Terraform Style Guide

This is Gruntwork’s style guide for Terraform. It aims to help us ensure that the code we write is clear, readable, idiomatic Terraform code. The conventions detailed in this guide are our preferences and should be thought of as guidelines rather than hard rules.

Starting point

We follow the official HashiCorp style guide for Terraform.

All of these are enforced using terraform fmt. All Gruntwork Terraform repos should enforce this using pre-commit hooks; please add if missing.

On top of the official guide, Gruntwork follows some additional conventions.

Additional conventions

General

2 space indentations

Block contents should be indented with 2 spaces.

120 column limit

We follow a 120 column line-length limit, except for description strings in variable and output blocks, where single line strings are preferred.

Block Labels, Variables, and Outputs should be snake case

The label for blocks should be in snake case. E.g. example_instance , not ExampleInstance or example-instance.

Labels are the strings that follow block names. For example, in the following, aws_instance and example_instance are labels for the resource block.

resource "aws_instance" "example_instance" {
  # Omitted for brevity
}

This includes variables and outputs as well:

variable "vpc_id" {}
output "instance_name" {}

Module folder conventions

Each module repo should have the following 3 folders:

modules: Terraform modules that are designed to be consumed by users. The intention is that users should pull the modules in the modules folder in their terraform code using module blocks.
examples: Folder that contains top level Terraform modules that provide an example of how to use the modules in the modules folder. The examples folder often has subfolders for-learning-and-testing and for-production that contain corresponding example code. See Testing: Terratest for more info on how these examples should be organized.
test: Terratest Go files for testing the code in the repo. See Testing: Terratest for specific conventions around Terratest.

Additionally, each module in modules should be organized with the following files:

variables.tf: All variable blocks should go in here and they specify the inputs.
outputs.tf: All output blocks should go in here and they specify the outputs.
main.tf: All other logic should be added here.
dependencies.tf (optional): Any external references that are pulled in by a data source block should go in here. This allows consumers of the module to quickly scan for what resources need to already exist to deploy the module.

Any nonstandard file structure should be called out in the README (e.g., if main.tf is split up into multiple smaller terraform files).

`variables.tf` conventions

Each variable block should always define a description and type, even if it is of the string type (the default), in that order. E.g.:

variable "example" {
  description = "This is an example"
  type        = string
  default     = "example"  # NOTE: this is optional
}

Complex types

Prefer concrete objects (object type) over free form maps. However, for particularly large objects it is useful to support optional attributes. This is currently not supported in terraform, so workaround by using any type.

When using any type, always use comments to describe the supported attributes. Example.

`outputs.tf` conventions

Each output block should always define a description, before the value:

output "greeting" {
  description = "This is a greeting for everyone."
  value       = "hello world!"
}

`main.tf` conventions

main.tf should (loosely) be organized by sections that correspond to components. There is no standard on grouping, but as a rule of thumb each section should be focused on a specific component of the module. For example, an ECS service module may consist of the following sections:

The ECS service resource, and any locals logic for setting up the attributes of the resource.
The ECS task definition resource, and any locals and template logic for setting up the attributes of the resource (e.g. the container definition).
Any resources related to configuring ELBs to forward traffic to the ECS service (e.g., listeners and target groups).
Any resources related to configuring IAM permissions for the ECS service.
Any resources related to configuring network access (e.g., security group rules) for the ECS service.

There is no standard on ordering the sections, but as a rule of thumb the following sections should be placed first, in order:

Version constraints for the module
Provider blocks, if needed.
The main component of the module (e.g., the aws_ecs_service resource for the ECS service module).
All other sections.
Any data blocks (at the bottom).

Conditionals

Use () to break up conditionals across multiple lines.

Examples:

locals {
  elb_id = (
    var.elb_already_exists
    ? var.elb_id
    : module.elb.elb_id
  )

  excluded_child_account_ids = (
    var.config_create_account_rules
    ? []
    : [
      for account_name, account in module.organization.child_accounts
      : account.id if lookup(lookup(var.child_accounts, account_name, {}), "enable_config_rules", false) == false
    ]
  )
}

Comments

This section lists the Gruntwork conventions around comments in Terraform code.

Prefer `#` over `//`

Use # for comment strings, not // or /**/.

Prefer `# -` over `# ~`

Delimit section header comment blocks with # ---- instead of # \~~~~.

`variables.tf`

variables.tf files should clearly indicate required environment variables, and separate out required variables from optional variables (with defaults) using block comments.

Example:

## ---------------------------------------------------------------------------------------------------------------------
## ENVIRONMENT VARIABLES
## Define these secrets as environment variables
## ---------------------------------------------------------------------------------------------------------------------

## TF_VAR_master_password

## ---------------------------------------------------------------------------------------------------------------------
## MODULE PARAMETERS
## These variables are expected to be passed in by the operator
## ---------------------------------------------------------------------------------------------------------------------

variable "required_var" {
  description = "This variable must be set in order to create the resource."
  type        = string
}

## ---------------------------------------------------------------------------------------------------------------------
## OPTIONAL PARAMETERS
## These variables have defaults and may be overridden
## ---------------------------------------------------------------------------------------------------------------------

variable "optional_var" {
  description = "This variable has a sensible default so it is not necessary to set it explicitly for this module to work."
  type        = string
  default     = "Hello world"
}

`main.tf`

Section comments

Each section (as described in conventions) of main.tf should have block comments describing the component managed in the section.

Example:

## ---------------------------------------------------------------------------------------------------------------------
## ONE LINE SUMMARY DESCRIBING WHAT IS BEING MANAGED IN THIS SECTION IN ALL CAPS
## The rest of the comments should be in standard casing. This section should contain an overall description of the
## component that is being managed, and highlight any unconventional workarounds or configurations that are in place.
## ---------------------------------------------------------------------------------------------------------------------

Testing: Terratest

Gruntwork uses Terratest to write tests for Terraform modules. Terratest is a Go library that provides patterns and helper functions for testing infrastructure code.

Terratest best practices

Follow all the best practices listed in Terratest best practices.

The rest of the items below are additional conventions on top of the documented best practices that Gruntwork follows when writing tests using Terratest for terraform modules.

Code formatting

Terratest is a Go library, so each test will be written in Go. All Go source files should be formatted using goimports and go fmt.

`examples` and `tests`

In many cases the individual modules in the modules folder are narrowly focused to a specific subset of the overall infrastructure. This means that in many cases you will need to provide dependent resources externally to the module in order to actually deploy them. The Terraform modules in the examples folder serves this purpose, specifying test resources that are injected as dependencies to the modules.

As such, the tests should be written against the examples folder, as opposed to the modules folder directly. In other words:

Every module in modules should have a corresponding example module in examples that calls it. (NOTE: you can have a single example call multiple modules).
Every example should have at least one test that calls it.
Tests should not directly call modules in the modules folder. Always go through the examples.

Parallel

Every test should have the t.Parallel call in the test function unless there is a specific need to run tests serially, e.g. manipulating process global resources, like environment variables. This is so that tests run as quickly as possible.

To facilitate this, every reference to a terraform example should use test_structure.CopyTerraformFolderToTemp to create a copy of the example module in a temp directory. Then as the test runs, any stateful changes to the example module directory are isolated across tests, so that there’s no conflict on parallel runs.

Use TestStages for faster development

Use test stages in the test code, unless you only have 1 or 2 steps in the test code (e.g. a plan verification test).

It’s very tedious to build and deploy resources over and over when you only want to tweak a validation step. TestStages make it flexible and convenient to skip stages, making development much faster.

For each test stage you introduce, add a commented out series of os.Setenv calls to make it convenient to skip stages as you develop.

func TestJenkins(t *testing.T) {
        t.Parallel()

        // Uncomment the items below to skip certain parts of the test
        //os.Setenv("SKIP_build_ami", "true")
        //os.Setenv("SKIP_deploy_terraform", "true")
        //os.Setenv("SKIP_validate", "true")
        //os.Setenv("SKIP_cleanup", "true")
        //os.Setenv("SKIP_cleanup_ami", "true")

        defer test_structure.RunTestStage(t, "cleanup_ami", deleteAMI)
        defer test_structure.RunTestStage(t, "cleanup", destroyInfra)
        test_structure.RunTestStage(t, "build_ami", buildAMI)
        test_structure.RunTestStage(t, "deploy_terraform", deployInfra)
        test_structure.RunTestStage(t, "validate", validateInfra)
}

To use the stages, here’s an example workflow. The first time you run the test, you’ll want to skip only the cleanup stages:

// Uncomment the items below to skip certain parts of the test
//os.Setenv("SKIP_build_ami", "true")
//os.Setenv("SKIP_deploy_terraform", "true")
//os.Setenv("SKIP_validate", "true")
os.Setenv("SKIP_cleanup", "true")
os.Setenv("SKIP_cleanup_ami", "true")

Let’s say building and deploying were successful, but validation failed. Since resources were not cleaned up, we can run only the validate stage. We skip the resource and time intensive build and deploy stages, and also continue to skip the cleanup stages.:

// Uncomment the items below to skip certain parts of the test
os.Setenv("SKIP_build_ami", "true")
os.Setenv("SKIP_deploy_terraform", "true")
//os.Setenv("SKIP_validate", "true")
os.Setenv("SKIP_cleanup", "true")
os.Setenv("SKIP_cleanup_ami", "true")

Once you’ve established that validation works, you can then run only the cleanup stages as below. Your workflow may vary.

// Uncomment the items below to skip certain parts of the test
os.Setenv("SKIP_build_ami", "true")
os.Setenv("SKIP_deploy_terraform", "true")
os.Setenv("SKIP_validate", "true")
//os.Setenv("SKIP_cleanup", "true")
//os.Setenv("SKIP_cleanup_ami", "true")

When committing the final version of the test, all should be commented out so all stages run.

// Uncomment the items below to skip certain parts of the test
//os.Setenv("SKIP_build_ami", "true")
//os.Setenv("SKIP_deploy_terraform", "true")
//os.Setenv("SKIP_validate", "true")
//os.Setenv("SKIP_cleanup", "true")
//os.Setenv("SKIP_cleanup_ami", "true")

Setup and Teardown pattern

In some cases you will want to write a group of tests that use a common resource, such as a Docker image or VPC. In this case, you will want to setup the common resource once, run a bunch of tests, and then teardown the resource. To achieve this, you can follow the subtest pattern of Go.

Use table driven tests where possible to make the subtest routines maintainable. Briefly, this means that you group your test cases using a test struct that reflects the unique parameters of the test cases. Then you can conveniently loop over the test cases in parallel, taking advantage of uniformity and speed.

Note that the subtest pattern has gotchas when running tests in parallel:

The main test function will not wait for the subtest to run if it uses t.Parallel. To avoid this, you need to wrap the parallel subtests in a synchronous, blocking subtest. In the example below, the group subtest is synchronous (no call to t.Parallel) and thus the main function will wait until that test finishes. The group test does not finish until all the subtests it spawns are finished, even if they are non-blocking and parallel, and thus the tearDownVPC call does not happen until all subtests are done.
If you are using table driven tests, the range variable will be updated to the next iteration before it is used within the subtest. That is, in the example below, if we did not have the testCase := testCase line in the range block, the testCase reference used in the subtest after the t.Parallel call will correspond to the last testCase in the testCases list. To avoid this, we create a new variable in the scope of the range block so that it does not get updated during the loop.

Example:

func TestECS(t *testing.T) {
    t.Parallel()

    defer tearDownVPC()
    deployVPC()

    // Wrap the parallel tests in a synchronous test group to ensure that the main test function (the one calling
    // `tearDownVPC` and `deployVPC`) waits until all the subtests are done before running the deferred function.
    t.Run("group", func(t *testing.T) {
        for _, testCase := range testCases {
            // To avoid the range variable from getting updated in the parallel tests, we bind a new name that is within
            // the scope of the for block.
            testCase := testCase
            t.Run(testCase.name, func(t *testing.T) {
                t.Parallel()
                testCase.testCode()
            })
        }
    })
}

Terraform Style Guide

Starting point​

Additional conventions​

General​

2 space indentations​

120 column limit​

Block Labels, Variables, and Outputs should be snake case​

Module folder conventions​

variables.tf conventions​

Complex types​

outputs.tf conventions​

main.tf conventions​

Conditionals​

Comments​

Prefer # over //​

Prefer # - over # ~​

variables.tf​

main.tf​

Section comments​

Testing: Terratest​

Terratest best practices​

Code formatting​

examples and tests​

Parallel​

Use TestStages for faster development​

Setup and Teardown pattern​

Starting point

Additional conventions

General

2 space indentations

120 column limit

Block Labels, Variables, and Outputs should be snake case

Module folder conventions

`variables.tf` conventions

Complex types

`outputs.tf` conventions

`main.tf` conventions

Conditionals

Comments

Prefer `#` over `//`

Prefer `# -` over `# ~`

`variables.tf`

`main.tf`

Section comments

Testing: Terratest

Terratest best practices

Code formatting

`examples` and `tests`

Parallel

Use TestStages for faster development

Setup and Teardown pattern