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Orthogonality and the DRY Principle
A Conversation with Andy Hunt and Dave Thomas, Part II
by Bill Venners
March 10, 2003

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Pragmatic Programmers Andy Hunt and Dave Thomas talk with Bill Venners about maintenance programming, the DRY principle, code generators and orthogonal systems, and a story about one highly coupled control system: the helicopter.

Andy Hunt and Dave Thomas are the Pragmatic Programmers, recognized internationally as experts in the development of high-quality software. Their best-selling book of software best practices, The Pragmatic Programmer: From Journeyman to Master (Addison-Wesley, 1999), is filled with practical advice on a wide range of software development issues. They also authored Programming Ruby: A Pragmatic Programmer's Guide (Addison-Wesley, 2000), and helped to write the now famous Agile Manifesto.

In this interview, which is being published in ten weekly installments, Andy Hunt and Dave Thomas discuss many aspects of software development:

All Programming is Maintenance Programming

Bill Venners: You say in your book, The Pragmatic Programmer (Addison-Wesley, 1999), that "programmers are constantly in maintenance mode." Why?

Dave Thomas: All programming is maintenance programming, because you are rarely writing original code. If you look at the actual time you spend programming, you write a bit here and then you go back and make a change. Or you go back and fix a bug. Or you rip it out altogether and replace it with something else. But you are very quickly maintaining code even if it's a brand new project with a fresh source file. You spend most of your time in maintenance mode. So you may as well just bite the bullet and say, "I'm maintaining from day one." The disciplines that apply to maintenance should apply globally.

Andy Hunt: It's only the first 10 minutes that the code's original, when you type it in the first time. That's it.

The DRY Principle

Bill Venners: What's the DRY principle?

Dave Thomas: Don't Repeat Yourself (or DRY) is probably one of the most misunderstood parts of the book.

Bill Venners: How is DRY misunderstood and what is the correct way to understand it?

Dave Thomas: Most people take DRY to mean you shouldn't duplicate code. That's not its intention. The idea behind DRY is far grander than that.

DRY says that every piece of system knowledge should have one authoritative, unambiguous representation. Every piece of knowledge in the development of something should have a single representation. A system's knowledge is far broader than just its code. It refers to database schemas, test plans, the build system, even documentation.

Given all this knowledge, why should you find one way to represent each feature? The obvious answer is, if you have more than one way to express the same thing, at some point the two or three different representations will most likely fall out of step with each other. Even if they don't, you're guaranteeing yourself the headache of maintaining them in parallel whenever a change occurs. And change will occur. DRY is important if you want flexible and maintainable software.

The problem is: how do you represent all these different pieces of knowledge only once? If it's just code, then you can obviously organize your code so you don't repeat things, with the help of methods and subroutines. But how do you handle things like database schemas? This is where you get into other techniques in the book, like using code generation tools, automatic build systems, and scripting languages. These let you have single, authoritative representations that then generate non-authoritative work products, like code or DDLs (data description languages).

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