This post originated from an RSS feed registered with Ruby Buzz by Rick DeNatale. Original Post: Ward Cunningham on the Debt Metaphor Feed Title: Talk Like A Duck Feed URL: http://talklikeaduck.denhaven2.com/articles.atom Feed Description: Musings on Ruby, Rails, and other topics by an experienced object technologist. Latest Ruby Buzz Posts Latest Ruby Buzz Posts by Rick DeNatale Latest Posts From Talk Like A Duck

I just discovered that my old friend, Ward Cunningham has started putting some short interviews, and “video essays” up on YouTube

In the embedded video above, Ward relates how he came up with the metaphor of debt to explain the need for refactoring in interative development. The key insight is that you can proceed faster with an incomplete knowledge of the problem you are solving, and learn as you go. In many cases this is essential because for many problems the problem itself changes as implementation proceeds, because of effects like new feedback from users once they see the system starting to emerge. Ward compares implementation with incomplete knowledge to working with borrowed money. As the system implementation proceeds, the assumptions made on incomplete or out of date knowledge, which drove the design, or “architecture” if you prefer, of the system, start to make implementation of the more complete or evolved requirements progressively harder. In the debt metaphor, this increased cost of making progress corresponds to paying interest on the loan. Refactoring is a process of redesigning the software to reflect this new problem knowledge, without breaking it, thus “re-financing” the system.

Before I’d heard Ward express this metaphor, I’d come up with another for the same idea. One of my hobbies is metalworking. I saw an evolving system as a piece of metal which starts out as flexible and malleable. But like metal, as you start to form it by hammering, and bending, and other operations, it starts to “work-harden.” This means that the metal becomes more and more resistant to further shaping, because of local changes to it’s crystal structure. Eventually it resists all further efforts, often by failing with a stress fracture. This is why you can take a piece of, say copper, and bend it back and forth several times until it finally breaks at the bend.

In metalworking, you can use a process called annealing which returns the metal to a ductile, maleable state. Of course the analogy here is to refactoring.

I still like both metaphors, I tend to use my work hardening/annealing metaphor when talking to programmers, and Ward’s debt metaphor when talking to business people (the “gold donors.”)