Make number of VMs customizable

deployment/modules/gcp/gce/tesseract/main.tf

@@ -268,7 +268,7 @@ resource "google_compute_region_instance_group_manager" "instance_group_manager"
   }
 
   base_instance_name = var.base_name
-  target_size        = "3"
+  target_size        = var.num_vms
 
   update_policy {
     type                           = "PROACTIVE"

deployment/modules/gcp/gce/tesseract/tests/variables_validation.tftest.hcl

@@ -148,3 +148,43 @@ run "invalid_dates_not_after_limit_lowercase" {
     var.not_after_limit,
   ]
 }
+
+run "valid_num_vms" {
+  command = plan
+
+  variables {
+    num_vms = 4
+  }
+}
+
+run "valid_num_vms_zero" {
+  command = plan
+
+  variables {
+    num_vms = 0
+  }
+}
+
+run "invalid_num_vms_negative" {
+  command = plan
+
+  variables {
+    num_vms = -1
+  }
+
+  expect_failures = [
+    var.num_vms,
+  ]
+}
+
+run "invalid_num_vms_float" {
+  command = plan
+
+  variables {
+    num_vms = 2.5
+  }
+
+  expect_failures = [
+    var.num_vms,
+  ]
+}

deployment/modules/gcp/gce/tesseract/variables.tf

@@ -39,6 +39,18 @@ variable "machine_type" {
   default     = "n2-standard-4"
 }
 
+variable "num_vms" {
+  description = "Number of VM instances to run in the regional managed instance group"
+  type        = number
+  default     = 3
+  nullable    = false
+
+  validation {
+    condition     = var.num_vms >= 0 && floor(var.num_vms) == var.num_vms
+    error_message = "num_vms must be a non-negative integer."
+  }
+}
+
 variable "bucket" {
   description = "Log GCS bucket"
   type        = string

deployment/modules/gcp/tesseract/gce/main.tf

@@ -34,6 +34,7 @@ module "gce" {
   docker_repo                                = var.docker_repo
   server_docker_image                        = var.server_docker_image
   machine_type                               = var.machine_type
+  num_vms                                    = var.num_vms
   not_after_start                            = var.not_after_start
   not_after_limit                            = var.not_after_limit
   bucket                                     = module.storage.log_bucket.id

deployment/modules/gcp/tesseract/gce/variables.tf

@@ -57,6 +57,18 @@ variable "machine_type" {
   default     = "n2-standard-4"
 }
 
+variable "num_vms" {
+  description = "Number of VM instances to run in the regional managed instance group"
+  type        = number
+  default     = 3
+  nullable    = false
+
+  validation {
+    condition     = var.num_vms >= 0 && floor(var.num_vms) == var.num_vms
+    error_message = "num_vms must be a non-negative integer."
+  }
+}
+
 variable "spanner_pu" {
   description = "Amount of Spanner processing units"
   type        = number