Improve apiserver backend service zone spanning

* google_compute_backend_services use nested blocks to define
backends (instance groups heterogeneous controllers)
* Use Terraform v0.12.x dynamic blocks so the apiserver backend
service can refer to (up to zone-many) controller instance groups
* Previously, with Terraform v0.11.x, the apiserver backend service
had to list a fixed set of backends to span controller nodes across
zones in multi-controller setups. 3 backends were used because each
GCP region offered at least 3 zones. Single-controller clusters had
the cosmetic ugliness of unused instance groups
* Allow controllers to span more than 3 zones if avilable in a
region (e.g. currently only us-central1, with 4 zones)

Related:

* https://www.terraform.io/docs/providers/google/r/compute_backend_service.html
* https://www.terraform.io/docs/configuration/expressions.html#dynamic-blocks

CHANGES.md

@@ -6,6 +6,11 @@ Notable changes between versions.
 
 * Update Calico from v3.7.3 to [v3.7.4](https://docs.projectcalico.org/v3.7/release-notes/)
 
+#### Google Cloud
+
+* Allow controller nodes to span more than 3 zones if available in a region
+* Eliminate extraneous controller instance groups in single-controller clusters
+
 #### Addons
 
 * Update Grafana from v6.2.4 to v6.2.5

google-cloud/container-linux/kubernetes/apiserver.tf

@@ -45,16 +45,11 @@ resource "google_compute_backend_service" "apiserver" {
   timeout_sec      = "300"
 
   # controller(s) spread across zonal instance groups
-  backend {
-    group = google_compute_instance_group.controllers[0].self_link
-  }
-
-  backend {
-    group = google_compute_instance_group.controllers[1].self_link
-  }
-
-  backend {
-    group = google_compute_instance_group.controllers[2].self_link
+  dynamic "backend" {
+    for_each = google_compute_instance_group.controllers
+    content {
+      group = backend.value.self_link
+    }
   }
 
   health_checks = [google_compute_health_check.apiserver.self_link]
@@ -64,11 +59,7 @@ resource "google_compute_backend_service" "apiserver" {
 resource "google_compute_instance_group" "controllers" {
   count = length(local.zones)
 
-  name = format(
-    "%s-controllers-%s",
-    var.cluster_name,
-    element(local.zones, count.index),
-  )
+  name = format("%s-controllers-%s", var.cluster_name, element(local.zones, count.index))
   zone = element(local.zones, count.index)
 
   named_port {

google-cloud/container-linux/kubernetes/controllers.tf

@@ -20,9 +20,7 @@ data "google_compute_zones" "all" {
 }
 
 locals {
-  # TCP proxy load balancers require a fixed number of zonal backends. Spread
-  # controllers over up to 3 zones, since all GCP regions have at least 3.
-  zones = slice(data.google_compute_zones.all.names, 0, 3)
+  zones = data.google_compute_zones.all.names
 
   controllers_ipv4_public = google_compute_instance.controllers.*.network_interface.0.access_config.0.nat_ip
 }