Allow passing in EIPs for the NAT Gateways (#38)

* Allow passing in EIPs for the NAT Gateways

* Document how to pass external IPs for NAT Gateways

* Update README.md to reduce example code

README.md

@@ -46,10 +46,45 @@ module "vpc" {
 }
 ```
 
+External NAT Gateway IPs
+------------------------
+
+By default this module will provision new Elastic IPs for the VPC's NAT Gateways.
+This means that when creating a new VPC, new IPs are allocated, and when that VPC is destroyed those IPs are released.
+Sometimes it is handy to keep the same IPs even after the VPC is destroyed and re-created.
+To that end, it is possible to assign existing IPs to the NAT Gateways.
+This prevents the destruction of the VPC from releasing those IPs, while making it possible that a re-created VPC uses the same IPs.
+
+To achieve this, allocate the IPs outside the VPC module declaration.
+```hcl
+resource "aws_eip" "nat" {
+  count = 3
+
+  vpc = true
+}
+```
+
+Then, pass the allocated IPs as a parameter to this module.
+```hcl
+module "vpc" {
+  source = "terraform-aws-modules/vpc/aws"
+
+  # The rest of arguments are omitted for brevity
+
+  enable_nat_gateway  = true
+  single_nat_gateway  = false
+  external_nat_ip_ids = ["${resource.aws_eip.nat.*.id}"]   # <= IPs specified here as input to the module
+}
+```
+
+Note that in the example we allocate 3 IPs because we will be provisioning 3 NAT Gateways (due to `single_nat_gateway = false` and having 3 subnets).
+If, on the other hand, `single_nat_gateway = true`, then `aws_eip.nat` would only need to allocate 1 IP.
+Passing the IPs into the module is done by setting variable `external_nat_ip_ids = ["${resource.aws_eip.nat.*.id}"]`.
+
 Terraform version
 -----------------
 
-Terraform version 1.0.0 or newer is required for this version to work.
+Terraform version 0.10.13 or newer is required for this module to work.
 
 Examples
 --------

main.tf

+terraform {
+  required_version = ">= 0.10.13" # introduction of Local Values configuration language feature
+}
+
 ######
 # VPC
 ######
@@ -152,8 +156,20 @@ resource "aws_elasticache_subnet_group" "elasticache" {
 ##############
 # NAT Gateway
 ##############
+# Workaround for interpolation not being able to "short-circuit" the evaluation of the conditional branch that doesn't end up being used
+# Source: https://github.com/hashicorp/terraform/issues/11566#issuecomment-289417805
+#
+# The logical expression would be
+#
+#    nat_gateway_ips = var.reuse_nat_ips ? var.external_nat_ip_ids : aws_eip.nat.*.id
+#
+# but then when count of aws_eip.nat.*.id is zero, this would throw a resource not found error on aws_eip.nat.*.id.
+locals {
+  nat_gateway_ips = "${split(",", (var.reuse_nat_ips ? join(",", var.external_nat_ip_ids) : join(",", aws_eip.nat.*.id)))}"
+}
+
 resource "aws_eip" "nat" {
-  count = "${var.enable_nat_gateway ? (var.single_nat_gateway ? 1 : length(var.azs)) : 0}"
+  count = "${(var.enable_nat_gateway && !var.reuse_nat_ips) ? (var.single_nat_gateway ? 1 : length(var.azs)) : 0}"
 
   vpc = true
 }
@@ -161,7 +177,7 @@ resource "aws_eip" "nat" {
 resource "aws_nat_gateway" "this" {
   count = "${var.enable_nat_gateway ? (var.single_nat_gateway ? 1 : length(var.azs)) : 0}"
 
-  allocation_id = "${element(aws_eip.nat.*.id, (var.single_nat_gateway ? 0 : count.index))}"
+  allocation_id = "${element(local.nat_gateway_ips, (var.single_nat_gateway ? 0 : count.index))}"
   subnet_id     = "${element(aws_subnet.public.*.id, (var.single_nat_gateway ? 0 : count.index))}"
 
   tags = "${merge(var.tags, map("Name", format("%s-%s", var.name, element(var.azs, (var.single_nat_gateway ? 0 : count.index)))))}"

variables.tf

@@ -65,6 +65,17 @@ variable "single_nat_gateway" {
   default     = false
 }
 
+variable "reuse_nat_ips" {
+  description = "Should be true if you don't want EIPs to be created for your NAT Gateways and will instead pass them in via the 'external_nat_ip_ids' variable"
+  default     = false
+}
+
+variable "external_nat_ip_ids" {
+  description = "List of EIP IDs to be assigned to the NAT Gateways (used in combination with reuse_nat_ips)"
+  type        = "list"
+  default     = []
+}
+
 variable "enable_dynamodb_endpoint" {
   description = "Should be true if you want to provision a DynamoDB endpoint to the VPC"
   default     = false