Prevent Unwanted Updates in Terraform with ignore_changes

2 May 2024

In the dynamic world of DevOps, managing infrastructure as code using Terraform has become a staple. However, certain challenges such as preventing unintended updates to your infrastructure necessitate a deeper understanding of Terraform’s capabilities. One such capability is the ignore_changes argument in the lifecycle block of a resource. In this article, I will demystify this argument, providing insights and practical examples from my experiences as a Senior DevOps Engineer and Docker Captain.

What is the ignore_changes Argument?

Within Terraform, the ignore_changes argument is part of the lifecycle meta-argument, crucial for managing how resources react to configuration changes. By specifying certain attributes within this argument, you can shield these attributes from triggering any updates or recreation of the resource, thereby maintaining stability and consistency in your infrastructure.

Practical Scenarios for Using ignore_changes

The ignore_changes attribute proves invaluable in various scenarios:

1. Dynamic Attributes: For attributes prone to frequent change like timestamps or ephemeral IDs, using ignore_changes prevents unnecessary updates that could disrupt your operations.

2. Immutable Attributes: Certain attributes, once set, do not change and are best left untouched, such as the Amazon Resource Name (ARN). Using ignore_changes here avoids accidental modifications.

3. Metadata Preservation: Attributes that serve as metadata, such as creation timestamps or IDs, should not influence the functional configuration of resources. Ignoring these can enhance the consistency of your infrastructure without triggering needless updates.

4. Sensitive Information: For attributes containing sensitive data like passwords or keys, ignoring changes can safeguard against accidental leaks in your Terraform state files.

5. Externally Managed Resources: When resources or certain attributes are managed outside Terraform, ignore_changes can prevent conflicts and allow seamless integration with external management systems.

6. Tag Management: Ignoring tag changes can be crucial, especially when tags are managed through policies or other mechanisms independent of Terraform.

Implementing ignore_changes: Key Considerations

When implementing ignore_changes, remember:

• The attributes listed must exactly match those defined in the resource schema.
• ignore_changes operates at the attribute level; you must explicitly specify each attribute to be ignored.
• It is resource-specific and must be defined within each resource block where you wish to ignore changes.
• It cannot be applied to the lifecycle block itself or any other meta-arguments.

Examples of ignore_changes in Action

Example 1: Azure Virtual Machine

resource "azurerm_virtual_machine" "example" {
  name                  = "example-vm"
  location              = "UK South"
  resource_group_name   = azurerm_resource_group.example.name
  network_interface_ids = [azurerm_network_interface.example.id]
  vm_size               = "Standard_DS1_v2"

  storage_os_disk {
    name              = "example-os-disk"
    caching           = "ReadWrite"
    create_option     = "FromImage"
    managed_disk_type = "Premium_LRS"
  }

  os_profile {
    computer_name  = "examplevm"
    admin_username = "adminuser"
    admin_password = "3c19uA53FsTcLrB36g56"  # It's best to manage passwords via secrets management tools
  }

  lifecycle {
    ignore_changes = [
      network_interface_ids,
      storage_os_disk,
      os_profile[0].computer_name,
    ]
  }
}

Example 2: Ignoring All Changes

resource "azurerm_storage_account" "example" {
  name                     = "examplestorageaccount"
  resource_group_name      = azurerm_resource_group.example.name
  location                 = "East US"
  account_tier             = "Standard"
  account_replication_type = "LRS"

  lifecycle {
    ignore_changes = all
  }
}

Conclusion

Understanding and effectively utilizing the ignore_changes argument in Terraform can significantly enhance your infrastructure management practices. By selectively ignoring changes to specific attributes, you can ensure stability, security, and efficiency in your deployments. As Terraform evolves, staying updated with these nuances will empower you to manage your infrastructure more adeptly.

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