Conditional expressions with Terraform deploying AWS resources

Question

My current Terraform setup consists of 3 template files. Each template file is linked to a launch configuration resource which is then used to launch instances via auto scaling events. In each template file, there is an AWS CLI command used to attach an existing EBS volume to the new instance being launched via autoscaling. I am having some trouble writing a conditional expression to pass in a variable to this AWS CLI command being used to attach a specific drive. Since I have 3 template files and 3 EBS volumes I'm looking to attach to each instance in its own autoscaling group, I don't believe I can have more than 2 expressions within my conditional expression. Any advice on how I can achieve this would be helpful.

Template_file

data "template_file" "ML_10_user_data" {
count    = "${(var.enable ? 1 : 0) * var.number_of_zones}" // 3 templates
template = "${file("userdata.sh")}

  vars {
    ebs_volume = "${count.index == 0 ? vol-xxxxxxxxxxxxxxxxx : vol-xxxxxxxxxxxxxxxxx}" // how to include 3rd EBS volume?
  }
}

Userdata.sh

#!/bin/bash
aws ec2 attach-volume --volume-id ${EBS_VOLUME} --instance_id `curl http://169.254.169.254/latest/meta-data/instance-id` --device /dev/sdf

EBS_VOLUME=${ebs_volume}

Any advice on how I can fulfill what I am trying to accomplish would be appreciated.

Answer 1

The best way would be to put it in a list:

variable "volumes" {
  default = ["vol-1111","vol-2222","vol-333"]
}

data "template_file" "user_data" {
  count    = "${(var.enable ? 1 : 0) * var.number_of_zones}"
  template = "${file("userdata.sh")}"

  vars {
    ebs_volume = "${var.volumes[count.index]}"
  }
}

However, if these instances should be in ASG, this is not a very good design. Instances in ASG should be identical, interchangeable and disposable. They can be terminated at any time by AWS or scaling activities, and you should treat those instances as a group, not as individual entities.