Writing modules

expected time	requirements
60 minutes	A computer with Terraform installed.

Goal: learn how to write Terraform modules.

Explanation

You’ve learned how to write Terraform code. When you write code, you’ll see that you will be repeating yourself. That can be a waste.

Modules to the rescue; with Terraform modules you can write (small) pieces of Terraform code that other (or yourself) can reuse. This prevents double code and makes maintaining your code much easier.

Imaging this example:

resource "azurerm_resource_group" "rg" {
  name     = "rg-robertdebock-sbx"
  location = "west europe"
}

# Create a virtual network
resource "azurerm_virtual_network" "vnet" {
    name                = "myTFVnet-robert"
    address_space       = ["10.0.0.0/16"]
    location            = "west europe"
    resource_group_name = azurerm_resource_group.rg.name
}

There are a lot of double values in the example above:

1. The name has robert in it.
2. The location is set in both resources.

You can make a module where some of these values are set to a default value, so you can prevent naming them.

The benefit of using a module is that you can fix mistakes just once, (in the module) and you do not need to change your Terraform code that uses the module.

As you can see the version = 1.0.0 is used. Versioning is important, you can indicate a few things:

1. The MAJOR version (1) indicates the compatibility. 2.0.0 is not compatible with 1.0.0.
2. The MINOR version (0) indicates the features. If this number increases, a new feature should be available.
3. The PATCH version (0) indicates the bugfixes. If this number increases, a problem has been fixed.

Number can go beyond 9, so a version a 3.2.13 is possible.

Many vendors do not use SemVer, rather use a new version for marketing purposes.

Howto

1. Make a new directory, maybe terraform-azurerm-resource_group.
2. Describe all input in variables.tf.
3. Describe all output in outputs.tf
4. Add a README.md.
5. Describe all resources in main.tf.
6. Describe all versions in versions.tf
7. Add examples/default/main.tf the uses the module.

• HashiCorp teaches.
• Terraform modules.

The documentation may also help.

Demo

• Azure.
• AWS.
• GCP.

Assignment

• Make a module that can create azurerm_resource_groups. Solution
• Try your module by running terraform init from the examples/default directory.

Bonus assignment

• Modify your root-module (learn-terraform-azure) to use the modules you’ve created. Solution

Best practices

1. Use published modules when possible
2. Make modules as small as possible.
3. Use sane defaults. (for example region and size.)
4. Use no configuration in modules. (It should be usable by anybody.)
5. Use explicit variable names. (Not r but region for example.)
6. Create a README.md explaining the features.
7. Add a LICENSE before publishing.
8. Put terraform.tfstate, terraform.tfstate.backup, .terraform and *.tfvars in .gitignore.
9. Bonus: add an examples directory, and in there default (or others). Write examples/*/main.tf to try your own module.

Questions:

1. Why would you write a module?
2. Where can you find and download modules?
3. By looking at the downloads, what is the most used provider? (AWS, Azure or GCP)

Extra

To use a module stored on a version control system, use this code:

module "resourcegroup" {
  source              = "git::ssh://[email protected]/NAMESPACE/REPOSITORY.git"
  resource_group_name = local.resource_group_name
  location            = var.location
  rg_tags             = var.rg_tags
}