Extract default target group to variable

terraform/app/ecs.tf

@@ -146,7 +146,7 @@ module "web_service" {
     vpc_id  = aws_vpc.application_vpc.id
   }
   loadbalancer = {
-    target_group_arn = aws_lb_target_group.green.arn
+    target_group_arn = local.ecs_initial_lb_target_group
     container_port   = 4000
   }
   cluster_id            = aws_ecs_cluster.cluster.id

terraform/app/env/preview.tfvars

@@ -26,3 +26,4 @@ http_hosts = {
 
 minimum_replicas = 3
 appspec_bucket   = "nhse-mavis-appspec-bucket-preview"
+ecs_initial_lb_target_group = "green"

terraform/app/loadbalancer.tf

@@ -150,7 +150,7 @@ resource "aws_lb_listener_rule" "forward_to_app" {
   priority     = 50000
   action {
     type             = "forward"
-    target_group_arn = aws_lb_target_group.blue.arn
+    target_group_arn = local.ecs_initial_lb_target_group
   }
   condition {
     path_pattern {

terraform/app/variables.tf

@@ -246,6 +246,17 @@ variable "maximum_replicas" {
   description = "Maximum amount of allowed replicas"
 }
 
+variable "active_lb_target_group" {
+  type        = string
+  description = "The actual loadbalancer target group is set by Codedeploy. However in scenarios where new resources behind the load balancer are created, terraform already needs to know the current target group. In this case, set the variable to the currently active target group."
+  default     = "blue"
+  validation {
+    condition     = contains(["blue", "green"], var.active_lb_target_group)
+    error_message = "Valid target groups: blue, green"
+  }
+}
+
 locals {
-  ecs_sg_ids = [module.web_service.security_group_id, module.good_job_service.security_group_id]
+  ecs_initial_lb_target_group = var.active_lb_target_group == "green" ? aws_lb_target_group.green.arn : aws_lb_target_group.blue.arn
+  ecs_sg_ids                  = [module.web_service.security_group_id, module.good_job_service.security_group_id]
 }