Backing Up the Waterfall

The first thing that happens to the waterfall model is that it changes into the waterfall with feedback, shown in Figure 2-3. This is an admission that a straight-line waterfall doesn't work and that you need the ability to back up to a previous phase when you discover a problem in the current phase.

Figure 2-3. Waterfall with feedback process model

The waterfall with feedback model recognizes that you have to start work with incomplete requirements, design, test plan, and so on. It also explicitly builds in the idea that you will have to go back to previous process steps as new information about your project is uncovered. The new information can be new requirements, updated requirements, design flaws, defects in testing plans, and the like. Any of these will require that you revisit a previous process step to rectify the problem.