Writing a Terraform Module

External

• https://www.terraform.io/docs/modules/index.html

Internal

• Terraform Concepts

Reference Example

• https://registry.terraform.io/modules/hashicorp/consul/aws/0.7.3
• https://github.com/hashicorp/terraform-aws-consul

Playground

https://github.com/ovidiuf/playground/tree/master/hashicorp/terraform/blue-module

Procedure

To define a module, create a new root directory for it and place one or more .tf files inside it.

Standard Structure

Everything is optional, except for the root module:

• Root module - this is the only required element for the standard module structure, and it consists in .tf files - which must exist - in the root directory of the module.
• README or README.md. Should contain a description of the module. Examples can be included in an example directory.
• LICENSE
• Recommended file (they should exist even if they are empty):
  • main.tf - this must contain all nested module calls, if any. If such calls exist, the should use paths like ./modules/my-internal-module/... so that Terraform will consider them to be part of the same repository or package, rather than downloading them again separately.
  • variables.tf - this must contain input variable declarations. Input variables should have descriptions.
  • outputs.tf - this must contain outputs. Output variables should have descriptions.
• For a complex module, resource creation may be split into multiple files, but any nested module calls should be in the main file.
• modules/ directory should contain the nested modules. 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.
• examples/ directory should contain examples. 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.

.
|-- main.tf
|-- variables.tf
|-- outputs.tf
|-- README.md
|-- modules
|      |
|      |-- nestedA     
|      |       |
|      |       |-- main.tf
|      |       |-- variables.tf
|      |       |-- outputs.tf
|      |       |-- README.md
|      |       +- ...
|      |
|      |-- nestedB     
|              |
|              |-- main.tf
|              |-- variables.tf
|              |-- outputs.tf
|              |-- README.md
|              +- ...
|
+-- examples

Module Composition

https://www.terraform.io/docs/modules/composition.html

Module Versioning

Modules intended for public consumption must be versioned. Hashicorp publishes the Consul module, which is supposed to be a reference implementation, using the following versioning scheme:

https://releases.hashicorp.com/consul/version/consul_version_linux_amd64.zip

where "version" is a semantic version string: "0.8.0"

However, the versioning scheme seems to be entirely "external", in that there is no metdata in the module that says what version the module is.