is a principle of software development aimed at reducing the repetition of software patterns, replacing them with abstractions, or using data normalization to avoid redundancy.
The DRY principle is stated as "Every piece of knowledge must have a single, unambiguous, authoritative representation within a system". When the DRY principle is applied successfully, a modification of any single element of a system does not require a change in other logically unrelated elements.
is a code refactoring rule of thumb to decide when similar pieces of code should be refactored to avoid duplication? It states that two instances of similar code do not require refactoring, but when similar code is used three times, it should be extracted into a new procedure. Duplication is considered a bad practice in programming because it makes the code harder to maintain. When the rule encoded in a replicated piece of code changes, whoever maintains the code will have to change it in all places correctly.
YAGNI is a principle of extreme programming that states a programmer should not add functionality until deemed necessary. YAGNI is a principle behind the XP practice or does the simplest thing that could possibly work. It is meant to be used in combination with several other practices, such as continuous refactoring, continuous automated unit testing, and continuous integration. Used without continuous refactoring.
is a version of a product with just enough features to be usable by early customers who can then provide feedback for future product development. means that developers potentially avoid lengthy and (ultimately) unnecessary work. Instead, they iterate on working versions and respond to feedback, challenging and validating assumptions about a product's requirements.