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Evolutionary Design
A Conversation with Martin Fowler, Part III
by Bill Venners
November 18, 2002

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Summary
Martin Fowler, chief scientist at Thoughtworks, Inc. and author of numerous books on software design and process, talks with Bill Venners about planned and evolutionary design, refactoring bad smells in code, and how good design can help you program faster.

Over the last decade, Martin Fowler pioneered many software development techniques in the development of business information systems. He's well known for his work on object-oriented analysis and design, software patterns, Unified Modeling Language, agile software processes (particularly extreme programming), and refactoring. He is the author of Analysis Patterns (Oct. 1996), Refactoring (June 1999; coauthored with Kent Beck, et al.), UML Distilled (Aug. 1999; with Kendall Scott), Planning Extreme Programming (Oct. 2000; with Kent Beck), and the soon to be released Patterns of Enterprise Application Architecture (Nov. 2002), all published by Addison Wesley.

In this six-part interview, which is being published in weekly installments, Fowler gives his views on many topics, including refactoring, design, testing, and extreme programming (XP). In Part I, Fowler makes the business case for refactoring and testing, and describes the interplay between refactoring, design, and reliability. In Part II, Fowler discusses design principles of avoiding duplication, separating presentation and domain logic, being explicit, and describes how refactoring depends on code ownership. In this third installment, Fowler differentiates between planned and evolutionary design, suggests that focusing on superficial problems can lead to the discovery of substantial problems, and claims that doing a good job won't slow you down.

Planned versus Evolutionary Design

Bill Venners: In your paper, "Is Design Dead?", you talk about planned design. What is planned design?

Martin Fowler: I distinguish between planned and evolutionary design. Planned design says that when you think about a piece of software, you create the design first, then you code it. A planned design could take the form of UML diagrams. Or you could express it in terms of dividing a system into subsystems and defining the interfaces between those subsystems. With planned design, there's a definite switch between the two modes of creating the design and then coding it. And those tasks may often be performed by different people. Architects come up with design. The developer then code it. The design is not necessarily considered completely fixed, but it is considered mostly fixed. You can argue that the better the design, the less it will change as you code it.

With evolutionary design, you expect the design to evolve slowly over the course of the programming exercise. There's no design at the beginning. You begin by coding a small amount of functionality, adding more functionality, and letting the design shift and shape.

The point I make in "Is Design Dead?" is that most people have encountered evolutionary design in an unconstrained and ill-disciplined environment and it doesn't work. You end up with a crappy design. And that's one reason why people gravitate towards planned design.

But in my view, extreme programming's practices of continuous integration, testing, and refactoring actually make evolutionary design work, and more effectively than planned design. Planned design's weakness is that creating a well-planned design is actually really tough.

Bill Venners: Why?

Martin Fowler: I don't know. Why is composing symphonies tough? I don't know. It's just very few people in the world can do it well. And I think that's the case with upfront design. It is very hard to do well.

It's interesting that many of evolutionary design's major proponents, like Kent Beck and Ward Cunningham, are stunningly good designers. But they came to the conclusion that their upfront designs were often not very good. They tended to over-engineer things, adding unnecessary complexity to their designs. They would often make a design flexible in areas that didn't need flexibility and inflexible in areas that did need it. So they've adopted an approach where, by applying a set of disciplines, evolutionary design works instead. As a result, they have created better designs, and at a faster rate. I think 80 percent of the time evolutionary design works for me as well. And I have the arrogant opinion that I am an above average designer, so I think evolutioary design would be even more useful for a wider range of people.

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