API design, SOLID and patterns

API design is the craft of shaping the interface other code uses to talk to yours: the functions, classes, endpoints and data they accept and return. A good API is small, obvious to use correctly, hard to use wrongly, and stable, because once others depend on it every change can break them. SOLID is a set of five guidelines for object-oriented code, such as giving each class one job and depending on abstractions rather than concrete details, so pieces can change independently. Design patterns are named, reusable solutions to recurring problems, like an adapter that makes one interface look like another. They are a shared vocabulary, not rules to apply everywhere.