🐶
Terraform

Terraform: Ignore Attribute Changes in Blocks

By Filip on 10/09/2024

Learn how to leverage lifecycle meta-arguments in Terraform to effectively ignore changes in specific attributes within resource blocks, ensuring smooth deployments even with dynamic values.

Terraform: Ignore Attribute Changes in Blocks

Table of Contents

Introduction

In Terraform, you can instruct the provider to disregard changes made to specific attributes within a resource block. This is particularly useful when you have attributes that might be modified outside of your Terraform management, and you want to prevent Terraform from reverting those changes.

Step-by-Step Guide

To ignore changes to a specific attribute within a block in Terraform, you'll use the lifecycle block with the ignore_changes argument.

Let's say you have an Azure web app and want to ignore changes to the scm_type attribute within the site_config block. Here's how you'd do it:

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

  site_config {
    # ... other configurations ...
  }

  lifecycle {
    ignore_changes = [site_config.0.scm_type]
  }
}

In this example:

  • lifecycle: This block is used to control the behavior of a resource during its lifecycle.
  • ignore_changes: This argument takes a list of attribute names that Terraform should ignore changes to.
  • site_config.0.scm_type: This specifies the exact attribute to ignore. The 0 represents the first element in the site_config block (if it were a list).

Important Considerations:

  • Specificity: You need to provide the full path to the attribute within the block, including any list indices if applicable.
  • Trade-offs: Using ignore_changes means Terraform won't manage that attribute anymore. Any changes made outside of Terraform will persist, and Terraform won't revert them.
  • Alternatives: Before using ignore_changes, consider if there are alternative solutions like using variables, dynamic blocks, or adjusting your workflow to avoid external modifications.

If you need to ignore changes to an attribute within nested blocks or lists of blocks, the syntax becomes more complex and might require workarounds. Refer to the provided resources for examples and discussions on handling these scenarios.

Code Example

This Terraform code defines infrastructure on Azure. It creates a resource group, an app service plan, and an app service. The app service is configured to ignore changes to the scm_type attribute within its site_config. This means if the scm_type is manually changed outside of Terraform, Terraform will not revert it.

# Configure the Azure provider
terraform {
  required_providers {
    azurerm = {
      source  = "hashicorp/azurerm"
      version = "~> 3.0"
    }
  }
}

# Create a resource group
resource "azurerm_resource_group" "example" {
  name     = "example-resources"
  location = "West Europe"
}

# Create an App Service Plan
resource "azurerm_app_service_plan" "example" {
  name                = "example-app-service-plan"
  location            = azurerm_resource_group.example.location
  resource_group_name = azurerm_resource_group.example.name
  sku {
    tier = "Free"
    size = "F1"
  }
}

# Create an App Service 
resource "azurerm_app_service" "example" {
  name                = "example-app-service"
  location            = azurerm_resource_group.example.location
  resource_group_name = azurerm_resource_group.example.name
  app_service_plan_id = azurerm_app_service_plan.example.id

  site_config {
    scm_type = "LocalGit" # Initial value, changes will be ignored
    # ... other site configurations ...
  }

  lifecycle {
    ignore_changes = [site_config.0.scm_type]
  }
}

Explanation:

  1. Resource Setup: The code first sets up the necessary Azure resources: a resource group, an app service plan, and an app service.
  2. site_config Block: Inside the azurerm_app_service resource, the site_config block is where you define settings related to your web app. Here, scm_type is initially set to "LocalGit".
  3. lifecycle Block: The lifecycle block is added to the azurerm_app_service resource.
  4. ignore_changes Argument: Within the lifecycle block, ignore_changes is used to specify the attribute to ignore changes to. site_config.0.scm_type targets the scm_type attribute within the first (and in this case, only) element of the site_config block.

How it Works:

  • When you apply this Terraform code for the first time, it will create the Azure web app with scm_type set to "LocalGit".
  • If you later manually change the scm_type in the Azure portal or through other means, Terraform will not revert it back to "LocalGit" during subsequent runs. This is because you've instructed Terraform to ignore changes to that specific attribute.

Caution:

  • Be mindful that using ignore_changes can lead to configuration drift between your Terraform code and the actual infrastructure.
  • Only use it when you have a clear understanding of the implications and when alternative solutions are not suitable.

Additional Notes

When to Use ignore_changes:

  • External Modifications: The primary use case is when you anticipate external systems or processes might modify specific attributes of your infrastructure.
  • Temporary Overrides: You might use it temporarily while troubleshooting or during a transition phase where you don't want Terraform to interfere with manual changes.
  • Provider Limitations: In some cases, providers might not fully support drift detection for certain attributes, making ignore_changes a workaround.

Best Practices:

  • Document Thoroughly: Always document why you're using ignore_changes for a particular attribute. This helps avoid confusion and ensures others understand the reasoning.
  • Use Sparingly: Avoid overusing ignore_changes. It should be an exception, not the rule. Excessive use can lead to significant configuration drift and make your infrastructure harder to manage with Terraform.
  • Explore Alternatives: Before resorting to ignore_changes, thoroughly investigate alternative solutions like:
    • Variables: If the changing value can be parameterized, use variables to manage it.
    • Dynamic Blocks: For more complex scenarios, dynamic blocks offer flexibility in defining resource configurations.
    • Workflow Adjustments: Review your workflow and identify if changes can be made through Terraform instead of external tools.

Additional Considerations:

  • State Drift: Using ignore_changes increases the risk of state drift. Regularly reconcile your Terraform state with the actual infrastructure to identify and address discrepancies.
  • Debugging: Debugging issues related to ignore_changes can be tricky. Ensure you have proper logging and monitoring in place to track changes and identify the source of any problems.
  • Terraform Version: The behavior and syntax related to ignore_changes might vary slightly across different Terraform versions. Always refer to the documentation for the specific version you're using.

Summary

This table summarizes how to use the lifecycle block with the ignore_changes argument in Terraform to prevent Terraform from managing specific attributes within resource blocks:

Feature Description Example
Purpose Prevents Terraform from managing changes to a specific attribute within a resource block. Ignore changes to the scm_type attribute within the site_config block of an azurerm_app_service resource.
Syntax terraform <br> resource "resource_type" "resource_name" { <br> ... <br> lifecycle { <br> ignore_changes = [ "block_name.attribute_name" ] <br> } <br> } terraform <br> resource "azurerm_app_service" "example" { <br> ... <br> site_config { <br> ... <br> } <br> lifecycle { <br> ignore_changes = [ "site_config.0.scm_type" ] <br> } <br> }
Specificity The full path to the attribute, including block names and list indices (if applicable), must be provided. site_config.0.scm_type refers to the scm_type attribute within the first element (index 0) of the site_config block.
Trade-offs Terraform will not manage the specified attribute. Any external changes will persist and won't be reverted by Terraform.
Alternatives - Using variables
- Dynamic blocks
- Adjusting workflows to avoid external modifications
Complex Scenarios Handling nested blocks or lists of blocks within ignore_changes can be complex and may require workarounds. Refer to Terraform documentation and community resources for guidance.

Note: Using ignore_changes should be carefully considered. It can lead to configuration drift and make it harder to manage your infrastructure with Terraform.

Conclusion

The lifecycle block in Terraform, specifically the ignore_changes argument, provides a way to manage situations where external modifications to your infrastructure might conflict with your Terraform state. By specifying attributes to ignore, you instruct Terraform not to revert changes made outside its control. However, this powerful feature should be used judiciously. Overreliance on ignore_changes can lead to configuration drift and make your infrastructure harder to manage with Terraform. Always explore alternative solutions like variables, dynamic blocks, or workflow adjustments before resorting to ignoring changes. When you do use ignore_changes, ensure thorough documentation and a clear understanding of the potential implications to maintain predictability and manageability of your infrastructure.

References

Were You Able to Follow the Instructions?

😍Love it!
😊Yes
😐Meh-gical
😞No
🤮Clickbait

Related posts

Terraform: Combine Variables and Strings in Configurations

Learn how to seamlessly combine Terraform variables with strings to create dynamic and reusable infrastructure configurations.

Terraform if else elsif: Conditional Logic Guide

Learn how to use if, else, and elsif conditional statements in Terraform to create dynamic and flexible infrastructure configurations.

Terraform State Locked: How to Unlock It Safely

Learn how to troubleshoot and safely unlock your Terraform state file to resume infrastructure management.