ALB not in available state when trying to attach as target to NLB

I am using Terraform version 1.8.0 and creating resources in AWS. My parent module calls modules to create an application load balancer a network load balancer and the target groups. I am trying to attach the application load balancer as a target for the target group that is used by the network load balancer. The problem I am running into is that the application load balancer is not in an available state when the target group is tries to attach it as a target.

I need help to figure out how to delay the creation of the aws_lb_target_group_attachment resource until the application load balancer is in an available state. Is there a way to do this?

This is the module call from the parent module that creates the target group. I create the application load balancer in a separate module and pass the ARN of the application load balancer to this module.

module "nlb_target_group_443" {
  source  = ""
  version = "0.0.2"

  health_check                  = var.nlb_tg_443.health_check
  load_balancing_algorithm_type = var.nlb_tg_443.lb_algorithm_type
  target_group_name             = var.nlb_tg_443.target_group_name
  target_group_port             = var.nlb_tg_443.port
  target_group_protocol         = var.nlb_tg_443.protocol
  attach_target                 = true
  target_id                     = module.application_load_balancer.lb_arn
  target_type                   = var.nlb_tg_443.target_type
  vpc_id                        = module.vpc.vpc_id

This is the attachment block from the module that creates the target group. I have the target_id set as the ARN of the application load balancer.

resource "aws_lb_target_group_attachment" "this" {
  count = var.attach_target == true ? 1 : 0

  target_group_arn = aws_lb_target_group.this.arn
  target_id        = var.target_id
  port             = var.target_group_port

When I apply the plan I get the following error:

The ALB does have a listener configured that matches the target group port. I believe I receive this error because the ALB is not available yet. When I re-apply this configuration after the application load balancer becomes available and I do not receive an error.

In your nlb_target_group_443 module, target_id refers to module.application_load_balancer.lb_arn which is likely available as soon as the underlying aws_lb resource is created while the rest of the downstream resources are still being provisioned.

If the application_load_balancer module can expose its aws_lb_target_group_attachment resources as outputs, you can set an explicit dependency on them in the nlb_target_group_443 module to ensure that the ALB setup is fully ready.

I’ve not used it myself, but perhaps you can also use explicit output dependencies within the application_load_balancer module to ensure that application_load_balancer is available only after the entire ALB including targets are fully set up.

Hope this gives you some ideas.

Thanks for the response, @acwwat.

I figured out what I had done wrong but I was inspired by your response. The target group module is using the arn output from the load balancer module to provide the target id. I mistakenly thought that this would create a dependency on the load balancer being completed. Since the arn for the lb is created immediately that meant the target group module was free to begin. All I needed to do was add a depends_on = [ module.application_load_balancer ] to this module and it waited for the load balancer to finish creation before it began.

Thanks again for your help.

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