Domain-Modeling 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.

Time Zones Are Part of the Data Model

Thu, 01 Jun 2017 00:00:00 +0000

One metering path assigns an event to a billing month after converting its occurrence time into the account’s timezone. The same path derives its daily usage key directly from the occurrence timestamp without that conversion.

Near midnight, those two fields can disagree:

event instant: May 31, 21:30 UTC
local business time: June 1, 00:30
billing period: June
stored usage date: May 31

Each expression looks like a date calculation. They model different things. The defect appears only when the difference changes a boundary.

Null Is a Domain Decision

Sat, 01 Apr 2017 00:00:00 +0000

We added four nullable fields to preserve a pricing snapshot: net amount, tax rate, tax amount, and gross amount.

Each field could now be absent. That did not mean each field was independently optional.

For older records, all four nulls meant one thing: this record predates the snapshot and does not contain captured pricing evidence. For new records, all four values had to arrive together. A row containing gross and net but no tax rate was not “partly known.” It was invalid.