In my experience, scope creep is one of the greatest threats to the completion of an iteration. I would even suggest that the most dangerous "scope creep" is the kind inflicted by engineers.
Most discussions of "scope creep" tend to focus on the customer inflicted variety. For agile teams, working iteratively, this sort of strategic creep has been managed (see embracing change). The creep we don't always catch is the creep at the tactical level.
Have you ever been faced with a simple story and somehow ended up making a Rube Goldberg machine? Damn the schedule and designed a complex framework in hopes of using it again later? Regretted wasting time on a complex API that did no more than paint your project into a corner?
In the end, all of these cases are examples of one thing...tactical scope creep. I myself have never fallen prey to these vices (yeah right). Seriously, I can't count the number of times I was going to write the next great framework that would solve ALL future problems......I wish someone had been there to say YAGNI!
So, what do we do? We apply the XP practices and principles.....duh. Remember, XP is a somewhat wholistic methodology. You MUST plan iteratively, have users write stories and play the planning game. Ok, disclaimer out of the way, here are a few XP principles/practices that will help you conquer tactical scope creep:
- Pair Programming
No sh@t...it works. One navigator, one driver. Two minds are working on the code and the design. This creates a natural 2-way sounding board for new ideas (as opposed to having NO sounding board in normal programming). Bad ideas and transgressions of the other practices are identified and squashed quickly.
You Aren't Gonna Need It! This one is a no-brainer. If the story states "Users can write coments to blog entries", then write a solution that accomplishes this! Notice that this story doesn't suggest that users can view comments. It doesn't suggest that authors can manage comments either. SO...don't worry about it! Suggest that a user write stories for these other features. Writing very small stories is a good practice anyway. Small stories are easier to estimate and easier to focus on. Of course, differentiating between YAGNI and flexibility is no science.
The Simplest Thing That Could Possibly Work. This practice dives deeper into the tactical design than YAGNI does. TSTTCPW asks us to simply make a duck a duck. If writing a file to the disk is simpler than writing it to a database....I guess you should write it to the disk. Later if requirements change, and you have properly encapsulated responsibilities, you can refactor to a database. Of course, like most of the XP practices it's application is subject to team arbitration. If a pair can't agree then let the WHOLE TEAM (see below) decide if something really is TSTTCPW.
- Simple Design
Over engineering a problem is the most frequent cause of tactical scope creep. If you can get away without a factory/service tier then do so. If you can use an O/R mapper (such as ActiveRecord) then do so. Stay away from glitzy user interfaces....wait until someone writes a story that requires a "glitzy" user interface.
- Whole Team
Especially the idea of putting all engineers in one room. Let the team police itself. Inform the other pairs when you are about to make a complex design decision. If your argument sways ALL of them then go for it!
- Fail Fast
Ok, this is actually a Test Driven Development (TDD) practice but it will help. Get something up and running quickly. Don't let yourself ponder the intricateness of the problem at hand. Over-engineering almost always starts in the "minds" of smart people.
- Small Frequent Releases
Your users will love you AND you will love being loved! That killer new framework would have to be pretty damned good to give you the same validation as a satisfied user. More likely the time you waste designing the killer API will overshadow any benefit derived from it.
Now, some things can slip through the cracks. Most will be unimportant. Some will be VERY important. Here are some things to keep in mind when you are getting ready to drop the YAGNI bomb:
This is the most serious potential problem. Security, especially on the low level, almost never makes it's way into a user story. I would suggest putting a security expert on your QA team. Have them write stories describing the security holes that require attention.
- Accesibilty / Multi-cultural / Usability
Once again these can be handled by placing an expert on your QA team.
Well, I hope I haven't wasted too much of your time. Please feel free to comment, complain or flame!