Cross reference variable from another module

[Kenan Virtucio]

Hi, 

I have this directory structure:

|-- modules

|   |-- network

|   |   |-- main.tf

|   |   |-- outputs.tf

|   |   `-- vars.tf

|   `-- services

|       `-- frontend-app

|           |-- main.tf

|           |-- outputs.tf

|           `-- vars.tf

`-- sample_customer

    |-- network

    |   |-- main.tf

    |   `-- vars.tf

    |-- services

    |   `-- frontend-app

    |       |-- main.tf

    `       `-- vars.tf

I just a question regarding cross referencing variables:

Under modules -> network -> main.tf, I have this resource declared:

resource "aws_subnet" "subnet1" {

  cidr_block        = "${element(var.subnet, 0)}"

  vpc_id            = "${aws_vpc.example_vpc.id}"

  availability_zone = "${element(var.availability_zone, 0)}"

}

Under modules -> network -> output.tf, I have this declaration:

output "subnet1" {

  value = "${aws_subnet.example_subnet1.id}"

}

I need to reference this variable subnet output under network module to be used in frontend-app module.

So here's how i cross reference it:

Under sample_customer -> services -> frontend-app -> main.tf

module "frontend" {

  source  = "../../../modules/services/frontend/" 

  subnet1 = "${module.network.subnet1}"

}

If i do this, i get an error: * module 'frontend': unknown module referenced: network

So my question is, how can i reference a variable from another module? It's really causing me confusion.

Thanks!

[Lowe Schmidt]

How and where do you instantiate the network module? 

--

Lowe Schmidt | +46 723 867 157

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[Kenan Virtucio]

Actually that is my question. How should instantiate it? Because it's really throwing me off.

Here, let me explain.

The whole content of sample_customer -> services -> frontend-app -> main.tf

module "frontend" {

  source  = "../../../modules/services/frontend/" 

  subnet1 = "${module.network.subnet1}"

}

According to the documentation of terraform, when i declare the variable subnet1 i have first to declare/instantiate the module first. So what would happen to the main.tf is:

module "network" {

  source = "../modules/network/"

}

module "frontend" {

  source  = "../../../modules/services/frontend/" 

  subnet1 = "${module.network.subnet1}"

}

This new declaration would present new problems:

1) Whenever i declare the module network (i'm assuming they'll become sibling modules), it will re-instantiate the entire network and i don't want that to happen.

2) I said in #1 that the VPC was already instantiated because of sample_customer -> network -> main.tf:

module "network" {

  source = "../modules/network/"

}

Here's my use case: I want to re-use my entire infrastructure, this includes the VPC as well. Also, the newly spawned web apps should be within this VPC network.

The only way i do is i have to add the option vpc_zone_identifier: https://www.terraform.io/docs/providers/aws/r/autoscaling_group.html. So i have to cross reference the newly spawned VPC resource (using output of terraform) to  another module.

I got this idea from: 

• https://blog.gruntwork.io/how-to-create-reusable-infrastructure-with-terraform-modules-25526d65f73d
  
• https://www.safaribooksonline.com/library/view/terraform-up-and/9781491977071/ch04.html

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[Lowe Schmidt]

Right, so you have one VPC + network subnets and multiple customer modules instantiated correct?

So, to share the network part without instantiating the VPC/Network modules on every customer you will either have to use the remote state data source https://www.terraform.io/docs/providers/terraform/d/remote_state.html or the aws_subnet_id data source https://www.terraform.io/docs/providers/aws/d/subnet_ids.html 

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Lowe Schmidt | +46 723 867 157

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[Kenan Virtucio]

Ideally, my goal is that for each customer thy will have their own VPC (including their own subnets) and all their corresponding modules will be within their own VPC.

[Kenan Virtucio]

Thanks Lowe for the suggestion. I finally made it work.

I published it at https://github.com/kenanbiel/terraform_template