Automatic Terraform documentation with terraform-docs

Terraform code reuse leads to modules. Modules lead to variables and outputs. Variables and outputs lead to massive amount of boilerplate documentation. terraform-docs lets you shortcut some of these steps and jump straight to consistent, easy to use, automatically generated documentation instead.

Terraform-docs, a self-contained binary implemented in Go, and released by Segment, provides an efficient way to add documentation to your terraform code without requiring large changes to your workflow or massive amounts of additional boilerplate. In its simplest invocation it reads the descriptions provided in your variables and outputs and displays them on the command line:

    * A sample terraform file with a variable and output

variable "greeting" {
  type        = "string"
  description = "The string used as a greeting"
  default     = "hello"

output "introduction" {
  description = "The full, polite, introduction"
  value       = "${var.greeting} from terraform"

Running terraform-docs against this code produces:

A sample terraform file with a variable and output

  var.greeting (hello)
  The string used as a greeting

  The full, polite, introduction

This basic usage makes it simpler to use existing code by presenting the official interface without over-burdening you with implementation details. Once you’ve added descriptions to your variables and outputs, something you should really already be doing, you can start to expose the documentation in other ways. By adding the markdown option -

terraform-docs markdown .

you can generate the docs in a GitHub friendly way that provides an easy, web based, introduction to what your code accepts and returns. We used this quite heavily in the GOV.UK AWS repo and it’s been invaluable. The ability to browse an overview of the terraform code makes it simpler to determine if a specific module does what you actually need without requiring you to read all of the implementation.

A collection of terraform variables and their defaults

When we first adopted terraform-docs we hit issues with the code being updated without the documentation changing to match it. We soon settled on using git precommit hooks, such as this terraform-docs githook script by Laura Martin or the heavy handed GOV.UK update-docs script. Once we had these in place the little discrepancies stopped slipping through and the reference documentation became a lot more trusted.

As an aside if you plan on using terraform-docs as part of your automated continuous integration pipeline you’ll probably want to create a terraform-docs package. I personally use FPM Cookery for this and it’d been an easy win so far.

I’ve become a big fan of terraform-docs and it’s great to see such self-contained tools making such a positive impact on the terraform ecosystem. If you’re writing tf code for consumption by more than just yourself (and even then) it’s well worth a second look.