"What I cannot create, I do not understand."

-- Richard Feynman

This is the source for the book 500 Lines or Less, the fourth in the Architecture of Open Source Applications series. As with other books in the series, all written material will be covered by the Creative Commons - Attribution license, and all code by the MIT License: please see the license description for details. In addition, all royalties from paid-for versions will all go to Amnesty International.

The production of this book has been made possible by the financial support of PagerDuty.

Every architect studies family homes, apartments, schools, and other common types of buildings during her training. Equally, every programmer ought to know how a compiler turns text into instructions, how a spreadsheet updates cells, and how a database efficiently persists data.

Previous books in the AOSA series have done this by describing the high-level architecture of several mature open-source projects. While the lessons learned from those stories are valuable, they are sometimes difficult to absorb for programmers who have not yet had to build anything at that scale.

"500 Lines or Less" focuses on the design decisions and tradeoffs that experienced programmers make when they are writing code:

• Why divide the application into these particular modules with these particular interfaces?
• Why use inheritance here and composition there?
• How do we predict where our program might need to be extended, and how can we make that easy for other programmers?

Each chapter consists of a walkthrough of a program that solves a canonical problem in software engineering in at most 500 source lines of code. We hope that the material in this book will help readers understand the varied approaches that engineers take when solving problems in different domains, and will serve as a basis for projects that extend or modify the contributions here.