How to use output of one child module as an input to another child module in Terraform

Question

I have the below directory structure

├── main.tf
├── output.tf
├── variables.tf
├── modules
│   ├── ServicePrincipal
│   │   ├── variables.tf
│   │   ├── outputs.tf
│   │   ├── main.tf
│   ├── aks
│   │   ├── main.tf
│   │   ├── output.tf
│   │   └── variables.tf
...

Issue: I want to use client_id and client_secret generated from service principal module as an input to create my aks cluster. I am able to reference the below output variables from my root main.tf by module.modulename.outputvarname however, I cannot access it in another child module(aks) as var.client_id or module.serviceprincipal.client_id

main.tf of root module where I am able to use client_id and client_secret

module "ServicePrincipal" {
  source                 = "./modules/ServicePrincipal"
  service_principal_name = var.service_principal_name
  redirect_uris          = var.redirect_uris

}

module "aks" {
  source                 = "./modules/aks/"
  service_principal_name = var.service_principal_name
  serviceprinciple_id    = module.ServicePrincipal.service_principal_object_id
  serviceprinciple_key   = module.ServicePrincipal.client_secret
  location            = var.location
  resource_group_name = var.rgname
  depends_on = [
    module.ServicePrincipal
  ]

}

main.tf of aks module

service_principal  {
    client_id = var.client_id
    client_secret = var.client_secret
  }

output.tf for my ServicePrincipal module

output "client_id" {
  description = "The application id of AzureAD application created."
  value       = azuread_application.main.application_id

}

output "client_secret" {
  description = "Password for service principal."
  value       = azuread_service_principal_password.main.*.value
  
}

Below is the error I am getting:

Error: Missing required argument

  on main.tf line 136, in module "aks":
 136: module "aks" {

The argument "client_id" is required, but no definition was found.


Error: Missing required argument

  on main.tf line 136, in module "aks":
 136: module "aks" {

The argument "client_secret" is required, but no definition was found.

I already defined those as variables in aks module and root module, am I missing something here?

Thanks in advance!

Piyush

Answer 1

Child modules can't reference each others outputs. You have to explicitly pass them in the root module from one module to the second, e.g.

in root:

module "ServicePrincipal" {
}

module "aks" {
   client_id = module.ServicePrincipal.client_id
}

Answer 2

You're using output name as client_id and client_secret but in module you're calling with different names ?

module.ServicePrincipal.service_principal_object_id