Architects look at thousands of buildings during their training, and study critiques of those buildings written by masters. In contrast, most software developers only ever get to know a handful of large programs well - usually programs they wrote themselves - and never study the great programs of history. As a result, they repeat one another's mistakes rather than building on one another's successes. This second volume of The Architecture of Open Source Applications aims to change that. In it, the authors of twenty-four open source applications explain how their software is structured, and why. What are each program's major components? How do they interact? And what did their builders learn during their development? In answering these questions, the contributors to this book provide unique insights into how they think.