Extreme Programming

Extreme Programming, usually called XP, is an Agile software development method that takes proven engineering practices and turns them up to the extreme. If code review is good, review code all the time by pairing; if testing is good, test constantly by writing tests first. Kent Beck created XP in the late 1990s while working on a payroll project at Chrysler and described it in the 1999 book Extreme Programming Explained. Its values are communication, simplicity, feedback, courage, and respect.

XP teams work in short iterations, often one week, and release small changes frequently. Core practices include planning with user stories, pair programming, test-driven development, continuous integration many times a day, frequent refactoring, simple design that avoids building for imagined future needs, collective code ownership so anyone can improve any code, and a sustainable pace without constant overtime. XP also asks for a customer or customer representative who is always available to answer questions and set priorities.

XP is like a professional kitchen where every cook tastes each dish as it is made, two chefs work on the most important plates together, and the menu is adjusted every week based on what diners say. It suits small to medium teams facing changing requirements, and many practices it popularized, such as TDD, continuous integration, and refactoring, are now standard even on teams that have never used the name XP.

Extreme Programming is often compared with Scrum. Scrum is a framework for organizing work, with roles, events, and a backlog, and it says nothing about how to write code, while XP focuses mainly on engineering practices. The two combine well, and many teams use Scrum to structure their process and XP practices to keep code quality high.