How to create a resource group that can be shared between modules in terraform?

Question

What is the proper way to create a resource group in terraform for azure that can be shared across different modules? I've been banging my head against this for a while and it's not working. As you can see in this image. I have a resource group in a separate folder. In my main.tf file i load the modules appservice and cosmosdb. I cant seem to figure out how to make the appservice and cosmosdb tf files reference the resource group here that is in this location. How is this done? Any suggestions would be greatly appreciated. Thank you.

Answer 1

In general, it is not recommended to have a module with a single resource like you have organized your code. However, in this situation, you would need to provide the exported resource attributes as an output for that module. In your resource_group module:

output "my_env_rg" {
  value       = azurerm_resource_group.rg
  description = "The my-env-rg Azure resource group."
}

Then, the output containing the map of exported resource attributes for the resource becomes accessible in a config module where you have declared the module. For example, in your root module config (presumably containing your main.tf referenced in the question):

module "azure_resource_group" {
  source = "resource-group"
}

would make the output accessible with the namespace module.<MODULE NAME>.<OUTPUT NAME>. In this case, that would be:

module.azure_resource_group.my_env_rg

Answer 2

There's two different kinds of sharing that require different solutions. You need to decide which kind of sharing you're looking for because your example isn't very illustrative.

The first is where you want to make a pattern of creating things that you want to use twice. The goal is to create many different things, each with different parameters. The canonical example is a RDS instance or an EC2 instance. Think of the Terraform module as a function where you execute it with different inputs in different places and use the different results independently. This is exactly what Terraform modules are for.

The second is where you want to make a thing and reference it in multiple places. The canonical example is a VPC. You don't want to make a new VPC for every autoscaling group - you want to reuse it.

Terraform doesn't have a good way of stitching the outputs from one set of resources as inputs to another set. Atlas does and Cloudformation does as well. If you're not using those, then you have to stitch them together yourself. I have always written a wrapper around Terraform which enables me to do this (and other things, like validation and authentication). Save the outputs to a known place and then reference them as inputs later.