ec2.asg.tf

# Auto Scaling Pool
resource "aws_autoscaling_group" "asg" {

  name = "${var.cluster_name}-asg"

  availability_zones = ["${var.availability_zone}"]

  min_size = "${var.cluster_min_size}"
  max_size = "${var.cluster_max_size}"
  desired_capacity = "${var.cluster_size}"
  
  load_balancers = ["${var.load_balancer}"]

  # here we link the launch configuration below
  launch_configuration = "${aws_launch_configuration.asg_pool.name}"
}

# Instances Launch Configuration
resource "aws_launch_configuration" "asg_pool" {
  # use system generated name to allow changes of launch_configuration
  name_prefix = "${var.cluster_name}-asg-"

  # From our base instance configuration
  image_id = "${var.ami}"
  instance_type = "${var.instance_type}"
  key_name = "${var.key_pair_name}"

  lifecycle { create_before_destroy = true }
}

ec2.variables.tf

variable "region" {} 
variable "availability_zone" { }

variable "cluster_name" {}
variable "load_balancer" {}

variable "ami" {} 
variable "instance_type" { default = "t2.micro" }
variable "key_pair_name" {}

variable "cluster_min_size" { default = 1 }
variable "cluster_max_size" { default = 4 }
variable "cluster_size" { default = 2 }

main.snippet.tf

#....
module "cluster" {
  source = "./ec2"

  region = "${var.region}"
  availability_zone = "${var.availability_zone}"

  cluster_name = "${var.cluster_name}"
  ami = "${var.ami}"
  instance_type = "${var.instance_type}"

  # From keys module
  key_pair_name = "${module.keys.key_name}"

  # From load balancer module
  load_balancer = "${module.load_balancer.elb_name}"
}
#....
  

variables.snippet.tf

#....
# ASG Pool and Launch Configuration=========================
variable "ami" { default = "ami-a9d276c9" } # Ubuntu Server 16.04 LTS (HVM)
variable "instance_type" { default = "t2.micro" } # Free tier
variable "availability_zone" { default = "us-west-2a" }
#....