Open/Closed: Add a Channel Without Touching the Core

Every quarter it’s the same routine: a new channel arrives, someone opens NotificationService, adds another case to the switch, and ships. Sooner or later one of those edits touches a shared line and breaks a channel that was already working in production. The new feature was never the risk — changing the old, tested code was.

From here the members-only walkthrough picks up: an interactive board where you add a channel in “edit the switch” vs. “plug in a class” mode, the full before/after refactor to a Channel registry, the tradeoffs a registry buys and costs, and the interview corner with a discount-engine challenge and quiz.