How to reference a value of module in a resource in Terraform

Question

I am trying to use ALB module value in http_listener_rule resource. I found only one way to do this that is by using this syntax: “${module.alb.http_tcp_listener_arns}” in resource But this is throwing the following error: Inappropriate value for attribute “listener_arn”: string required.

The following error occurs: Error: Incorrect attribute value type on main.tf line 197, in resource "aws_lb_listener_rule" "host_based_routing": 197: listener_arn = "${module.alb.http_tcp_listener_arns}" ├──────────────── │ module.alb.http_tcp_listener_arns is empty tuple

Inappropriate value for attribute "listener_arn": string required.

resource "aws_lb_listener_rule" "host_based_routing" {
  listener_arn = "${module.alb.http_tcp_listener_arns}"
  priority     = 99

  action {
    type             = "forward"
    target_group_arn = "${module.alb.target_group_arns}"
  }

  condition {
    host_header {
      values = ["example.com"]
    }
  }
}

module "alb" {
  source = "git@github.com:terraform-aws-modules/terraform-aws-alb.git?ref=v6.0.0"

  name = "demo-alb"

  load_balancer_type = "application"

  vpc_id          = module.vpc.vpc_id
  subnets         = module.vpc.public_subnets
  security_groups = [module.security_group_asg.security_group_id]

  target_groups = [
    {
      name             = "target-group"
      backend_protocol = "HTTP"
      backend_port     = 80
      target_type      = "instance"
      health_check = {
        enabled             = true
        interval            = 110
        path                = "/drupal"
        port                = "traffic-port"
        healthy_threshold   = 3
        unhealthy_threshold = 3
        timeout             = 100
        protocol            = "HTTP"
        matcher             = "200-399"
      }
    }
  ]
}

Answer 1

module.alb.http_tcp_listener_arns is a list of ARNs, so you have to specify individual ARN for your host_based_routing. If you have only one, then you can try:

listener_arn = module.alb.http_tcp_listener_arns[0]