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Tracer Bullets and Prototypes
A Conversation with Andy Hunt and Dave Thomas, Part VIII
by Bill Venners
April 21, 2003

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Starting with a Skeleton Application

Bill Venners: You say in your book, "In tracer code development, developers tackle use cases one by one." What is a use case, and why do you recommend that we do tracer bullets in use case chunks?

Andy Hunt: Central to tracer bullet development is the idea of a skeleton application, in which one thin line of execution goes end to end. In a skeleton application, you have some bit of functionality—even if it's just the equivalent of "Hello, world!"—that goes all the way from the UI, through business logic, through whatever else is in the middle, all the way to a database. It may do nothing more than put a checkbox value into the database as a Boolean. Whatever the skeleton application does, it does end to end, skeletally thin.

From the skeleton application forward, you might start adding one feature at a time, where each feature implements one use case. You implement the feature, go all the way through, and see if it hangs together. As you steadily grow your application, one feature at a time—this is where tracer bullets come in—you may realize, you're off by a little bit. Well, you still don't have that much code. It's still easier to adjust it and fix your aim. As you grow the application fatter and fatter over time, it will be increasingly harder and harder to make gross changes. But you'll also be getting smarter as the project progresses, because you are firing tracer bullets and getting feedback. So when the requirements are more volatile, the code is easiest to change. Later the code does become harder to change, but by then you know more and less change is required.

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