Terraform and GCS : how to create a multi zone LB - google-cloud-platform

Question

I want to create a load balancer adressing two or more instances on several zones of the same region in GCP
I start like that :
- create a backend service adressing two instance groups :
resource "google_compute_backend_service" "www-service" {
name = "${var.environment}-www-service"
protocol = "HTTP"
port_name = "http"
backend {
group = "${google_compute_instance_group.instance-group-0.self_link}"
}
backend {
group = "${google_compute_instance_group.instance-group-1.self_link}"
}
health_checks = ["${google_compute_health_check.health-check1.self_link}"]
}
Then I have two instance groups, each one with a one instance with that syntax :
resource "google_compute_instance_group" "instance-group-0" {
count = "${var.web_count}"
name = "${var.environment}-instance-group-0"
instances = ["${google_compute_instance.www.self_link}"]
named_port {
name = "http"
port = "80"
}
network = "${google_compute_network.platform-network.self_link}"
}
I get an error :
google_compute_backend_service.www-service: Resource 'google_compute_instance_group.instance-group-0' not found for variable 'google_compute_instance_group.instance-group-0.self_link'
I see that switching the backend/group declarations in the backend_service moves the error to group-1, so I can guess that this is not the proper syntax, although you can create a backend_service with multiple instance groups in the Google GUI.
I have two questions :
Q1. How can I create backend_service with multiple instance groups ?
what is the right syntax ?
Q2. Is it possible to reference a compute_instance in a compute_instance_group via a syntax like :
instances = ["${google_compute_instance.www.[count.index].self_link}"]
(the above syntax does not work)

Answer 1

Thanks for your answer
Finally I found the syntax across multiple Github tickets :
resource "google_compute_instance_group" "instance-group-0" {
name = "${var.environment}-instance-group-0"
zone = "${data.google_compute_zones.available.names[0]}"
instances = ["${slice(google_compute_instance.www.*.self_link, 0, floor(var.web_count/2)-1)}"]
named_port {
name = "http"
port = "80"
}
network = "${google_compute_network.platform-network.self_link}"
}
same for instance group 1, but different slice
Then :
resource "google_compute_instance" "www" {
count = "${var.web_count}"
zone = "${data.google_compute_zones.available.names[floor((2*count.index)/var.web_count)]"}
I must say that different design decisions can be taken :
- use a region managed instance group is simpler, except the template is static
- use Kubernetes

Answer 2

A1 - each backend service can only use a single instance group. You need to create a new backend for each group. You can then assign multiple backends to your load balancer.
On another note, the error message is with regards to the resource type you are using. Here is the resource page for unmanaged Instance groups, Managed instance groups and for backend services. You can find a list of the different compute APIs here.
A2- you should use $(ref.name.selfLink) instead, I believe this syntax should work and you can still use your variable. Also, make sure to update the API call syntax for adding instances to the group