Terraform variable validation is often misunderstood.
It is not a usability feature.
It is not input sanitization.
It is not a safety net added for convenience.
Validation exists for a much deeper reason.
Terraform does not execute infrastructure. It reasons about it. Before any plan can exist, Terraform must believe the world it is about to model is possible. Variable validation is the line where that belief is tested.
Without validation, any value is acceptable as long as it matches a type. A string is a string, even if it is empty. A number is a number, even if it violates the assumptions of the system. Terraform will still build a dependency graph. It will still produce a plan. The result can be internally consistent and externally wrong.
Validation changes that.
A validation block is a declaration of legitimacy. It tells Terraform which values are allowed to exist before dependency relationships are evaluated. If the condition fails, Terraform does not partially continue. It does not degrade. It refuses to reason about an impossible world.
This is not error handling.
This is identity enforcement.
In the video, we explore where validation sits in Terraform’s reasoning lifecycle, how it differs from type checking, and why weak validation rules can be more dangerous than none at all. Code is used only as evidence of Terraform’s behavior, not as instructions to follow.
If you want to understand why Terraform sometimes stops before planning even begins, this lesson explains the boundary.