Terraform Child module dependency on a calling resource

Question

I am trying to create dependency between multiple sub modules which should be able to create the resource individually as well as should be able to create the resource if they are dependent on each other.

basically i am trying to create multiple VMs, and based on the ip addresses and vip ip address returned as the output i want to create the lbaas pool and lbaas pool members.

i have kept the project structure as below

 - Root_Folder
    - main.tf // create all the vm's 
    - output.tf
    - variable.tf
    - calling_module.tf
    - modules
        - lbaas-pool
            - lbaas-pool.tf // create lbaas pool
            - variable.tf
            - output.tf
        - lbaas-pool-members  
            - lbaas-pool-members.tf // create lbaas pool member
            - variable.tf
            - output.tf

calling_module.tf contains the reference to the lbaas-pool module and lbaas-pool-members, as these 2 modules are dependent on the output of the resource generated by main.tf file. It is giving below error:

A managed resource has not been declared.

As the resource has not been generated yet, and while running terraform plan and apply command is trying to load the resource object which has not been created. Not sure with his structure declare the module implicit dependency between the resources so the module can work individually as well as when required the complete stack.

Expected behaviour:

main.tf output parameters should be create the dependency automatically in the terraform version 0.14 but seems like that is not the case from the above error.

Answer 1

Let's say you have a module that takes an instance ID as an input, so in modules/lbaas-pool you have this inside variable.tf

variable "instance_id" {
   type = string
}

Now let's say you define that instance resource in main.tf:

resource "aws_instance" "my_instance" {
  ...
}

Then to pass that resource to any modules defined in calling_module.tf (or in any other .tf file in the same folder as main.tf), you would do so like this:

module "lbaas-pool" {
  src="modules/lbaas-pool"
  instance_id = aws_instance.my_instance.id
  ...
}

---

Notice how there is no output defined at all here. Any output at the root level is for exposing outputs to the command line console, not for sending things to child modules.

Also notice how there is no data source defined here. You are not writing a script that will run in a specific order, you are writing templates that tell Terraform what you want your final infrastructure to look like. Terraform reads all that, creates a dependency graph, and then deploys everything in the order it determines. At the time of running terraform plan or apply anything you reference via a data source has to already exist. Terraform doesn't create everything in the root module, then load the submodule and create everything there, it creates things in whatever order is necessary based on the dependency graph.