[BUG] terraform plan replaces compute instance instead of doing nothing

[vparpoil]

Describe the bug

I deployed an openstack_compute_instance_v2 with image_name = "Ubuntu 22.04" a few days ago.
OVH deprecated the image on 18/04/2024 (it's shown in ovh manager UI: Ubuntu 22.04 - deprecated - 2024-04-18)
Running terraform now causes a replacement of the compute instance, which is not the expected behavior

This is a blocking on using terraform to deploy and maintain OVH infra on the long run

Terraform Version

Terraform v1.7.1

OVH Terraform Provider Version

• Reusing previous version of terraform-provider-openstack/openstack from the dependency lock file
• Reusing previous version of ovh/ovh from the dependency lock file
• Reusing previous version of hashicorp/aws from the dependency lock file
• Reusing previous version of hashicorp/google from the dependency lock file
• Using previously-installed hashicorp/aws v4.67.0
• Using previously-installed hashicorp/google v5.25.0
• Using previously-installed terraform-provider-openstack/openstack v1.42.0
• Using previously-installed ovh/ovh v0.42.0

Affected Resource(s)

Please list the resources as a list, for example:

• openstack_compute_instance_v2

Terraform Configuration Files

resource "openstack_compute_instance_v2" "INSTANCE_NAME" {
  count       = 1
  name        = "INSTANCE_NAME"
  key_pair    = openstack_compute_keypair_v2.my_sskkey.name
  flavor_name = "b3-8"
  image_name  = "Ubuntu 22.04"
  region      = "SBG7"
  network {
    name = "Ext-Net"
  }
  network {
    name = ovh_cloud_project_network_private.my_network.name
  }
  /*metadata = {
    "ovh-monthly-instance" = 1
  }*/
}

Debug Output

terraform plan gives

  # openstack_compute_instance_v2.INSTANCE_NAME[0] must be replaced
-/+ resource "openstack_compute_instance_v2" "INSTANCE_NAME" {
      ~ access_ip_v4        = "RETRACTED" -> (known after apply)
      ~ access_ip_v6        = "[RETRACTED]" -> (known after apply)
      ~ all_metadata        = {} -> (known after apply)
      ~ all_tags            = [] -> (known after apply)
      ~ availability_zone   = "nova" -> (known after apply)
      ~ flavor_id           = "945a6e11-2563-475b-bb92-c41fa5ef1020" -> (known after apply)
      ~ id                  = "RETRACTED" -> (known after apply)
      ~ image_id            = "610542c0-65bb-4251-9225-44d73feea678" -> (known after apply)
      ~ image_name          = "Image not found" -> "Ubuntu 22.04" # forces replacement
        name                = "INSTANCE_NAME"
      ~ security_groups     = [
          - "default",
        ] -> (known after apply)
      - tags                = [] -> null
        # (6 unchanged attributes hidden)

      ~ network {
          ~ fixed_ip_v4    = "RETRACTED" -> (known after apply)
          ~ fixed_ip_v6    = "[RETRACTED]" -> (known after apply)
          + floating_ip    = (known after apply)
          ~ mac            = "RETRACTED" -> (known after apply)
            name           = "Ext-Net"
          + port           = (known after apply)
          ~ uuid           = "RETRACTED" -> (known after apply)
            # (1 unchanged attribute hidden)
        }
      ~ network {
          ~ fixed_ip_v4    = "10.3.153.10" -> (known after apply)
          + fixed_ip_v6    = (known after apply)
          + floating_ip    = (known after apply)
          ~ mac            = "RETRACTED" -> (known after apply)
            name           = "RETRACTED"
          + port           = (known after apply)
          ~ uuid           = "RETRACTED" -> (known after apply)
            # (1 unchanged attribute hidden)
        }
    }

Expected Behavior

Since no change has been made to terraform, neither to the instance configuration, there should be no change

[vparpoil]

On solution would be to update the images after deprecation for this :

  image_name  = "Image not found"

But this is not satisfying

[amstuta]

Hello @vparpoil,

Indeed the image name can be updated to indicate deprecations for example. To solve your issue, you should not rely on the image_name but only on the image_id, which is immutable.

You should use image_name at instance creation but ignore it afterwards using ignore_changes (https://developer.hashicorp.com/terraform/language/meta-arguments/lifecycle#ignore_changes).

[vparpoil]

Hi @amstuta ,
Thanks for your answer. Indeed, ignore_changes solves my issue.
I think those best practices should be highlighted in the docs , I didn't came with image_name on my own ;)

[amstuta]

Great :) You're right, the info has been raised to the right team and they will update the docs.