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AGENT-ORIENTED INFORMATION SYSTEMS ANALYSIS AND DESIGN 99
Tropos adopts the i* (Yu, 1995) modeling framework for analyzing requirements. In i* (which
stands for “distributed intentionality”), stakeholders are represented as (social) actors who depend
on each other for goals to be achieved, tasks to be performed, and resources to be furnished. The
i* framework includes the strategic dependency model (actor diagram in Tropos) for describing
the network of interdependencies among actors, as well as the strategic rationale model (ratio-
nale diagram in Tropos) for describing and supporting the reasoning that each actor goes through
concerning relationships with other actors. These models have been formalized using intentional
concepts from artificial intelligence, such as goal, belief, ability, and commitment (e.g., Cohen
and Levesque, 1990). The framework has been presented in detail (Yu, 1995) and has been related
to different application areas, including requirements engineering (Yu, 1993), software processes
(Yu, 1994), and business process reengineering (Yu and Mylopoulos, 1996).
During early requirements analysis, the requirements engineer identifies the domain stakeholders
and models them as social actors who depend on one another for goals to be fulfilled, tasks to be
performed, and resources to be furnished. Through these dependencies, one can answer why ques-
tions, besides what and how, regarding system functionality. Answers to why questions ultimately
link system functionality to stakeholder needs, preferences, and objectives. Actor diagrams and
rationale diagrams are used in this phase.
An actor diagram is a graph involving actors who have strategic dependencies on each other. A
dependency represents an “agreement” (called dependum) between two actors: the depender and
the dependee. The depender depends on the dependee, to deliver on the dependum. The dependum
can be a goal to be fulfilled, a task to be performed, or a resource to be delivered. In addition, the
depender may depend on the dependee for a softgoal to be fulfilled. Softgoals represent vaguely
defined goals, with no clear-cut criteria for their fulfillment. Graphically, actors are represented
as circles; dependums—goals, softgoals, tasks, and resources—are respectively represented as
ovals, clouds, hexagons, and rectangles; and dependencies have the form depender → dependum
→ dependee.
Actor diagrams are extended during early requirements analysis by incrementally adding more
specific actor dependencies, discovered by a means–ends analysis of each goal. This analysis is
specified using rationale diagrams. A rationale diagram appears as a balloon within which goals
of a specific actor are analyzed and dependencies with other actors are established. Goals are de-
composed into subgoals, and positive/negative contributions of subgoals to goals are specified.
During late requirements analysis, the conceptual model developed during early requirements
is extended to include the system-to-be as a new actor, along with dependencies between this actor
and others in its environment. These dependencies define functional and nonfunctional requirements
for the system-to-be. Actor diagrams and rationale diagrams are also used in this phase.
Architectural Design

System architectural design has been the focus of considerable research during the past fifteen
years that has produced well-established architectural styles and frameworks for evaluating their
effectiveness with respect to particular software qualities. Examples of styles are pipes-and-filters,
event-based, layered, control loops, and the like (Shaw and Garlan, 1996). In Tropos, we are in-
terested in developing a suitable set of architectural styles for multi-agent software systems. Since
the fundamental concepts of a multi-agent system (MAS) are intentional and social, rather than
implementation-oriented, we turn to theories that study social structures resulting from a design
process, namely, organization theory and strategic alliances. Organization theory (e.g., Scott, 1998)
describes the structure and design of an organization; strategic alliances (e.g., Morabito, Sack,