Feedback and Efficient Guide

The control system changes the missile's course, continually reducing the distance between the missile and the target measured by the sensors. An efficient guide should give orders to the missile actuators very quickly. In software development, this is the task of the development process. In XP, the practices related to feedback and the process are these.

• Planning game. This activity enables controlling the whole development. Development occurs in iterations, through implementing user stories. The customer chooses which stories must be implemented in each iteration. The time needed by the various pairs to implement the system features they choose is recorded and used to estimate project velocity. Overall, it is a light and efficient way to plan software development.

• Short iterations. These are the essence of the feedback. Systems are implemented in iterations, each providing a system with features of business value for the customer. Iterations (or feedback loops) must be kept as short as possible. In XP, an iteration typically lasts from one to three weeks. Only in this way it is possible to efficiently track a target moving at high speed.

• Split business and technical. As mentioned earlier, the customer chooses which user stories must be implemented in each iteration. The developers, however, are responsible for estimating the time needed to implement the various stories. The customer and management cannot impose on developers schedules that they cannot follow. On the other hand, only customers can assign business value to the various features. In this way, the system implementation can produce the highest value for the customer, following a realistic time schedule and not pushing our missile off-track.

• Task-based development. The user stories chosen by the customer and to be implemented at each iteration are decomposed into engineering tasks. Each developer commits to implementing certain tasks. The task is thus the basic unit of system development and control.

This classification of practices must not be viewed as exclusive. Many practices could be assigned to more than one feature that a missile must have to be able to hit the target. For instance, the metaphor is also a way to improve productivity and thus enhance thrust; the on-site customer is also a feedback mechanism; and so on.