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Creating Modules

Creating modules in Terraform is easy. You may want to do this to better organize your code, to make a reusable component, or just to learn more about Terraform. For any reason, if you already know the basics of Terraform, creating a module is a piece of cake.

Modules in Terraform are just folders with Terraform files. In fact, when you run terraform apply, the current working directory holding the Terraform files you're applying comprise what is called the root module. It itself is a valid module.

Therefore, you can enter the source of any module, run terraform apply, and expect it to work (assuming you satisfy the required variables, if any).

An Example

Within a folder containing Terraform configurations, create a subfolder "child". In this subfolder, make one empty "main.tf" file. Then, back in the root folder containing the "child" folder, add this to one of the Terraform files:

module "child" {
	source = "./child"
}

This will work. You've created your first module! You can add resources to the child module to see how that interaction works.

Inputs/Outputs

To make modules more useful than simple isolated containers of Terraform configurations, modules can be configured and also have outputs that can be consumed by the configuration using the module.

Inputs of a module are variables and outputs are outputs. There is no special syntax to define these, they're defined just like any other variables or outputs.

In the "child" module we created above, add the following:

variable "memory" {}

output "received" {
	value = "${var.memory}"
}

This will create a required variable "memory" and then an output "received" that will simply be the value of the memory variable.

You can then configure the module and use the output like so:

module "child" {
	source = "./child"

	memory = "1G"
}

output "child_memory" {
	value = "${module.child.received}"
}

If you run apply, you'll again see that this works.

And that is all there is to it. Variables and outputs are used to configure modules and provide results. Resources within a module are isolated, and the whole thing is managed as a single unit.

Nested Modules

You can use a module within a module just like you would anywhere else. This module will be hidden from the root user, so you'll have re-expose any variables if you need to, as well as outputs.

The get command will automatically get all nested modules as well.

You don't have to worry about conflicting versions of modules, since Terraform builds isolated subtrees of all dependencies. For example, one module might use version 1.0 of module "foo" and another module might use version 2.0 of module "foo", and this would all work fine within Terraform since the modules are created separately.