Goal-Based Domain Modeling as a Basis for Cross-Disciplinary Systems Engineering - Intentional Perspectives on Information Systems Engineering - page 88

i ∗ [45] is a well-known formalism in the requirements engineering field that
combines the features of goal- and agent-orientation, and additionally has built-
in support for representing non-functional requirements. Furthermore, the formal
foundation in the knowledge representation language Telos [ 26] enables formally
sound, automated analysis and transformation support. In detail, i ∗ supports the
notion of agent to represent all relevant stakeholders, dependency to indicate mutual
interactions, and goal to capture the internal rationales of stakeholders. The strategic
dependency (SD) diagram focuses the stakeholders and their dependencies while the
strategic rationale (SR) diagram targets the modeling of internal rationales. Figure 3
mixes the two modeling levels while presenting the requirements of a common rail
injection system (see Fig. 2) within a combustion engine.
The common rail is supposed to reliably provide the injectors with fuel; a rail
pressure controller must therefore ensure a nearly constant pressure within the rail.
The injectors are the disturbances since each injection causes a pressure loss. The
controller acts against that by suitably activating the pump.
i ∗ puts much emphasis on its ability to capture the context of a system to be devel-
oped. In our case, the “controlled system” can be considered the most important part
of the environment of a “controller” to be designed. In Fig. 3 the “controlled system”
is refined into the “rail”, the “pump”, and the “injector”. Is-part-of and inheri-
tance relationships (is-a) can be used as known from object-oriented approaches.
Regarding dependencies, i ∗ distinguishes four different kinds: task , goal , resource ,
and softgoal dependency , the latter suited to capture non-functional requirements.
They vary according to the degree of freedom they leave to the dependee. While in a
task dependency the depender prescribes the concrete steps, for a goal dependency
only a desired situation is specified. Similarly for the resource dependency: it is up to
the dependee how to bring the situation (or resource, resp.) about. In control systems
development, all these kinds of dependencies apply as well. For example, actua-
tors and sensors can be captured via resource dependencies, see “actuator: pressure
valve” and “sensor: pressure” in Fig. 3.
At the strategic rationale level, the types of links on SD level (task, goal, resource,
and softgoal dependencies) become modeling elements. They are used to cap-
ture the individual goals and processes of stakeholders and systems as well as
their relation to external dependencies. Therefore, the SR diagram provides new
kinds of links to detail out a complex task ( and- and or-decomposition ), to model
common rail
pump
injectors
Fig. 2 Schematic
representation of a common
rail injection system