Validating Variables in Terraform

There are always new features being added to Terraform and a more recent set of features is the ability to put validations on your variables. I am going to add validation to my Terraform weather application called Terra Weather. I am going to put some constraints on the zipcode variable to ensure that it is only a number and that it’s only five digits. This limits it to just the USA right now. Let’s get into it.

Validation

Here is our variable.

variable "zipcode" {
    type = string
    default = "90210"
}

Now we can add in our validation. We are going to ensure that we have the required five digits. We will also leverage two functions can and tonumber to validate that it doesn’t contain any alpha or special characters.

variable "zipcode" {
  type    = string
  default = "90210"
  validation {
    condition     = length(var.zipcode) == 5 && can(tonumber(var.zipcode))
    error_message = "The zip code isn't a valid USA zip code."
  }
}

Now we can execute our TF passing in an invalid zip code to verify that it works.

$ terraform apply --auto-approve --var zipcode=123
╷
│ Error: Invalid value for variable
│ 
│   on app.tf line 14:
│   14: variable "zipcode" {
│     ├────────────────
│     │ var.zipcode is "123"
│ 
│ The zip code isn't a valid USA zip code.
│ 
│ This was checked by the validation rule at app.tf:17,3-13.

If we passed in a value that has a length of five, but with any non-numeric, then it fails again.

Wrapping Up

That’s all that it takes to add in some validation of your variables. This goes a long way in ensuring that users know exactly what you are expecting as input. We also added a nice feature to Terra Weather. I have some other ideas on how to build out my Terraform app.

Thanks for reading,

Jamie

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