A fourth channel, three classes

transcript

Your app sends three kinds of message: order updates, codes, promotions. By three channels: email, text, push. Nine classes. Then product asks for a fourth channel, chat. How many new classes? Three. Why three?

One class for every pair. Three kinds times three channels. Nine classes. Add a fourth channel. It needs one class for every kind. Twelve classes. Now keep kinds and channels in separate hierarchies. It's three plus four. Seven.

What is said, and how it goes out. Those are two different things. So, two hierarchies. One arrow joins them. The notification holds a channel, and delivers through it. Chat is one new class. And not one kind is edited.

Four kinds, five channels. A class for every pair: twenty classes. A bridge: four plus five. Nine classes. A product becomes a sum.

Split what is said from how it goes out. Join them with one reference. Do it when both sides really grow.

Full breakdown in the lecture.