DRY

Don’t Repeat Yourself

“Every piece of knowledge must have a single, unambiguous, authoritative representation within a system.” – Andy Hunt & Dave Thomas

Duplication is the root of all evil in software.

When to use

When you find yourself copying and pasting code, or when you realized you need to make the same logical change in two different places.

Why it matters

Maintainability: Fix a bug once, and it’s fixed everywhere.
Consistency: Prevents business logic from drifting apart in different parts of the application.

Signs of Violation

Copy-pasted code blocks.
Multiple classes representing the same concept (e.g., UserDTO, UserEntity, UserModel that are identical).
Magic numbers repeated throughout the code.

Explanation

Problem

If logic is duplicated, it eventually diverges. One copy gets updated, the other doesn’t. This leads to subtle bugs where feature X works in the App but not in the API.

Solution

Abstraction. Extract the common logic into a function, class, or constant.

Real world analogy

Government forms. You shouldn’t have to fill out your Name, Address, and birthdate on 5 different pages of the same application. You fill it once (Single Source of Truth), and it’s referenced everywhere else.

Pros and Cons

Pros	Cons
Consistency Less code	“Premature DRY” can lead to wrong abstractions (coupling unrelated things just because they look similar)

Comparison

WET (Write Everything Twice): The opposite of DRY. Sometimes acceptable in tests or decoupling phases.
Rule of Three: Wait until you see duplication 3 times before you abstract it.

DRY