Most Terraform modules fail for the same reason.
They expose too much.
When a module leaks internal decisions through variables and outputs, every caller becomes dependent on implementation details. That dependency spreads risk across environments.
Reusable design fixes this by shrinking the interface.
Fewer inputs force consistency.
Internal defaults remove duplication.
Outputs expose identity, not structure.
The result is controlled change.
When the module evolves, callers remain stable because the contract did not change.
This lesson shows how to design that boundary correctly.