DRY (Don't Repeat Yourself)

The idea of DRY is simple: each fact or piece of logic in your system should be expressed once and only once. If you find the same code, formula, or rule in multiple modules, you’re violating DRY. Such repetition creates hidden maintenance cost. When the business rule changes, you must hunt down every duplicate instance.

By following DRY, developers strive for a single, unique implementation of any given behavior. This often means abstracting common code into a function or class, or consolidating configuration into a single file.

DRY also applies to database schemas, tests, and documentation.

However, remember that DRY is about duplicated intent, not just similar-looking code. Two pieces of code can look identical but serve different purposes in an application.

• The DRY principle is about avoiding duplication of knowledge in code. If the same idea or logic is in multiple places, it's a signal to refactor.
• When requirements change, you should update logic in only one place. Repeated code increases the risk of inconsistencies and bugs (you might fix a bug in one copy and forget about the others).
• Be careful to apply DRY wisely. It's about knowledge repetition, not just duplication. Sometimes two pieces of code look alike but do different jobs; merging them could over-complicate things.