Reusable module in Terraform

Question

I want to get an autoscaling group name variable in module_apps and use it in other modules - for example module_init. However when I try to run terraform plan it does not work.

content of module_apps

resource "aws_autoscaling_group" "example" 
{
  name = "asg-ad"
}

output "asg_name" 
{
  value = "${aws_autoscaling_group.example.name}"
}

content of module_init

module "module_init" 
{
  source = "../module_apps"
  asg_name = "${module.module_init.asg_name}"
}

Trying to run terraform plan seems that it can't get the source module:

o:Get: file:///home/user/terraform/application/module_apps
o:Get: file:///home/user//terraform/application/module_init
o:Get: file:///home/user//terraform/application/module_elb
o:Get: file:///home/user//terraform/application/module_deloy
o:Get: file:///home/user/terraform/application/module_apps
o:Get: file:///home/user//terraform/application/module_init
o:Get: file:///home/user//terraform/application/module_elb
o:Get: file:///home/user//terraform/application/module_deloy
o:Get: file:///home/user//terraform/application/module_init
o:Get: file:///home/user//terraform/application/module_elb
o:Get: file:///home/user//terraform/application/module_deloy
o:Get: file:///home/user/terraform/application/module_apps
o:Get: file:///home/user//terraform/application/module_init
o:Get: file:///home/user//terraform/application/module_elb
o:Get: file:///home/user//terraform/application/module_deloy
o:Get: file:///home/user//terraform/application/module_init
o:Get: file:///home/user//terraform/application/module_elb
o:Get: file:///home/user//terraform/application/module_deloy
o:Get: file:///home/user/terraform/application/module_apps
o:Get: file:///home/user//terraform/application/module_init
o:Get: file:///home/user//terraform/application/module_elb
o:Get: file:///home/user//terraform/application/module_deloy

Answer 1

If it doesn't outright fail you can expect to see a cycle warning where you've created a loop in Terraform and it's trying to fulfil your request for asg-name before the asg has been created.

To get what you need you'll need to have something like the following:

module_apps/module_apps.tf

resource "aws_autoscaling_group" "example" {
  name = "${var.asg_name}"
}

module_init.tf

variable "asg_name" { default = "asg-ad" }

module "module_init" {
  source = "../module_apps"

  asg_name = "${var.asg_name}"
}

output "asg_name" {
  value = "${var.asg_name}"
}

This way, if you call module_init somewhere else you'll have asg_name available to you. Always be careful of trying to use an output of a module to create the module, Terraform doesn't handle loops like that well and the errors can be confusing to diagnose.