Organizing roles

You can define a “base” module that determines many values based on some input.

In this example we’ll use a env_shortcode as the input. The env_shortcode refers to a three letter abbreviation that refers to the environment.

There are a limited number of valid values; in this example: ‘dev’ and ‘prd’.

Base module.

1. Create a directory terraform-module-base and step into the directory.
2. Create a file called versions.tf:

# The `validation` of variables was introduced since Terraform 0.13."
terraform {
  required_version = ">= 0.13"
}

1. Create a file variables.tf:

# This is the mapping beteween:
# - env_shortcode (left)
# - subscription (right)
variable "subscriptions" {
  type    = map
  default = {
    dev = "xyz-dev"
    prd = "abc-prd"
  }
}

# This is the required variable (string) to make the mapping.
variable "env_shortcode" {
  type    = string
  description = "The short (3 letter string) code for the environment. i.e. 'dev'."
  validation {
    condition     = contains(["dev", "prd"], var.env_shortcode)
    error_message = "Only 'dev' or 'prd' are supported."
  }
}

# This is a primitive variable, always returning "westeurope".
variable "location" {
  type    = string
  default = "westeurope"
}

1. Create a file output.tf:

# To use variables generated in this module, you need to `output` them.

# Here the mapping is done between `subscriptions` and `env_shortcode`.
output "subscription" {
  value = var.subscriptions[var.env_shortcode]
}

# Here the location is returned.
output "location" {
  value = var.location
}

1. Try the module by running terraform init and terraform apply. You’ll be asked for env_shortcode.

The base module is ready.

A sub module.

Now that the base module is ready, you can use it from another module.

1. Create a directory terraform-module-xyz and step into the directory.
2. Create a file called versions.tf:

# The `validation` of variables was introduced since Terraform 0.13."
terraform {
  required_version = ">= 0.13"
}

module "base-module" {
  source        = "../terraform-module-base"
  env_shortcode = "prd"
}

1. Create a file called variables.tf:

variable "name" {
  description = "The name of something."
}

1. Create a file called output.tf:

output "subscription" {
  value = module.base-module.subscription
}

output "location" {
  value = module.base-module.location
}

output "name" {
  value = var.name
}

Nota bene: this last step is just to show how the two roles work together, normally you would use the variables in main.tf.

1. Try the module by running terraform init and terraform apply. You’ll be asked for name.

As you can see, the output show a mapped/calculated value for subscription:

$ terraform apply
var.name
  The name of something.

  Enter a value: my_name


An execution plan has been generated and is shown below.
Resource actions are indicated with the following symbols:

Terraform will perform the following actions:

Plan: 0 to add, 0 to change, 0 to destroy.

Changes to Outputs:
  + location     = "westeurope"
  + name         = "my_name"
  + subscription = "abc-prd"

Do you want to perform these actions?
  Terraform will perform the actions described above.
  Only 'yes' will be accepted to approve.

  Enter a value: yes


Apply complete! Resources: 0 added, 0 changed, 0 destroyed.

Outputs:

location = "westeurope"
name = "my_name"
subscription = "abc-prd"