Extend nat-gateways modules to enable support for EIP switching

This is a backward compatible change with no updates needed to
existing modules. The additional variables can be used for scenarions
where migrating a EIP from one NAT to another is required.

Author: psibi

modules/nat-gateways/main.tf

@@ -9,21 +9,32 @@
  *
  */
 
+locals {
+  total_nat_count = var.enable_nat_creation ? var.nat_count : 0
+  total_new_nat   = var.enable_nat_creation ? (length(var.nat_eip) == 0 ? local.total_nat_count : 0) : 0
+  nat_ids         = var.enable_nat_creation ? (length(var.nat_eip) == 0 ? aws_eip.nat.*.id : values(data.aws_eip.nat)[*].id) : []
+}
+
 # AWS Managed NAT Gateways
 resource "aws_eip" "nat" {
-  count = var.nat_count
+  count = local.total_new_nat
   vpc   = true
 }
 
+data "aws_eip" "nat" {
+  for_each  = length(var.nat_eip) != 0 ? toset(var.nat_eip) : toset([])
+  public_ip = each.value
+}
+
 data "aws_subnet" "public" {
   count = length(var.public_subnet_ids)
   id    = element(var.public_subnet_ids, count.index)
 }
 
 resource "aws_nat_gateway" "nat" {
-  count         = var.nat_count
+  count         = local.total_nat_count
   subnet_id     = element(data.aws_subnet.public.*.id, count.index)
-  allocation_id = element(aws_eip.nat.*.id, count.index)
+  allocation_id = element(local.nat_ids, count.index)
 
   tags = merge(
     {
@@ -35,7 +46,7 @@ resource "aws_nat_gateway" "nat" {
 
 # Route tables. One per NAT gateway.
 resource "aws_route_table" "private" {
-  count  = var.nat_count
+  count  = local.total_nat_count
   vpc_id = var.vpc_id
 
   tags = merge(
@@ -47,15 +58,15 @@ resource "aws_route_table" "private" {
 }
 
 resource "aws_route" "private_nat_gateway" {
-  count                     = var.nat_count
-  route_table_id            = aws_route_table.private[count.index].id
-  destination_cidr_block    = "0.0.0.0/0"
-  nat_gateway_id            = element(aws_nat_gateway.nat.*.id, count.index)
+  count                  = local.total_nat_count
+  route_table_id         = aws_route_table.private[count.index].id
+  destination_cidr_block = "0.0.0.0/0"
+  nat_gateway_id         = element(aws_nat_gateway.nat.*.id, count.index)
 }
 
+# https://github.com/terraform-providers/terraform-provider-aws/pull/6999
 resource "aws_route_table_association" "private-rta" {
-  count          = length(var.private_subnet_ids)
+  count          = var.enable_nat_creation ? length(var.private_subnet_ids) : 0
   subnet_id      = element(var.private_subnet_ids, count.index)
   route_table_id = element(aws_route_table.private.*.id, count.index)
 }
-

modules/nat-gateways/variables.tf

@@ -29,3 +29,14 @@ variable "extra_tags" {
   type        = map(string)
 }
 
+variable "enable_nat_creation" {
+  default     = true
+  description = "boolean, enable/disable NAT creation"
+  type        = string
+}
+
+variable "nat_eip" {
+  description = "The public IP of the specific EIP to retrieve. If non empty, this list should have same number of EIP as the number of var.public_subnet_ids."
+  type        = list(string)
+  default     = []
+}