Validation on Brian Faust

A Primitive Can Hide a Domain Decision

Wed, 01 Apr 2020 00:00:00 +0000

Uppercasing a country code looks like normalization:

return mb_strtoupper($input);

It makes fi and FI equal. It also turns finland into FINLAND and leaves FIN looking plausible.

The function has changed the string’s shape. It has not answered the domain question: is this a supported country identity in the representation this boundary promises?

A primitive becomes expensive when every caller must remember that answer. The reason to introduce a domain type is not that strings are unfashionable. It is that the scalar no longer carries enough of the contract.

Invalid State Should Be Difficult to Construct

Sat, 01 Feb 2020 00:00:00 +0000

A wallet reservation held one amount while a booking was in progress. When the booking later reached the billing system, an imported record could contain no price. The reservation knew how much money had been held, but not how that amount divided into net price and VAT.

There was no authoritative way to create the final sale from the hold alone. Inferring a tax rate would turn a guess into accounting data.

Validate at the Boundary You Control

Thu, 01 Mar 2018 00:00:00 +0000

A booking request contains a recipient email and a phone number made entirely of spaces.

For one delivery service, email is enough. For the same service with a particular handoff option, a phone number is mandatory. For another service, phone is always mandatory.

There is no honest global rule such as “phone is nullable” or “phone is required.” The invariant depends on the operation the request asks the system to perform.