Nodes are created across the given subnets.

modules/aws-network-existing/main.tf

@@ -22,6 +22,14 @@ data "aws_subnet" "subnets" {
   id = each.key
 }
 
+data "aws_subnet" "subnet-by-name" {
+  for_each = toset(var.existing-subnet-names)
+  filter {
+    name   = "tag:Name"
+    values = [each.key]
+  }
+}
+
 data "aws_security_group" "default" {
   name = var.default-security-group-name
 }

modules/aws-network-existing/outputs.tf

@@ -11,11 +11,16 @@ output "subnets" {
   value = data.aws_subnet.subnets
 }
 
-output "k8s-subnets-ids" {
-  description = "An array of subnets to be used for k8s VMs. These subnets were chosen by selecting a single subnet from each availability_zone."
+output "one-subnet-per-az" {
+  description = "An array of subnets that selects 1 subnet per az."
   value = [for k,v in local.az-to-subnets : v[0]]
 }
 
+output "subnet-by-name" {
+  description = "A map of subnet name to subnet resource."
+  value = data.aws_subnet.subnet-by-name
+}
+
 output "az-to-subnets" {
   description = "A map of availability zone to array of subnets that are in thet availability zone."
   value = local.az-to-subnets

modules/aws-network-existing/variables.tf

@@ -5,3 +5,9 @@ variable "default-security-group-name" {
 variable "default-vpc-name" {
   description = "The name of the existing default VPC. This module will query AWS for a VPC with this name,"
 }
+
+variable "existing-subnet-names" {
+  description = "A list of subnet names that already exist in default-vpc-name"
+  default     = []
+  type        = list(string)
+}

modules/aws-nodes/main.tf

@@ -3,7 +3,7 @@ resource "aws_instance" "nodes" {
   instance_type               = var.ec2-instance-type
   # TODO Make this a variable.
   associate_public_ip_address = true
-  subnet_id                   = var.subnet-id
+  subnet_id = element(var.subnet-ids, count.index % length(var.subnet-ids))
   vpc_security_group_ids = var.security-group-ids
   user_data = element(var.user-datas.*.rendered, count.index)
   count                       = var.num-nodes

modules/aws-nodes/variables.tf

@@ -25,9 +25,9 @@ variable "user-datas" {
   description = "A list of cloud-init configs that get applied to their corresponding node."
 }
 
-variable "subnet-id" {
-  description = "The ID of the subnet that all the nodes will be added to."
-  type = string
+variable "subnet-ids" {
+  description = "An array of subnet ids. These subnets will be round robined as the subnet to use for each node."
+  type = list(string)
 }
 
 variable "security-group-ids" {