Prefactoring: Don't Repeat Yourself (DRY)

Don't Repeat Yourself (DRY). Every piece of knowledge must have a single, unambiguous, authoritative representation within a system.

This is a really powerful idea to grasp and sometimes very hard to adhere to. Working on systems where you have to remember to change 4 different files just to make a single logical change is painful and error prone.  If you know you want to stay DRY, and who doesn't, then you start to see your solutions in a different light.  Sometimes you have to write a custom tool that churns through an XML file to generate the multiple pieces the system needs to support a new feature but at least you have a canonical definition of that feature.  One tool that might help to stay DRY is MPS.  You can create a custom language to describe the solution to a problem, such as generating Hibernate and JAXB aware Java classes from a single definition file.  Having to look multiple places to make sure a change gets through is going to cause a failure in the system at some point.