Added backend preference to global application load balancers (#3139)

modules/net-lb-app-ext/backend-service.tf

@@ -78,6 +78,7 @@ resource "google_compute_backend_service" "default" {
     for_each = { for b in coalesce(each.value.backends, []) : b.backend => b }
     content {
       group           = lookup(local.group_ids, backend.key, backend.key)
+      preference      = backend.value.preference
       balancing_mode  = backend.value.balancing_mode # UTILIZATION, RATE
       capacity_scaler = backend.value.capacity_scaler
       description     = backend.value.description

modules/net-lb-app-ext/variables-backend-service.tf

@@ -37,6 +37,7 @@ variable "backend_service_configs" {
     backends = list(object({
       # group renamed to backend
       backend         = string
+      preference      = optional(string, "DEFAULT")
       balancing_mode  = optional(string, "UTILIZATION")
       capacity_scaler = optional(number, 1)
       description     = optional(string, "Terraform managed.")
@@ -163,6 +164,15 @@ variable "backend_service_configs" {
     ]))
     error_message = "When specified, balancing mode needs to be 'RATE' or 'UTILIZATION'."
   }
+  validation {
+    condition = alltrue(flatten([
+      for backend_service in values(var.backend_service_configs) : [
+        for backend in backend_service.backends : contains(
+          ["DEFAULT", "PREFERRED"], coalesce(backend.preference, "DEFAULT")
+      )]
+    ]))
+    error_message = "When specified, balancing mode needs to be 'DEFAULT' or 'PREFERRED'."
+  }
   validation {
     condition = alltrue([
       for backend_service in values(var.backend_service_configs) :