Allow multiple subnet ids (#67)

* Allow specifying a list of subnet IDs to iterate instances over

README.md

@@ -84,6 +84,7 @@ data "aws_ami" "ubuntu-xenial" {
 * `network_interface` can't be specified together with `associate_public_ip_address`, which makes `network_interface`
   not configurable using this module at the moment
 * Changes in `ebs_block_device` argument will be ignored. Use [aws_volume_attachment](https://www.terraform.io/docs/providers/aws/r/volume_attachment.html) resource to attach and detach volumes from AWS EC2 instances. See [this example](https://github.com/terraform-aws-modules/terraform-aws-ec2-instance/tree/master/examples/volume-attachment).
+* One of `subnet_id` or `subnet_ids` is required. If both are provided, the value of `subnet_id` is prepended to the value of `subnet_ids`.
 
 <!-- BEGINNING OF PRE-COMMIT-TERRAFORM DOCS HOOK -->
 ## Inputs
@@ -111,7 +112,8 @@ data "aws_ami" "ubuntu-xenial" {
 | private\_ip | Private IP address to associate with the instance in a VPC | string | `""` | no |
 | root\_block\_device | Customize details about the root block device of the instance. See Block Devices below for details | list | `<list>` | no |
 | source\_dest\_check | Controls if traffic is routed to the instance when the destination address does not match the instance. Used for NAT or VPNs. | string | `"true"` | no |
-| subnet\_id | The VPC Subnet ID to launch in | string | n/a | yes |
+| subnet\_id* | The VPC Subnet ID to launch in | string | `""` | no |
+| subnet\_ids* | A list of VPC Subnet IDs to launch in | string | `<list>` | no |
 | tags | A mapping of tags to assign to the resource | map | `<map>` | no |
 | tenancy | The tenancy of the instance (if the instance is running in a VPC). Available values: default, dedicated, host. | string | `"default"` | no |
 | user\_data | The user data to provide when launching the instance | string | `""` | no |

main.tf

@@ -11,7 +11,7 @@ resource "aws_instance" "this" {
   ami                    = "${var.ami}"
   instance_type          = "${var.instance_type}"
   user_data              = "${var.user_data}"
-  subnet_id              = "${var.subnet_id}"
+  subnet_id              = "${element(distinct(compact(concat(list(var.subnet_id), var.subnet_ids))),count.index)}"
   key_name               = "${var.key_name}"
   monitoring             = "${var.monitoring}"
   vpc_security_group_ids = ["${var.vpc_security_group_ids}"]
@@ -50,7 +50,7 @@ resource "aws_instance" "this_t2" {
   ami                    = "${var.ami}"
   instance_type          = "${var.instance_type}"
   user_data              = "${var.user_data}"
-  subnet_id              = "${var.subnet_id}"
+  subnet_id              = "${element(distinct(compact(concat(list(var.subnet_id), var.subnet_ids))),count.index)}"
   key_name               = "${var.key_name}"
   monitoring             = "${var.monitoring}"
   vpc_security_group_ids = ["${var.vpc_security_group_ids}"]

variables.tf

@@ -57,6 +57,13 @@ variable "vpc_security_group_ids" {
 
 variable "subnet_id" {
   description = "The VPC Subnet ID to launch in"
+  default     = ""
+}
+
+variable "subnet_ids" {
+  description = "A list of VPC Subnet IDs to launch in"
+  default     = []
+  type        = "list"
 }
 
 variable "associate_public_ip_address" {