aws_lb_target_group_attachment: attaching multiple instances to per target_group

Question

I have multiple target_groups per NLB and I need to attach multiple instances to each target_group. The target_id being a string in aws_lb_target_group_attachment resource, I don't see any easy way to achieve that. This is what I'm doing atm:

vars.tf

variable "nlb_listeners" {
  default = [
    {
      protocol     = "TCP"
      target_port  = "80"
      health_port  = "1936"
    },
    {
      protocol     = "TCP"
      target_port  = "443"
      health_port  = "1936"
    }
  ]
}

instances .tf

// Get the instance ids of the NLB members  
data "aws_instances" "nlb_insts" {
  instance_tags = {
   Name = "${var.vpc_names[var.idx]}${var.inst_role}0*"
  }
  instance_state_names = ["running", "stopped"]
}

// EC2 instances
resource "aws_instance" "insts" {
  count         = var.inst_count
  instance_type = var.inst_type
  .......
}

balancer.tf

// Creates the target-group
resource "aws_lb_target_group" "nlb_target_groups" {
  count                = length(var.nlb_listeners)
  name                 = "nlb-tgr-${lookup(var.nlb_listeners[count.index], "target_port")}"
  deregistration_delay = var.deregistration_delay
  port                 = lookup(var.nlb_listeners[count.index], "target_port")
  protocol             = lookup(var.nlb_listeners[count.index], "protocol")
  vpc_id               = var.vpc_ids[var.idx]

  health_check {
    port                = lookup(var.nlb_listeners[count.index], "health_port")
    protocol            = lookup(var.nlb_listeners[count.index], "protocol")
    interval            = var.health_check_interval
    unhealthy_threshold = var.unhealthy_threshold
    healthy_threshold   = var.healthy_threshold
  }
}

// Attach the target groups to the instance(s)
resource "aws_lb_target_group_attachment" "tgr_attachment" {
  count            = length(var.nlb_listeners)
  target_group_arn = element(aws_lb_target_group.nlb_target_groups.*.arn, count.index)
  target_id        = [for sc in range(var.inst_count) : data.aws_instances.nlb_insts.ids[sc]]
  port             = lookup(var.nlb_listeners[count.index], "target_port")
}

so, I'm getting this error:

Error: Incorrect attribute value type

on ../../modules/elb/balencer.tf line 31, in resource "aws_lb_target_group_attachment" "tgr_attachment": 31: target_id = [for sc in range(2) : data.aws_instances.nlb_insts.ids[sc]]

Inappropriate value for attribute "target_id": string required.

Anyone has any idea how do I achieve that?

Answer 1

Quickly looking at your code I would say this is the mistake that was giving you your error:

data.aws_instances.nlb_insts.ids[sc]

Should be:

data.aws_instances.nlb_insts[sc].ids

The index goes on the resource, then you get the ids from your resource, in your case you were trying to get all the ids of all the resources and then get the index of that, which is not the way to do it!