Custom Variable Validation in Terraform 0.13

We’re excited to announce that custom variable validation is being released as a production-ready feature in Terraform 0.13. Custom Variable Validation was introduced as a language experiment in Terraform 0.12.20 and builds upon the type system introduced in Terraform 0.12 by allowing configurations to contain validation conditions for a given variable.

In the example below, we define a condition for the variable ami_id which requires the id to begin with the string ami:

variable "ami_id" {
    type = string

    validation {
    condition = (
        length(var.ami_id) > 4 &&
        substr(var.ami_id, 0, 4) == "ami-"
    )
    error_message = "The ami_id value must start with \"ami-\"."
    }
}

Each variable block can have zero or more validation blocks, allowing an author to potentially write a specific error message associated with each distinct check.

The condition expression is evaluated in a reduced evaluation context that supports all of Terraform's built-in functions but allows referring only to the variable being validated — in this case, var.ami_id — which evaluates to the value given by the caller after any automatic conversion to the given type constraint string. The expression can therefore safely assume var.ami_id is a string in this case.

If the expression returns true then the validation is considered to have succeeded. If it returns false then validation has failed and Terraform will return an error at the module call site, using the text given in error_message:

Error: Invalid value for variable

    on variable-validation-rfc.tf line 4, in module "example":
    4:   ami_id = "amo-abc123"

The ami_id value must start with "ami-".

This was checked by the validation rule at example/variables.tf:4,3-13.

Terraform’s custom variable validation takes the ergonomics of this feature one step further and will distinguish between an error produced by validation of the variable itself or an error with the expression used as the condition for validation. More information is available in the Terraform documentation.

Download the latest Terraform 0.13 pre-release today and give custom variable validation a try. Don’t forget to join us on our community forums for questions and discussion about this and the other new features of Terraform 0.13.