Produce repeating blocks inside a terraform resource - terraform

Question

I am fairly new to terraform and trying to create a google_compute_backend_service using terraform and there are multiple backend blocks inside the resource as shown below:
resource "google_compute_backend_service" "app-backend" {
log_config {
enable = "true"
sample_rate = "1"
}
name = "app-backend"
port_name = "http-34070"
project = "my-project"
protocol = "HTTP"
session_affinity = "NONE"
timeout_sec = "30"
backend {
group = "insatnce-group1"
}
backend {
group = "instance-group2"
}
backend {
group = "instance-group3"
}
health_checks = [google_compute_http_health_check.app-http-l7.name]
}
As seen in the code block above the backend block repeats multiple times. I want to make it dynamic so I do not have to write multiple blocks manually.
I tried the following:
Created a variable in the variables.tf file that contains all the instance groups:
variable "groups" {
type = list(object({
name = string
}))
default = [{ name = "instance-group1"},
{ name = "instance-group2"},
{ name = "instance-group3"}]
}
And modified my resource block to this:
resource "google_compute_backend_service" "app-backend" {
log_config {
enable = "true"
sample_rate = "1"
}
name = "app-backend"
port_name = "http-34070"
project = "my-project"
protocol = "HTTP"
session_affinity = "NONE"
timeout_sec = "30"
dynamic "backend" {
for_each = var.groups
iterator = item
group = item.value.name
}
health_checks = [google_compute_http_health_check.app-http-l7.name]
}
However, when I execute terraform plan I get the following error:
Error: Unsupported argument
│
│ on backend_service.tf line 15, in resource "google_compute_backend_service" "app-backend":
│ 15: group = item.value.name
│
│ An argument named "group" is not expected here.
Where am I going wrong? Is there a better way to achieve this?

Answer 1

You can check the dynamic blocks documentation for the syntax. Otherwise, you had the right idea.
dynamic "backend" {
for_each = var.groups
content {
group = backend.value.name
}
}
You can also simplify the variable structure to make this even easier.
variable "groups" {
type = set(string)
default = ["instance-group1", "instance-group2", "instance-group3"]
}
dynamic "backend" {
for_each = var.groups
content {
group = backend.value
}
}