Most Terraform failures don’t start in the resource.
They start at the input.
A module accepts a value.
That value spreads into naming, tagging, networking, and policy.
By the time something breaks, the root cause is buried.
Validation changes that.
It stops bad input at the boundary.
Not after the plan.
Not after the apply.
At the edge.
In this lesson, you’ll see why type constraints aren’t enough, and how variable validation acts as a control layer that prevents cascading failure.
One rejected value is cheap.
Letting it through is not.