GCP Terraform: Setting port for backend service - terraform

Question

Reading through the docs here:
https://www.terraform.io/docs/providers/google/r/compute_backend_service.html
We can define backend service:
resource "google_compute_backend_service" "kubernetes-nginx-prod" {
name = "kubernetes-nginx-prod"
health_checks = [google_compute_health_check.kubernetes-nginx-prod-healthcheck.self_link]
backend {
group = replace(google_container_node_pool.pool-1.instance_group_urls[0], "instanceGroupManagers", "instanceGroups")
# TODO missing port 31443
}
}
It seems we are unable to set backend service port via the Terraform settings:
Recreating the backend service without this settings actually leads to downtime for us and the port must be written manually.

Answer 1

We need to reference the port name that we gave in the instance group for e.g.
resource "google_compute_backend_service" "test" {
name = "test-service"
port_name = "test-port"
protocol = "HTTP"
timeout_sec = 5
health_checks = []
backend {
group = "${google_compute_instance_group.test-ig.self_link}"
}
}
resource "google_compute_instance_group" "test-ig" {
name = "test-ig"
instances = []
named_port {
name = "test-port"
port = "${var.app_port}"
}
zone = "${var.zone}"
}

Answer 2

I resolved this by using a terraform data source to extract the instance group data from my google_container_node_pool, and then appending a google_compute_instance_group_named_port resource to it.
NOTE my google_container_node_pool spans 3 zones (a,b,c) and the below code highlights the solution for only zone c
# extract google_compute_instance_group from google_container_node_pool
data "google_compute_instance_group" "instance_group_zone_c" {
# .2 refers to the last of the 3 instance groups in my node pool (zone c)
name = regex("([^/]+)/?$", "${google_container_node_pool.k8s_node_pool.instance_group_urls.2}").0
zone = regex("${var.region}-?[abc]", "${google_container_node_pool.k8s_node_pool.instance_group_urls.2}")
project = var.project_id
}
# define a named port to attach to the google_compute_instance group in my node pool
resource "google_compute_instance_group_named_port" "named_port_zone_c" {
group = data.google_compute_instance_group.instance_group_zone_c.id
zone = data.google_compute_instance_group.instance_group_zone_c.zone
name = "port31090"
port = 31090
}