Unable to reference vpc_id for a subnet within modules

Question

Have a modules/network/testvpc and modules/network/subnet module configurations .

mainfolder/modules/network/testvpc/main.tf

    variable "vpccidr" {type="list"}
    variable "vpcname" {type="list"}

   resource "aws_vpc" "customVpc" {
   count = "${length(var.vpccidr)}"
   cidr_block = "${element(var.vpccidr,count.index)}"

    tags {
    Name = "${element(var.vpcname,count.index)}"
    }

mainfolder/modules/network/subnet/main.tf

variable "subcidr" {type="list"}
variable "subname" {type="list"}

resource "aws_subnet" "subnet" {
count = "${length(var.subcidr)}"
vpc_id = "${element(aws_vpc.customVpc.*.id, count.index)}"
cidr_block = "${element(var.subcidr, count.index)}"

tags {

Name = "${element(var.subname, count.index)}"
}
 }

mainfolder/main.tf

module "testvpc" {
source = "./modules/network/testvpc"
vpccidr="${var.vpccidr}"
vpcname="${var.vpcname}"
}

module "subnet" {
source = "./modules/network/subnet"
subcidr = "${var.subcidr}"
subname = "${var.subname}"
} 

mainfolder/var.tf

  variable "vpccidr" {type="list"}
  variable "vpcname" {type="list"}
  variable "subcidr" {type="list"}
  variable "subname" {type="list"}

mainfolder/terraform.tfvars

 - vpccidr=["10.1.0.0/16","10.2.0.0/16","10.3.0.0/16"]   
   vpcname=["vpc-shared","vpc-sand","vpc-preprod"]   
   subcidr=["10.1.1.0/24","10.2.1.0/24","10.3.1.0/24"]   
   subname=["sub-shared","sub-sand","sub-preprod"]
 - 

While running terraform validate -var-file=terraform.tfvars getting the following error

            Error: resource 'aws_subnet.subnet' config: unknown 
           resource 'data.aws_vpc.customVpc' referenced in variable 
           data.aws_vpc.customVpc.*.id

Is it because aws_subnet is not able to locate vpc_id since the resource aws_vpc is not created it . I am calling both the testvpc and subnet as modules in the mainfolder/main.tf . What am i missing .

Secondly is the loop in the aws_vpc and aws_subnet proper . It should create vpc-shared 10.1.0.0/16 and sub-shared within that vpc and so on

Answer 1

You need to use module outputs because you are trying to reference resources in a separate module. That won't work because

Modules encapsulate their resources. A resource in one module cannot directly depend on resources or attributes in other modules, unless those are exported through outputs.

So in mainfolder/modules/network/testvpc/main.tf, add an output like so

output "vpc_ids" { value=["${aws_vpc.customVpc.*.id}"] }

Then add a variable in mainfolder/modules/network/subnet/main.tf like so

variable "vpc_ids" {type="list"}

and use it within that module (instead of trying to directly reference resources from the /testvpc/main.tf module)

resource "aws_subnet" "subnet" {
  count = "${length(var.subcidr)}"
  vpc_id = "${element(var.vpc_ids, count.index)}"

  etc, etc

}

and finally now from your mainfolder/main.tf

module "testvpc" {
  source = "./modules/network/testvpc"
  vpccidr="${var.vpccidr}"
  vpcname="${var.vpcname}"
}

module "subnet" {
  source = "./modules/network/subnet"
  subcidr = "${var.subcidr}"
  subname = "${var.subname}"
  vpc_ids = "${module.testvpc.vpc_ids}"
}