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Design Spikes Sometimes you'll need to test some approach to your production code. Perhaps you want to see how a design possibility will work in practice, or you need to see how a persistence framework will work on your production code. In this case, go ahead and work on production code. Be sure to check in your latest changes before you start the spike and be careful not to check any of your spike code.

If you anticipate the need for a spike when estimating a story, include the time in your story estimate. Sometimes, you won't be able to estimate a story at all until you've done your research; in this case, create a spike story and estimate that instead

This is difficult! Experienced programmers think in abstractions. In fact, the ability to think in abstractions is often a sign of a good programmer. Coding for one specific scenario will seem strange, even unprofessional.

Continuous Design Incremental design initially creates every design element—method, class, namespace, or even architecture—to solve a specific problem. Additional customer requests guide the incremental evolution of the design. This requires continuous attention to the design, albeit at different time-scales. Methods evolve in minutes; architectures evolve over months. No matter what level of design you're looking at, the design tends to improve in bursts. Typically, you'll implement code into the existing design for several cycles, making minor changes as you go. Then something will give you an idea for a new design approach, requiring a series of refactorings to support it. [Evans] calls this a breakthrough (see Figure). Breakthroughs happen at all levels of the design, from methods to architectures.

Don't let design discussions turn into long, drawn-out disagreements. Follow the ten-minute rule: if you disagree on a design direction for ten minutes, try one and see how it works in practice. If you have a particularly strong disagreement, split up and try both as spike solutions. Nothing clarifies a design issue like working code.

Risk-Driven Architecture Architecture may seem too essential not to design up front. Some problems do seem too expensive to solve incrementally, but I've found that nearly everything is easy to change if you eliminate duplication and embrace simplicity. Common thought is that distributed processing, persistence, internationalization, security, and transaction structure are so complex that you must consider them from the start of your project. I disagree; I've dealt with all of them incrementally [Shore 2004a]. Two issues that remain difficult to change are choice of programming language and platform. I wouldn't want to make those decisions incrementally!

Limit your efforts to improving your existing design

To apply risk-driven architecture, consider what it is about your design that concerns you and eliminate duplication around those concepts

Your power lies in your ability to chooose which refactorings to work on. Although it would be inappropriate to implement features your customers haven't asked for, you can direct your refactoring efforts towards reducing risk. Anything that improves the current design is okay—so choose improvements that also reduce future risk.

design is so important in XP that we do it all the time

Don't try to use incremental design without a commitment to continuous daily improvement (in XP terms, merciless refactoring.) This requires self-discipline and a strong desire for high-quality code from at least one team member. Because nobody can do that all the time, pair programming, collective code ownership, energized work, and slack are important support mechanisms.

Test-driven development is also important for incremental design. Its explicit refactoring step, repeated every few minutes, gives pairs continual opportunities to stop and make design improvements. Pair programming helps in this area, too, by making sure that half of the team's programmers—as navigators—always have an opportunity to consider design improvements.

Alternatives If you are uncomfortable with XP's approach to incremental design, you can hedge your bets by combining it with up-front design. Start with an up-front design stage and then commit completely to XP-style incremental design. Although it will delay the start of your first iteration (and may require some up-front requirements work, too), this approach has the advantage of providing a safety net without incurring too much risk.