AWS VPC module public and private subnets - Terraform

Question

I'm trying to create AWS vpc environment with public and private subnet which I create with "template_file", however, I'm facing issues with defining the public and private subnets inside the VPC module.

I'v generated my subnets using:

data "template_file" "cidrsubnets" {
  count = var.subnet_count[terraform.workspace]

  template = "$${cidrsubnet(vpc_cidr,8,current_count)}"

  vars = {
    vpc_cidr      = var.network_address_space[terraform.workspace]
    current_count = count.index
  }
}

Inside the VPC module, the above code adds all my subnets into public subnets (works):

module "vpc" {
  source = "terraform-aws-modules/vpc/aws"
  name   = "${local.prefix}-vpc"

  cidr            = var.network_address_space[terraform.workspace]
  azs             = slice(data.aws_availability_zones.available.names, 0, var.subnet_count[terraform.workspace])
  public_subnets  = data.template_file.cidrsubnets[*].rendered
  private_subnets = []

  create_database_subnet_group = false
  enable_dns_hostnames         = true
  enable_nat_gateway           = true
  enable_s3_endpoint           = true
  enable_dynamodb_endpoint     = true

  tags = local.common_tags
}

However, I want to add the even subnets in the list to the public subnets and the odd subnets in the list to the private subnets. I still haven't split the subnets into private and public automatically (that what I need), but tried to add specific elements inside the lists and got the error above:

Can you help with this? Thank you for the response.

Answer 1

using a template_file data source here seems overly complicated. I guess what you want to achieve is something like the following (using locals for better readability) [untested]:

locals {
  cidr    = var.network_address_space[terraform.workspace]
  netnums = range(var.subnet_count[terraform.workspace])
}

module "vpc" {
  source = "terraform-aws-modules/vpc/aws"

  ....
  cidr           = local.cidr
  public_subnets = [for i in local.netnums : cidrsubnet(local.cidr, 8, i)]
  ....
}

netnums will be an array of numbers starting at 0 .. see range() terraform function on how to start at a different number.