How to conditionally populate an argument value in terraform? - terraform

Question

I am writing a Terraform script to spin up resources in Google Cloud Platform.
Some resources require one argument only if the other one set, how to populate one argument only if the other one is populated (or any other similar condition)?
For example:
resource "google_compute_router" "compute_router" {
name = "my-router"
network = "${google_compute_network.foobar.name}"
bgp {
asn = 64514
advertise_mode = "CUSTOM"
advertised_groups = ["ALL_SUBNETS"]
advertised_ip_ranges {
range = "1.2.3.4"
}
advertised_ip_ranges {
range = "6.7.0.0/16"
}
}
}
In the above resource (google_compute_router) the description for both advertised_groups and advertised_ip_ranges says This field can only be populated if advertise_mode is CUSTOM and is advertised to all peers of the router.
Now if I keep the value of advertise_mode as DEFAULT, my code looks something like below:
resource "google_compute_router" "compute_router" {
name = "my-router"
network = "${google_compute_network.foobar.name}"
bgp {
asn = 64514
#Changin only the value below
advertise_mode = "DEFAULT"
advertised_groups = ["ALL_SUBNETS"]
advertised_ip_ranges {
range = "1.2.3.4"
}
advertised_ip_ranges {
range = "6.7.0.0/16"
}
}
}
The above script however on running gives the following error:
* google_compute_router.compute_router_default: Error creating Router: googleapi: Error 400: Invalid value for field 'resource.bgp.advertiseMode': 'DEFAULT'. Router cannot have a custom advertisement configurati
on in default mode., invalid
As a workaround to the above, I have created two resources with different names doing almost the same thing. The script looks something like below:
resource "google_compute_router" "compute_router_default" {
count = "${var.advertise_mode == "DEFAULT" ? 1 : 0}"
name = "${var.router_name}"
region = "${var.region}"
network = "${var.network_name}"
bgp {
asn = "${var.asn}"
advertise_mode = "${var.advertise_mode}"
#Removed some codes from here
}
}
resource "google_compute_router" "compute_router_custom" {
count = "${var.advertise_mode == "CUSTOM" ? 1 : 0}"
name = "${var.router_name}"
region = "${var.region}"
network = "${var.network_name}"
bgp {
asn = "${var.asn}"
advertise_mode = "${var.advertise_mode}"
advertised_groups = ["${var.advertised_groups}"]
advertised_ip_ranges {
range = "${var.advertised_ip_range}"
description = "${var.advertised_ip_description}"
}
}
}
The above script works fine, however it seems like a lot of code repetition to me and a hack. Also, for two options (of dependent attributes) is fine, however, if there are more options say 5, the code repetition for such a small thing would be too much.
Is there a better way to do what I am trying to achieve?

Answer 1

This is pretty much what you are restricted to in Terraform < 0.12. Some resources allow you to use an empty string to omit basic values and the provider will interpret this as a null value, not passing it to the API endpoint so it won't complain about it not being set properly. But from my brief experience with the GCP provider this is not the case for most things there.
Terraform 0.12 introduces nullable arguments which would allow you to set these conditionally with something like the following:
variable "advertise_mode" {}
resource "google_compute_router" "compute_router" {
name = "my-router"
network = "${google_compute_network.foobar.name}"
bgp {
asn = 64514
advertise_mode = "${var.advertise_mode}"
advertised_groups = ["${var.advertise_mode == "DYNAMIC" ? ALL_SUBNETS : null}"]
advertised_ip_ranges {
range = "${var.advertise_mode == "DYNAMIC" ? 1.2.3.4 : null}"
}
advertised_ip_ranges {
range = "${var.advertise_mode == "DYNAMIC" ? 6.7.0.0/16 : null}"
}
}
}
It will also introduce dynamic blocks that you are able to loop over so you can also have a dynamic number of advertised_ip_ranges blocks.

Answer 2

The above answer is incorrect as 'advertised_ip_ranges' wont accept a null value; the solution to this is to leverage a dynamic block which can handle a null value for this resource and further enables the resource to accept a variable number of ip ranges.
variable custom_ranges {
default = ["172.16.31.0/24","172.16.32.0/24"]
}
resource "google_compute_router" "router_01" {
name = "cr-bgp-${var.gcp_bgp_asn}"
region = var.gcp_region
project = var.gcp_project
network = var.gcp_network
bgp {
asn = var.gcp_bgp_asn
advertise_mode = var.advertise_mode
advertised_groups = var.advertise_mode == "CUSTOM" ? ["ALL_SUBNETS"] : null
dynamic "advertised_ip_ranges" {
for_each = var.advertise_mode == "CUSTOM" ? var.custom_ranges : []
content {
range = advertised_ip_ranges.value
}
}
}
}
additional search keys: google_compute_router "bgp.0.advertised_ip_ranges.0.range" wont accept a null value.