Good Sensors

To hit the target, we need to spot it and efficiently measure its distance from the missile. From the metaphor, XP provides practices to continually get the requirements from the customer and assess how many features of the system are implemented and to what extent. The practices related to this feature are these.

• Metaphor. This is one of the most controversial and least understood practices of XP. It enables better communication between the customer and the developers when they talk about the system. Therefore, it is a way to improve gathering the requirements (spotting the target).

• Continuous testing. XP prescribes writing automatic unit tests even before coding. These tests are run very often and are invaluable for detecting whether something has been broken somewhere when the system is modified. Therefore, unit tests are very important sensors for keeping the missile aligned with the target. The acceptance tests, on the other hand, are dictated by the customer and are used to verify whether the system complies with its requirements. They, too, are a feedback sensor, able to track the number of features implemented so far and whether they meet the requirements.

• On-site customer. Having a customer on-site who cooperates in developing the system helps clarify the requirements, makes it possible to have instantaneous feedback on the system features as soon as they are implemented, and minimizes the time needed to communicate to the development team the requisite changes. It is a powerful feedback sensor.

• User stories. These are short descriptions of the functional features of the system. They resemble use cases, but in general they are more concise. All system development is based on user stories, and each iteration strives to implement a number of them. They are a tool for gathering requirements and tracking the system implementation.