Variables & defaults

Give configuration inputs clear types

Lesson 2 of 20 · 12 min · Module 1: Declare a configuration contract

A typed input boundary makes environment differences visible.

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By the end, you’ll be able to…

Name the input instead of scattering literals
Preserve the declared type

The idea

Terraform accepts .tf.json as an alternative configuration representation. Here, built-in terraform_data values make state and plan concepts inspectable without provisioning a cloud resource.

{
  "variable": {
    "environment": {
      "type": "string",
      "default": "staging"
    }
  }
}

01Name the input instead of scattering literals

A variable makes a configurable choice explicit. Keep its meaning narrow enough that callers know what they are changing.

02Preserve the declared type

The JSON expression string ${var.replicas} represents a Terraform reference, not necessarily a string result. The lab resolves an exact variable reference to its typed value.

03Separate default from supplied value

A default is used when no input is supplied. A later lesson supplies a scenario override to demonstrate the same configuration in another environment.

Your turn

Declare variable replicas with type number and default 2. Set terraform_data.app input.replicas to "${var.replicas}". The resolved value must remain numeric.

What the checks look for

Declares a numeric variable
Resolves a number rather than text

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Take this with you

Declare input types and preserve them through configuration references.

Go a little further

Supply a string for the numeric scenario variable and inspect the type error.