How to use variables from other modules in Terraform: Adding Host Project id to the Service Projects. (GCP)

My infrastructure its composed by a Host Project and several Service Projects that are using its Shared VPC.

I have refactored my .tf files of my infrustructure as it follows:

├── env
|   ├── dev
│       ├── main.tf
│       ├── outputs.tf
│       └── variables.tf
│   ├── pre
│   └── pro
├── host
│   ├── main.tf
│   ├── outputs.tf
│   ├── terraform.tfvars
│   └── variables.tf
└── modules
    ├── compute
    ├── network
    └── projects

The order of creation of the infrastructure is:

1. terraform apply in /host
2. terraform apply in /env/dev (for instance)

In the main.tf of the host directory I have created the VPC and enabled Shared VPC hosting:

# Creation of the hosted network
resource "google_compute_network" "shared_network" {
  name                    = var.network_name
  auto_create_subnetworks = false
  project                 = google_compute_shared_vpc_host_project.host_project.project
  mtu                     = "1460"
 
}

# Enable shared VPC hosting in the host project.
resource "google_compute_shared_vpc_host_project" "host_project" {
  project    = google_project.host_project.project_id
  depends_on = [google_project_service.host_project]
}

The issue comes when I have refer to the Shared VPC Network in the Service Projects.

In the main.tf from env/dev/ I have set the following:

resource "google_compute_shared_vpc_service_project" "service_project_1" {
  host_project    = google_project.host_project.project_id
  service_project = google_project.service_project_1.project_id

  depends_on = [
    google_compute_shared_vpc_host_project.host_project,
    google_project_service.service_project_1,
  ]
}

QUESTION

How do I refer to the Host Project ID from another directory in the Service Project?

What I have tried so far

1. I have thought of using Ouput Values and Data Sources:

   In the host/outputs.tf declared as an output the Project ID as:

   output "project_id" {
       value = google_project.host_project.project_id
    }
   

   But then I end up not knowing how to implement this output in my env/dev/main.tf

2. I have thought on Data Sources and, in the env/dev/main.tf fetch for the Host Project ID. But then, in order to fetch it, I would need its name (which breaks the purpose of providing it in a programatic way if I have to hardcode it).

What should I try next? What I am missing?

Solution 1:

The files under the env/dev folder can't see anything above it, only any referenced modules.
You could refactor the host folder into a module to allow access to it's outputs... but that adds a risk that the host will be destroyed whenever you destroy a dev environment.
I would try running terraform output -raw project_id after creating the host and piping it to a text file or environment variable. Then using that as the input for a new "host_project" or similar variable in the 'env/dev' deployment.