Code That Fits in Your Head

Mark Seemann, a former economist, found a second career as a programmer and has worked as a web and enterprise developer since the late 1990s. He is a Certified Rockstar Developer and has written a Jolt Award-winning book about Dependency Injection, given more than a hundred international conference talks, and authored video courses for both Pluralsight and Clean Coders. Mark has regularly published his blog ( blog.ploeh.dk) since 2006.

"We progress in software by standing on the shoulders of those who came before us. Mark's vast experience ranges from philosophical and organisational considerations right down to the precise details of writing code. In this book, you're offered an opportunity to build on that experience. Use it." --Adam Ralph, speaker, tutor, and software simplifier, Particular Software "I've been reading Mark's blogs for years and he always manages to entertain while at the same time offering deep technical insights. Code That Fits in Your Head follows in that vein, offering a wealth of information to any software developer looking to take their skills to the next level." --Adam Tornhill, founder of CodeScene, author of Software Design X-Rays and Your Code as a Crime Scene "My favorite thing about this book is how it uses a single code base as a working example. Rather than having to download separate code samples, you get a single Git repository with the entire application. Its history is handcrafted to show the evolution of the code alongside the concepts being explained in the book. As you read about a particular principle or technique, you'll find a direct reference to the commit that demonstrates it in practice. Of course, you're also free to navigate the history at your own leisure, stopping at any stage to inspect, debug, or even experiment with the code. I've never seen this level of interactivity in a book before, and it brings me special joy because it takes advantage of Git's unique design in a new constructive way." --Enrico Campidoglio, independent consultant, speaker and Pluralsight author "Mark Seemann not only has decades of experience architecting and building large software systems, but is also one of the foremost thinkers on how to scale and manage the complex relationship between such systems and the teams that build them." --Mike Hadlow, freelance software consultant and blogger "Mark writes, 'Successful software endures'--this book will help you to write that kind of software." --Bryan Hogan, software architect, podcaster, blogger "Mark has an extraordinary ability to help others think deeply about the industry and profession of software development. With every interview on .NET Rocks! I have come away knowing I would have to go back and listen to my own show to really take in everything we discussed." --Richard Campbell, co-host, .NET Rocks!

• Series Editor Foreword xixPreface xxiiiAbout the Author xxix Part I: Acceleration 1Chapter 1: Art or Science? 3 1.1 Building a House 4 1.2 Growing a Garden 7 1.3 Towards Engineering 8 1.4 Conclusion 14 Chapter 2: Checklists 15 2.1 An Aid to Memory 15 2.2 Checklist for a New Code Base 17 2.3 Adding Checks to Existing Code Bases 29 2.4 Conclusion 32 Chapter 3: Tackling Complexity 33 3.1 Purpose 34 3.2 Why Programming Is Difficult 38 3.3 Towards Software Engineering 44 3.4 Conclusion 46 Chapter 4: Vertical Slice 49 4.1 Start with Working Software 50 4.2 Walking Skeleton 53 4.3 Outside-in 60 4.4 Complete the Slice 77 4.5 Conclusion 85 Chapter 5: Encapsulation 87 5.1 Save the Data 87 5.2 Validation 92 5.3 Protection of Invariants 105 5.4 Conclusion 108 Chapter 6: Triangulation 111 6.1 Short-Term versus Long-Term Memory 111 6.2 Capacity 114 6.3 Conclusion 127 Chapter 7: Decomposition 129 7.1 Code Rot 129 7.2 Code That Fits in Your Brain 136 7.3 Conclusion 153 Chapter 8: API Design 155 8.1 Principles of API Design 156 8.2 API Design Example 168 8.3 Conclusion 176 Chapter 9: Teamwork 177 9.1 Git 178 9.2 Collective Code Ownership 187 9.3 Conclusion 199 Part II: Sustainability 201Chapter 10: Augmenting Code 203 10.1 Feature Flags 204 10.2 The Strangler Pattern 209 10.3 Versioning 218 10.4 Conclusion 220 Chapter 11: Editing Unit Tests 223 11.1 Refactoring Unit Tests 223 11.2 See Tests Fail 233 11.3 Conclusion 234 Chapter 12: Troubleshooting 235 12.1 Understanding 235 12.2 Defects 240 12.3 Bisection 250 12.4 Conclusion 255 Chapter 13: Separation of Concerns 257 13.1 Composition 258 13.2 Cross-Cutting Concerns 267 13.3 Conclusion 274 Chapter 14: Rhythm 275 14.1 Personal Rhythm 276 14.2 Team Rhythm 282 14.3 Conclusion 285 Chapter 15: The Usual Suspects 287 15.1 Performance 288 15.2 Security 292 15.3 Other Techniques 300 15.4 Conclusion 308 Chapter 16: Tour 309 16.1 Navigation 309 16.2 Architecture 318 16.3 Usage 323 16.4 Conclusion 326 Appendix A: List of Practices 329 A.1 The 50/72 Rule 329 A.2 The 80/24 Rule 330 A.3 Arrange Act Assert 330 A.4 Bisection 330 A.5 Checklist for A New Code Base 331 A.6 Command Query Separation 331 A.7 Count the Variables 331 A.8 Cyclomatic Complexity 331 A.9 Decorators for Cross-Cutting Concerns 332 A.10 Devil's Advocate 332 A.11 Feature Flag 332 A.12 Functional Core, Imperative Shell 333 A.13 Hierarchy of Communication 333 A.14 Justify Exceptions from the Rule 333 A.15 Parse, Don't Validate 334 A.16 Postel's Law 334 A.17 Red Green Refactor 334 A.18 Regularly Update Dependencies 335 A.19 Reproduce Defects as Tests 335 A.20 Review Code 335 A.21 Semantic Versioning 335 A.22 Separate Refactoring of Test and Production Code 335 A.23 Slice 336 A.24 Strangler 336 A.25 Threat-Model 337 A.26 Transformation Priority Premise 337 A.27 X-driven Development 337 A.28 X Out Names 338 Bibliography 339Index 349