Terraform Conditional depending if a resource has been created

Question

I have created a compute module that has a conditional to create an external IP.

resource "google_compute_address" "external" {     
    count = "${var.EXT_IP_CREATE ? 1 : 0}"     
    name = "${var.NAME}-ext-ip"     
    address_type = "EXTERNAL"     
    region = "${var.REGION}"}

Within the compute instance resource block, I have the following network interface configuration:

network_interface {     
    network= "${var.NETWORK}"     
    network_ip = "${google_compute_address.internal.address}"         
    access_config {         
         nat_ip = "${var.EXT_IP_CREATE ? google_compute_address.external.address : 0 }"         
        }     
    }

If the resource google_compute_address.external has not been created, I need to set nat_ip to null or in other words 0.

That looks like it should work but it does not.

When setting EXT_IP_CREATE to true TF succeeds to create the resource. When setting it to false I receive the following error:

Error: Error running plan: 1 error(s) occurred:

* module.compute-dbma-dev.google_compute_instance.compute: 1 error(s) occurred:

* module.compute-dbma-dev.google_compute_instance.compute: Resource 'google_compute_address.external' not found for variable 'google_compute_address.external.address'

When I explicitly pass nat_ip = 0 TF recognizes the blank value and successfully creates the compute instances without the external IP.

Im currently on Terraform version Terraform v0.11. There is probably a super simple solution but I am just starting out with conditionals in TF and I getting stuck here.

Thanks in advance!

So your intention to do an optional argument with null is not possible before 0.12, but your real problem here is you need to set it for all of the network_interface. — Matt Schuchard

victor m victor m · Accepted Answer · 2019-03-13T19:30:07

Two ways to fix that:

TF_WARN_OUTPUT_ERRORS=1 terraform apply
${element(concat(google_compute_address.*.address, list("")), 0)}

Terraform Conditional depending if a resource has been created

2 Answers