Azure KeyVault Terraform cycle

Question

I am trying to write a Terraform descriptor for integration Azure Functions, KeyVault and CosmosDB.

On one hand I need Azure Functions identity id to create KeyVault access policy. On the other I need KeyVault's CosmosDB key reference to put into Azure Functions configuration. That causes cycle dependency Azure Functions <-> KeyVault. Is there a way to solve it some way? If I would do it manually, I would create Azure Functions App, create KeyVault, add access policy in KeyVault and update Azure Functions with KeyVault key reference. But as far as I know, Terraform doesn't allow to create and update resource later.

Some code snippets:

functions.tf

variable "db_key" {
   type = string
}

resource "azurerm_linux_function_app" "my_functions" {
   ...
   app_settings = {
      "DB_KEY": var.db_key
   }
} 

output "functions_app_id" {
   value = azurerm_linux_function_app.my_functions.identity[0].principal_id
}

keyvault.tf

variable "functions_app_id" {
  type = string
}

resource "azurerm_key_vault" "my_keyvault" {
   access_policy {
      tenant_id = ...
      object_id = var.functions_app_id

      secret_permissions {
         "Get"
      }
   }
}

resource "azurerm_key_vault_secret" "db_key" {
   ...
}

output "db_key" {
   value = "@Microsoft.KeyVault(SecretUri=${azurerm_key_vault_secret.db_key.id})"
}

main.tf

module "functions" {
   ...
   db_key = module.key-vault.db_key
}

module "key-vault" {
   ...
   functions_app_id = module.functions.functions_app_id
}

Answer 1

You can:

1. Create Key Vault with key
2. Create function with key reference
3. Add access policy or RBAC to vault for function

Answer 2

Ok I have figured out how to do this. Instead of using access_policy block in key_vault script, I should have used "azurerm_key_vault_access_policy" resource in functions.tf. Now it looks like this

functions.tf

variable "db_key" {
   type = string
}

resource "azurerm_linux_function_app" "my_functions" {
   ...
   app_settings = {
      "DB_KEY": var.db_key
   }

   identity {
      type = "SystemAssigned"
   }
} 

resource "azurerm_key_vault_access_policy" "functions_app_access_policy" {
   key_vault_id = ... //passed as output from key_vault.tf
   tenant_id = ...
   object_id = azurerm_linux_function_app.my_functions.identity[0].principal_id

   secret_permissions = ["Get"]
}

And there is no access_policy block in key_vault.tf file anymore