» Creating Modules

A module is a container for multiple resources that are used together. Modules can be used to create lightweight abstractions, so that you can describe your infrastructure in terms of its architecture, rather than directly in terms of physical objects.

The .tf files in your working directory when you run terraform plan or terraform apply together form the root module. That module may call other modules and connect them together by passing output values from one to input values of another.

To learn how to use modules, see the Modules configuration section. This section is about creating re-usable modules that other configurations can include using module blocks.

» Module structure

Re-usable modules are defined using all of the same configuration language concepts we use in root modules. Most commonly, modules use:

  • Input variables to accept values from the calling module.
  • Output values to return results to the calling module, which it can then use to populate arguments elsewhere.
  • Resources to define one or more infrastructure objects that the module will manage.

To define a module, create a new directory for it and place one or more .tf files inside just as you would do for a root module. Terraform can load modules either from local relative paths or from remote repositories; if a module will be re-used by lots of configurations you may wish to place it in its own version control repository.

Modules can also call other modules using a module block, but we recommend keeping the module tree relatively flat and using module composition as an alternative to a deeply-nested tree of modules, because this makes the individual modules easier to re-use in different combinations.

» When to write a module

In principle any combination of resources and other constructs can be factored out into a module, but over-using modules can make your overall Terraform configuration harder to understand and maintain, so we recommend moderation.

A good module should raise the level of abstraction by describing a new concept in your architecture that is constructed from resource types offered by providers.

For example, aws_instance and aws_elb are both resource types belonging to the AWS provider. You might use a module to represent the higher-level concept "HashiCorp Consul cluster running in AWS" which happens to be constructed from these and other AWS provider resources.

We do not recommend writing modules that are just thin wrappers around single other resource types. If you have trouble finding a name for your module that isn't the same as the main resource type inside it, that may be a sign that your module is not creating any new abstraction and so the module is adding unnecessary complexity. Just use the resource type directly in the calling module instead.

» Standard Module Structure

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

The standard module structure expects the layout documented below. The list may appear long, but everything is optional except for the root module. Most modules don't need to do any extra 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 repository. 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. It makes a lot of assumptions however, and we expect 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 it is not an open source license.

  • 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 any nested module calls 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.

If the root module includes calls to nested modules, they should use relative paths like ./modules/consul-cluster so that Terraform will consider them to be part of the same repository or package, rather than downloading them again separately.

If a repository or package contains multiple nested modules, they should ideally be composable by the caller, rather than calling directly to each other and creating a deeply-nested tree of modules.

  • 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.

Because examples will often be copied into other repositories for customization, any module blocks should have their source set to the address an external caller would use, not to a relative path.

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/
│   ├── .../