Design Rot and the Five SOLID Principles

Working code can still be badly designed, and the cost shows up the day someone has to change it. This topic names that cost and the five principles meant to keep it down.

It starts with design rot, the four symptoms that tell you a codebase has become hard to change, and with coupling and cohesion, the two measures that every principle moves. Then it names the five SOLID principles in one sentence each and shows a single checkout class that breaks all five at once.

Two tool cards follow: how to read the class diagrams used across the subject, and how Python, Java and TypeScript each declare an interface. The topic closes on the limit of the whole idea: where applying SOLID costs more than it saves.

By the end you can name the symptom in front of you, name the principle that answers it, and say when to leave the principles out.