to remain unchanged:

resource "aws_s3_bucket" "example_bucket" {
  bucket = "example-bucket"
  tags   = {
    Name        = "ExampleBucket"
    Environment = "Production"
  }

  lifecycle {
    ignore_changes = [tags]
  }
}

In this example, the ignore_changes = [tags] line ensures that any changes to the tags attribute of the aws_s3_bucket resource will be ignored by Terraform.

Example 2: Ignoring Changes to Instance Type

For an AWS EC2 instance, you might want to ensure that the instance type remains unchanged:

resource "aws_instance" "example_instance" {
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t2.micro"

  lifecycle {
    ignore_changes = [instance_type]
  }
}

Here, the ignore_changes = [instance_type] line prevents Terraform from modifying the instance_type attribute of the aws_instance resource.

Pros and Cons of Using `ignore_changes`

Pros:

Stability: By ignoring specific changes, you maintain stability in your environment.
Reduced Noise: Prevent unnecessary updates caused by benign changes.
Predictable Behavior: Explicitly define what Terraform should ignore, ensuring consistent behavior.

Cons:

Risk of Ignoring Important Changes: Ignoring critical changes can lead to unintended consequences.
Maintenance Overhead: As your infrastructure evolves, revisit and adjust ignore_changes as needed.

Best Practices

Use Sparingly

Only ignore changes that won’t impact the functionality or security of your infrastructure.

Document Your Choices

Always document the reasons for using ignore_changes and specify which attributes are being ignored to maintain transparency and clarity.

Test Thoroughly

After applying ignore_changes, run terraform plan and terraform apply to validate that the desired attributes are being ignored without any unintended side effects.

Conclusion

Terraform’s ignore_changes attribute is a powerful tool that can help you manage and control your infrastructure configurations more effectively. By understanding its usage, pros and cons, and best practices, you can leverage ignore_changes to enhance the stability, control, and manageability of your Terraform-managed infrastructure. Remember to use ignore_changes judiciously and always test thoroughly to ensure a smooth and predictable infrastructure deployment process.

What is ignore_changes in Terraform?

Understanding ignore_changes