7.6.1 Goals and scenarios
Goals prescribe admissible or preferred behaviours of the agents forming the system. They are related to each other through refinement/contribution links. As we will see in Pan ll. such links can be gathered in a goal model to support various fonns of early, declarative and incremental reasoning for goal refinement and operationalization, completeness checking, conflict management, hazard analysis, threat analysis, goal-oriented animation and generation of the requirements document. On the other hand, goals are sometimes felt to be too abstract by stakeholders. They cover classes of intended behaviours, but such behaviours are left implicit Goals may sometimes be hard to elicit and make fully precise in the first place. Scenarios capture typical examples or counterexamples of intended system behavioui through sequences of interactions among agents (see Sections 2.2.5 and 4.3-6). Their n.iu.uive and concrete style of description make them appropriate for requirements eliciution and validation, as they are easily accessible to stakeholders. On the other hand, scenarios are inherently partial and cover few behaviours of specific instances. They leave intended system properties implicit, and may entail premature decisions about event sequencing and distribution of responsibilities among agents. Goals and scenarios thus have complementary strengths and weaknesses. As Parts 11 and III will show in greater detail, they can be combined synergistically for building and analysing system models. The semantic relationship between goals and scenarios should become clear at this point. A behavioural goal implicitly defines a maximal set of admissible system behaviours tsee Section 7.3-1). Such behaviours are composed of parallel agent behaviours, each captured by a sequence of state transitions of items that the corresponding agent controls. We can therefore make these state transitions explicit along agent timelines through corresponding agent inter actions. A behavioural goal then covers the set of all positive scenario examples that illustrate
7.6.1 เป้าหมายและสถานการณ์ Goals prescribe admissible or preferred behaviours of the agents forming the system. They are related to each other through refinement/contribution links. As we will see in Pan ll. such links can be gathered in a goal model to support various fonns of early, declarative and incremental reasoning for goal refinement and operationalization, completeness checking, conflict management, hazard analysis, threat analysis, goal-oriented animation and generation of the requirements document. On the other hand, goals are sometimes felt to be too abstract by stakeholders. They cover classes of intended behaviours, but such behaviours are left implicit Goals may sometimes be hard to elicit and make fully precise in the first place. Scenarios capture typical examples or counterexamples of intended system behavioui through sequences of interactions among agents (see Sections 2.2.5 and 4.3-6). Their n.iu.uive and concrete style of description make them appropriate for requirements eliciution and validation, as they are easily accessible to stakeholders. On the other hand, scenarios are inherently partial and cover few behaviours of specific instances. They leave intended system properties implicit, and may entail premature decisions about event sequencing and distribution of responsibilities among agents. Goals and scenarios thus have complementary strengths and weaknesses. As Parts 11 and III will show in greater detail, they can be combined synergistically for building and analysing system models. The semantic relationship between goals and scenarios should become clear at this point. A behavioural goal implicitly defines a maximal set of admissible system behaviours tsee Section 7.3-1). Such behaviours are composed of parallel agent behaviours, each captured by a sequence of state transitions of items that the corresponding agent controls. We can therefore make these state transitions explicit along agent timelines through corresponding agent inter actions. A behavioural goal then covers the set of all positive scenario examples that illustrate
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