Commenting out optional parameter is not properly reflected in TF plan (DCNE-126)

[FrederikSuijs]

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Terraform Version

terraform -v Terraform v0.14.5 provider registry.terraform.io/ciscodevnet/aci v0.5.4

APIC version and APIC Platform

• v4.1.1 and all

Affected Resource(s)

• aci_tenant
• aci_vrf
• aci_???

I have only tested with these two resources, but I am expecting all aci resources are affected by this behavior. I am trying to write some of my own aci resources and observe similar behavior.

Terraform Configuration Files

terraform {
  required_providers {
    aci = {
      source  = "CiscoDevNet/aci"
      version = "0.5.4"
    }
  }
}

provider "aci" {
 username = "admin"
 password = "ciscopsdt"
 url      = "https://sandboxapicdc.cisco.com"
 insecure = true
}

Debug Output

https://gist.github.com/FrederikSuijs/ff3f31f4ea27151b70dae69135b0f11f

Panic Output

Expected Behavior

After commenting out the description field during step 3, the plan should fall back to the default behavior. In this case the description field should be updated to an empty string, to be consistent to the state after step 1.

Actual Behavior

Nothing happens. The Terraform plan/apply seems to calculate a change is not required.

Steps to Reproduce

1. Create resources

   
   resource "aci_tenant" "debug_tenant" {
   //description = "created from terraform"
   name = "debug_tenant"
   }

resource "aci_vrf" "debug_vrf" { //description = "created from terraform" name = "debug_vrf" tenant_dn = aci_tenant.debug_tenant.id }

`terraform apply`

2. Uncomment description
```hcl
resource "aci_tenant" "debug_tenant" {
    description = "created from terraform"
    name = "debug_tenant"
}

resource "aci_vrf" "debug_vrf" {
    description = "created from terraform"
    name = "debug_vrf"
    tenant_dn = aci_tenant.debug_tenant.id
}

terraform apply

3. Recomment description

   
   resource "aci_tenant" "debug_tenant" {
   //description = "created from terraform"
   name = "debug_tenant"
   }

resource "aci_vrf" "debug_vrf" { //description = "created from terraform" name = "debug_vrf" tenant_dn = aci_tenant.debug_tenant.id }

`terraform apply`

### Important Factoids

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* #0000

[lhercot]

This is the normal behavior. As a generic guideline, we do not define any default values for fields in the provider or the SDK. If you want to reset any field to its default value, you have to modify it yourself. This is to avoid re-writing fields that the provider might not be managing yet.

[andbyrne]

This also impacts object relationships. For example in an "aci_leaf_access_bundle_policy_group" resource you can optionally specify a relationship to a policy such as "relation_infra_rs_cdp_if_pol" and it will create that relationship.

Later if you change that value to null or an empty string, the relationship is not removed as one would expect. It is only possible to change the relationship to another valid policy.

Both the GUI and REST API support the removal of relationships. The inability to do this with the ACI provider is a concern for anyone wanting to use it for the ongoing management of objects vs their initial creation.

[akinross]

Resources created with plugin framework resolve this issue, however the plan output will not be rendered properly.

See details of render issue here: https://github.com/hashicorp/terraform/issues/31887#issuecomment-1958832973