» 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, then creating a module is a piece of cake.

Modules in Terraform are 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. This itself is a valid module.

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

Modules that are created for reuse should follow the standard structure. This structure enables tooling such as the Terraform Registry to inspect and generate documentation, read examples, and more.

» An Example Module

Within a folder containing Terraform configurations, create a subfolder called 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 your Terraform configuration files:

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

You've now created your first module! You can now add resources to the child module.

Note: Prior to running the above, you'll have to run the get command for Terraform to sync your modules. This should be instant since the module is a local path.

» 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 your Terraform configuration.

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. You can think about these variables and outputs as the API interface to your module.

Let's add a variable and an output to our child module.

variable "memory" {}

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

This will create a required variable, memory, and then an output, received, that will 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 now run terraform apply, you see how this works.

» Paths and Embedded Files

It is sometimes useful to embed files within the module that aren't Terraform configuration files, such as a script to provision a resource or a file to upload.

In these cases, you can't use a relative path, since paths in Terraform are generally relative to the working directory from which Terraform was executed. Instead, you want to use a module-relative path. To do this, you should use the path interpolated variables.

resource "aws_instance" "server" {
  # ...

  provisioner "remote-exec" {
    script = "${path.module}/script.sh"
  }
}

Here we use ${path.module} to get a module-relative path.

» Nested Modules

You can nest a module within another module. This module will be hidden from your root configuration, so you'll have to re-expose any variables and outputs you require.

The get command will automatically get all nested modules.

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, and this will all work fine within Terraform since the modules are created separately.

» Standard Module Structure

The standard module structure is a file and folder layout we recommend for reusable modules. Terraform tooling is built to understand the standard module structure and use that structure to generate documentation, index modules for the registry, and more.

The standard module expects the structure documented below. The list may appear long, but everything is optional except for the root module. All items are documented in detail. Most modules don't need to do any work to follow the standard structure.

  • Root module. This is the only required element for the standard module structure. Terraform files must exist in the root directory of the module. This should be the primary entrypoint for the module and is expected to be opinionated. For the Consul module the root module sets up a complete Consul cluster. A lot of assumptions are made, however, and it is fully expected that advanced users will use specific nested modules to more carefully control what they want.

  • README. The root module and any nested modules should have README files. This file should be named README or README.md. The latter will be treated as markdown. There should be a description of the module and what it should be used for. If you want to include an example for how this module can be used in combination with other resources, put it in an examples directory like this. Consider including a visual diagram depicting the infrastructure resources the module may create and their relationship. The README doesn't need to document inputs or outputs of the module because tooling will automatically generate this. If you are linking to a file or embedding an image contained in the repository itself, use a commit-specific absolute URL so the link won't point to the wrong version of a resource in the future.

  • LICENSE. The license under which this module is available. If you are publishing a module publicly, many organizations will not adopt a module unless a clear license is present. We recommend always having a license file, even if the license is non-public.

  • main.tf, variables.tf, outputs.tf. These are the recommended filenames for a minimal module, even if they're empty. main.tf should be the primary entrypoint. For a simple module, this may be where all the resources are created. For a complex module, resource creation may be split into multiple files but all nested module usage should be in the main file. variables.tf and outputs.tf should contain the declarations for variables and outputs, respectively.

  • Variables and outputs should have descriptions. All variables and outputs should have one or two sentence descriptions that explain their purpose. This is used for documentation. See the documentation for variable configuration and output configuration for more details.

  • Nested modules. Nested modules should exist under the modules/ subdirectory. Any nested module with a README.md is considered usable by an external user. If a README doesn't exist, it is considered for internal use only. These are purely advisory; Terraform will not actively deny usage of internal modules. Nested modules should be used to split complex behavior into multiple small modules that advanced users can carefully pick and choose. For example, the Consul module has a nested module for creating the Cluster that is separate from the module to setup necessary IAM policies. This allows a user to bring in their own IAM policy choices.

  • Examples. Examples of using the module should exist under the examples/ subdirectory at the root of the repository. Each example may have a README to explain the goal and usage of the example. Examples for submodules should also be placed in the root examples/ directory.

A minimal recommended module following the standard structure is shown below. While the root module is the only required element, we recommend the structure below as the minimum:

$ tree minimal-module/
.
├── README.md
├── main.tf
├── variables.tf
├── outputs.tf

A complete example of a module following the standard structure is shown below. This example includes all optional elements and is therefore the most complex a module can become:

$ tree complete-module/
.
├── README.md
├── main.tf
├── variables.tf
├── outputs.tf
├── ...
├── modules/
│   ├── nestedA/
│   │   ├── README.md
│   │   ├── variables.tf
│   │   ├── main.tf
│   │   ├── outputs.tf
│   ├── nestedB/
│   ├── .../
├── examples/
│   ├── exampleA/
│   │   ├── main.tf
│   ├── exampleB/
│   ├── .../

» Publishing Modules

If you've built a module that you intend to be reused, we recommend publishing the module on the Terraform Registry. This will version your module, generate documentation, and more.

Published modules can be easily consumed by Terraform, and in Terraform 0.11 you'll also be able to constrain module versions for safe and predictable updates. The following example shows how easy it is to consume a module from the registry:

module "consul" {
  source = "hashicorp/consul/aws"
}

You can also gain all the benefits of the registry for private modules by signing up for a private registry.