197 lines
9.2 KiB
Markdown
197 lines
9.2 KiB
Markdown
---
|
|
layout: "docs"
|
|
page_title: "Creating Modules"
|
|
sidebar_current: "docs-modules"
|
|
description: |-
|
|
A module is a container for multiple resources that are used together.
|
|
---
|
|
|
|
# Creating Modules
|
|
|
|
> **Hands-on:** Try the [Reuse Configuration with Modules](https://learn.hashicorp.com/collections/terraform/modules?utm_source=WEBSITE&utm_medium=WEB_IO&utm_offer=ARTICLE_PAGE&utm_content=DOCS) collection on HashiCorp Learn.
|
|
|
|
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`](/docs/commands/plan.html)
|
|
or [`terraform apply`](/docs/commands/apply.html) together form the _root_
|
|
module. That module may [call other modules](/docs/configuration/modules.html#calling-a-child-module)
|
|
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](/docs/configuration/modules.html).
|
|
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](/docs/configuration/) concepts we use in root modules.
|
|
Most commonly, modules use:
|
|
|
|
* [Input variables](/docs/configuration/variables.html) to accept values from
|
|
the calling module.
|
|
* [Output values](/docs/configuration/outputs.html) to return results to the
|
|
calling module, which it can then use to populate arguments elsewhere.
|
|
* [Resources](/docs/configuration/resources.html) 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](./composition.html)
|
|
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](https://www.consul.io/) 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](https://registry.terraform.io/modules/hashicorp/consul)
|
|
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](https://github.com/hashicorp/terraform-aws-consul/tree/master/examples).
|
|
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](/docs/configuration/variables.html) and
|
|
[output configuration](/docs/configuration/outputs.html) 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](https://registry.terraform.io/modules/hashicorp/consul)
|
|
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](./composition.html) 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:
|
|
|
|
```sh
|
|
$ 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:
|
|
|
|
```sh
|
|
$ 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/
|
|
│ ├── .../
|
|
```
|