Working Effectively with Legacy Code

by Michael C. Feathers

Preserving existing behavior is one of the largest challenges in software development. Even when we are changing primary features, we often have very large areas of behavior that we have to preserve.

When you break dependencies in legacy code, you often have to suspend your sense of aesthetics a bit. Some dependencies break cleanly; others end up looking less than ideal from a design point of view. They are like the incision points in surgery: There might be a scar left in your code after your work, but everything beneath it can get better.