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The Tropos metamodel and its use.

Tropos is a software development methodology founded on the key concepts of agent-oriented software development. Specifically, Tropos emphasizes concepts for modelling and analysis during the early requirements phase. This phase precedes the prescriptive requirements specification of the system-to-be. In this paper, we present the Tropos metamodel starting from the basic concepts of actor, goal, plan, resource and social dependency and then we illustrate its use by introducing an extension intended to introduce concepts for modelling security concerns. We also sketch the Tropos modelling environment and compare with the metamodels of other software development methodologies.

Keywords: Agent Oriented Software Engineering Methodology, Metamodel

Received: May 9, 2005

Povzetek: Podana je programska metodologija Tropos, temeljeca na agentnih pristopih.

1 Introduction

Software development paradigms have exploited a wealth of models to capture requirements and design information about a software system (the "system-to-be") throughout its development process. Structured software development used SADT and Data Flow Diagrams. Object-oriented software development has used a range of modelling languages which have been integrated into UML. Not surprisingly, agent-oriented software development is following on the same footsteps.

To formally analyze software models, we need a means to define their syntax and semantics. Metamodels have been used for the former task. Metamodels define a set of possible instantiations, which are all and only the syntactically correct models in some modelling language. As such, metamodels have been used for more than two decades as a basis for defining the syntax of (usually graph-theoretic) modelling languages, such as UML as well as Tropos.

The objective of this paper is to introduce the Tropos metamodel, discuss some of its uses, and compare it to other metamodels of agent/goal-oriented software development methodologies. Section 4 of the paper sketches the Tropos methodology, while Section 3 presents the metamodel and explains its features. Section 4 presents one extension of the metamodel to include security-related concepts. In Section 5 we sketch the Tropos development environment, which uses the metamodel in its basic core. Section 6 relates the proposed metamodel to others in the same family of modelling languages, while Section 7 concludes the paper.

2 Models and Methodology

Tropos is founded on the idea of using the agent paradigm and related mentalistic notions during all phases of the development software process. The methodology [6] adopts the i* [26] modelling framework, which proposes the concepts of (social) actor, goal, task, resource and social dependency to model both the system-to-be and its organizational operating environment. The i* framework includes the strategic dependency model (actor diagrams in Tropos) for describing the network of inter-dependencies among actors, as well as the strategic rationale model (goal diagrams in Tropos) for describing and supporting the means-ends analysis conducted by each actor as it attempts to ensure that--through delegations to other actors--its goals will eventually be fulfilled.

An actor diagram is a graph whose nodes represent actors (agents, positions, or roles), while edges represent dependencies among them. A dependency represents an agreement between two actors where one actor (the depender) depends on another (the dependee) to fulfill a goal, perform a task or deliver a resource (the dependum). Dependencies may also involve softgoals (such as "having a good quality meeting") which represent vaguely-defined goals, with no clear-cut criteria for their fulfillment.

A goal diagram is also a graph where nodes represent goals or plans (1), while edges represent goal/plan relationships, such as AND/OR-decomposition (i.e., a goal/plan can be decomposed into a set of other goals/plans. Goals/plans can also be related to softgoals through qualitative relationships (labelled "+" or "-") to indicate that the goal/plan contributes positively or negatively to the fulfillment of the softgoal. Goal diagrams appear inside a balloon associated with a single actor. This is the actor whose goals/plans are being analyzed to determine how they can be fulfilled/executed.

The Tropos methodology supports four phases of software development: Early Requirements Analysis, Late Requirements Analysis, Architectural Design, and Detailed Design. Early requirements is concerned with understanding the organizational context within which the system-to-be will eventually function. During early requirements analysis, the requirements engineer identifies the domain stakeholders (who have a stake in the system-to-be) and models them as social actors, who have goals and depend on each other for goals to be fulfilled, plans to be performed, and resources to be furnished. Late requirements, on the other hand, is concerned with a definition of the functional and non- functional requirements of the system-to-be. This is accomplished by treating the system as another actor (or a small number of actors) who are dependers/dependees in dependencies that relate them to external actors. The shift from early to late requirements occurs when the system actor is introduced and it participates in delegations from/to other actors.

Architectural design is concerned with the global structure of the system-to-be. Unsurprisingly, subsystems and system components are represented as actors too, and their dependencies to other system components are social, rather than procedural/structural. This means that system components need to have the ability to monitor dependencies to other actors to make sure they will be fulfilled. As well, system components need to be able to cancel dependencies that seem ineffective and replace them with new ones through planning, negotiation, etc. As with conventional software architectures, architectural styles constitute critical support for the software developer. Since the fundamental concepts of Tropos architectures are intentional and social, we have turned to theories which study social structures to define architectural styles: namely Organization Theory and Strategic Alliances.

Detailed design focuses on the specification of actor communication and behavior. To support this phase, we have adopted existing agent communication languages such as FIPA-ACL [20] or KQML [11]; also message transportation mechanisms and other related concepts and tools. We have also proposed and defined a set of stereotypes, tagged values, and constraints to accommodate Tropos concepts within UML [5].

Through the models constructed during these phases, one can answer "why" questions, in addition to "what" and "how" ones, regarding system functionality. For example, one can ask "Why does this component of the system need to notify library users when a book becomes available". Answers to why questions ultimately link system functionality to stakeholder needs, preferences and objectives. Such answers serve as ultimate justifications for all elements of a proposed design.

3 The Metamodel

Figure 1 shows the portion of the Tropos metamodel, where agent, role and position are specialization of the concept of actor. A position can cover 1 ... n roles, whereas an agent can play O ... n roles and can occupy O ... n positions. An actor can have 0 ... n goals, which can be both hard and softgoals and are wanted by 1 actor.

An actor dependency is a quaternary relationship and relates respectively a depender, dependee, and dependum (i.e. goal, plan, resource). It is possible to specify also a reason for the dependency (labeled as why).

A model is an instance of the metamodel and can have a graphical representation in terms of actor and goal diagrams.

Figure 2 depicts an example of an actor diagram for the domain of the Conference Review Process and represents a model that can be obtained instantiating the metamodel discussed so far. Three actors are involved: the Program Committee Chair (PC Chair), the Program Committee Member (PC Member) and the Reviewer. Dependencies take place between them; in particular the goal review papers is delegated by the PC Chair to the PC Member, moreover the PC Chair also expects to have the information of the possible conflicts (a resource dependency) between the PC Member and the authors of the papers. On the other hand, the PC Member depends on the PC Chair to obtain the papers to distribute and the review form. Many critical goal and resource dependencies occur between the PC Member and the Reviewer. In particular, the PC Member depends on the Reviewer for review the papers and to obtain the information about the possible conflicts on assigned papers. The Reviewer depends on the PC Member in order to obtain a set of assigned papers as well as the review form. Finally, the PC Member wants to be fair in the review assignment, and this is represented as a softgoal wanted by the PC Member.

The concepts related to the Tropos goal diagram are depicted in Figure 3. The central concept of goal is represented by the class Goal. Goals can be analyzed, from the point of view of an actor, by Means-end analysis, Contribution analysis and Boolean decomposition. Means-end Analysis is a ternary relationship defined among an Actor, whose point of view is represented in the analysis, a goal (the end), and a Plan, Resource or Goal (the means). Contribution Analysis is a ternary relationship between an actor, whose point of view is represented, and two goals. Contribution analysis strives to identify goals that can contribute positively or negatively towards the fulfillment of other goals (see association relationship labeled contribute in Figure 3). A contribution can be annotated with a qualitative metric, as proposed in [8], denoted by +, ++, -, --. In particular, if the goal g1 contributes positively to the goal g2, with metric ++ then if g1 is satisfied, so is g2. Analogously, if the plan p contributes positively to the goal g, with metric ++, this says that p fulfills g. A + label for a goal or plan contribution represents a partial, positive contribution to the goal being analyzed. With labels --, and - we have the dual situation representing a sufficient or partial negative contribution towards the fulfillment of a goal. Decomposition, whose metamodel is described in Figure 3, is also a ternary relationship which defines a generic boolean decomposition of a root goal into subgoals, that can be in particular an AND- or an OR-decomposition specified via the attribute Type in the class Boolean Decomposition specialization of the class Decomposition.

The concept of plan in Tropos is specified in Figure 2 and 3. Means-end analysis and AND/OR decomposition, defined above for goals, can be applied to plans also. In particular, AND/OR decomposition allows for modelling the plan structure.

Figure 4 gives a sketchy view of goal diagram for the actor PC Member and for the goal review papers and for the softgoal be fair in the review assignment.

The goal review papers has been AND-decomposed in two sub goals: assign papers to reviewers and collect the reviews. This latter represents the "Why" for the dependency review the papers between PC Member and Reviewer, as shown in Figure 1. The goal assign papers to reviewers is decomposed in two subgoals: send the papers, that is operationalized as send papers by e-mail, and select reviewers decomposed in verify the competences and verify conflicts. This latter represents the "Why" for the resource dependency conflicts between the PC Member and the reviewer. Moreover, the fulfillment of these two sub-goals can contribute positively to the fulfillment of the softgoal be fair in the review assignment as described by the positive contribution relationships in the diagram.

4 Metamodel Extension

Secure Tropos has been proposed in [16] as a formal framework for modelling and analyzing security. It enhances Tropos introducing four new concepts and relationships behind Tropos dependency: trust, delegation, provisioning, and ownership. The basic idea of ownership is that the owner of a resource (goal or plan) has full authority concerning access and disposition of his resource (goal or plan). The distinction between owning a resource makes it clear how to model situations in which, for example, a client is the legitimate owner of his/her personal data and a Web Service provider that stores customers' personal data, provides the access to her/his data. We use the relation for delegation when in the domain of analysis there is a formal passage of authority (e.g. a signed piece of paper, a digital credential is sent, etc.). The trust relations have their intuitive meaning among agents, namely the believe of an agent that the actor does not misuse some resources.

Figure 5 shows the the new part of the Tropos metamodel concerning trust and ownership. An actor (the truster) trusts another actor (the trustee) about the achievement of a goal, the fulfillment of a plan or the delivering of a resource. The content of the trust relationship is called trustum. An actor can be the owner of a resource, a plan and goal and he/she has authority concerning the use of the resource, the execution of the plan and achievement of the goal, respectively.

The metamodel describing delegation relationships is basically identical to the metamodel for the dependency relationship as presented in Figure 1. The delegater delegates the delegatee for the achievement of a goal, the execution of a plan or the delivering of a resource. As for the dependency relationship, it is also possible here to specify the reason (why) of a delegation.

We have shown in [17] how the original concept of Tropos dependency can be expressed in terms of trust and delegation. Roughly, when an actor depends on another actor to achieve a goal (to fulfill a task or to deliver a resource), it is implicitly intended that the actor trusts the other actor and delegates it for such activities. A precise formalization of dependency refinement in terms of trust and delegation has been presented in [17].

Figure 6 presents an example of application the extended metamodel. The Author trusts the PC Chair to implement a fair review process and he/she is the owner of the paper sent to the PC Member and reviewed by the Reviewer. The PC Chair trusts and delegates PC Member to review a certain number of papers, and in turn the PC Member trusts and delegates the Reviewer to review the papers. The PC member (Reviewer) depends on the PC Chair (PC Member) to receive the paper to review.

5 A Modelling Environment

In order to support the specific analysis techniques adopted in Tropos, different tools have been developed, such as a tool for the verification of requirements specification through model-checking technique (T-Tool) [13], a tool which supports forward and backward reasoning on the goal analysis structures (GR-Tool) [15]. In this section, we will give details of a modelling environment, called TAOM4e (Tool for Agent-Oriented Modelling for Eclipse), which is based on an implementation of the metamodel described in the previous sections. The metamodel has been specified following the OMG's MDA [21] standard for metamodel interoperability, that is the Meta Object Facility (MOF) (2) which offers a mechanism for automatically deriving a concrete syntax based on XML DTDs and/or schemas known as XML Model Interchange (XMI). This is a preliminary step towards the adoption of the model-to-model transformation approach proposed by MDA.

Among the main requirements we considered in developing this tool are the following [23]:

--Visual Modelling. The modelling environment should support the user during the specification of an AO model (e.g., according to the Tropos visual notation). Moreover, the environment should allow us to represent new entities that will be included in the Tropos metamodel, language variants, such as those presented in Section 4, as well as to restrict its use to a subset of entities of the modelling language.

--Specification of model entities properties. The modelling environment should allow us to easily annotate the visual model with model properties like invariants, creation or fulfillment conditions that are typically used in Formal Tropos specification.

--Automatic Model Translation. The modelling environment should allow us to save a model in a standard format (e.g., XML and XMI), and provide automatic transformation into a different specification language. The model-to-model transformation approach should be also compliant with Query/View/Transformation (QVT) requirements [14], as discussed in [24].

--Extensibility. The modelling environment should be extensible and allow for different configurations by easily integrating other tools at will. An effective solution to the requirement of a flexible architecture and to the component integration issue is offered by the Eclipse Platform.

New tools are integrated into the platform through plug-ins that provide the environment with new functionalities. A plug-in is the smallest unit of function in Eclipse and the Eclipse Platform itself is organized as a set of subsystems, implemented in one or more plug-ins, built on the top of a small runtime engine. The TAOM4e architecture is depicted in Figure 7. It follows the Model View Controller pattern and has been devised as an extension of two existing plug-ins. First, the EMF plug-in (3) offers a modelling framework and code generation facilities for building tools and other applications based on a structured data model. Given an XMI model specification, EMF provides functions and runtime support to produce a set of Java classes for the model. Most importantly, EMF provides the foundation for interoperability with other EMF-based tools and applications. The resulting plug-in, called TAOM4e model implements the Tropos metamodel. It represents the Model component of the MVC architecture. Second, the Graphical Editing Framework (GEF) plug-in4 allows developers to create a rich graphical editor around an existing metamodel. The functionality of the GEF plug-in helps to cover the essential requirement of the tool, that is supporting a visual development of Tropos models by providing some standard functions like drag & drop, undo-redo, copy & paste and others. The resulting plug-in, called TAOM4e platform represents both the Controller and the Viewer components of the tool. In Figure 8 a snapshot of the modeler: the diagram editor window on the right, the project and model browsers on the left, the entity properties window at the bottom.

6 Related Work

Many Agent-Oriented Software Engineering methodologies have been proposed and compared over the last few years [18, 25]. An analysis of the metamodels of three methodologies, ADELFE [4], GAIA [27] and PASSI [7] has been presented in [3]. The aim of this work was to face interoperability issues between different methodologies.

In this section we extend this analysis including Tropos. We will focus on four dimensions: Agent Structure, Agent Interaction, Agent Organization and Agent Development (e.g., CASE tools at support of the development process). Table 1 summarizes the comparison. In ADELFE the concept of agent (Cooperative Agent) is defined as the composition of aptitudes, skills, characteristic, communication and representation. Not explicit concept of role is given, the concept of goal is implicitly used to identify agent skills, but it is not representable as well as the concept of plan, since a plan is an entity that will be built at run time and which is not representable at design time. In GAIA, an agent (Agent Type) is specified as a composition of roles. Each role is responsible of a specific set of activities associated with the role. Goals cannot be explicitly modeled, but they are implicitly used to characterize a role. In PASSI, an agent (Agent) is defined as the composition of roles and each role is defined as the manifestation of the agent activity in some scenario. Goals are implicitly considered when specifying non-functional requirements attached to agent duties. In Tropos, the concept of Actor generalizes the concepts of agent and role (or set of roles), an actor can have individual goals and it can be able to execute plans to satisfy goals. Goal analysis in Tropos drives the modelling process, as discussed in Section 4 and allows us to represent goal decomposition, means to satisfy a goal or contribution towards goal satisfaction through different goal relationships.

The concepts used to specify the interactions of an agent with another agent or with the environment are similar in ADELFE, GAIA and PASSI. Basically, they use the concept of communication, role, and protocols. Tropos adopts the Agent Unified modelling Language (AUML) Agent Interaction Diagram, described in [2, 22] (proposed by the FIPA -Foundation for Physical Intelligent Agents-[12] and the OMG Agent Work group) where agent communicative acts are represented as messages in a UML sequence diagram.

In GAIA, the concept of organization is a primary concept, organization rules specify constraints that the organization should observe. In PASSI, agent organization aspects are modeled implicitly in terms of services that can be accessed by agents in a given scenario. In ADELFE, agent organization and society emerges from the evolving interactions between the agents which are compliant with cooperation rules.

In Tropos the strategic dependencies between actors in a domain makes explicit the organizational dimension and provide basic entities to model organizational patterns [ 19]. Moreover, the Tropos metamodel has been extended to include concepts of business processes and security.

Both ADELFE and PASSI provide CASE tools at support of modelling and for ad-hoc analysis on part of the resulting specification. Tropos provides modelling and analysis tools (details can be found in http://www.troposproject.org) as well as code generation tools [10].

This comparison shows that different metamodels (methodologies) may allow us to model different properties of a system (e.g., organizational aspects, communications and protocols). On the other hand, it shows that even if metamodels share a comparable set of concepts, they can be used in a different way by the different methodologies. This can be found also considering requirements engineering methodologies based on metamodels. For instance, in KAOS [9], the concept of agent is used to assign leaf goals resulting from goal analysis.

Finally, other related work on [i.sup.*] and Tropos metamodels are worth to be mentioned. The [i.sup.*] metamodel [26] represents the basis for the Tropos metamodel. Other extensions of the [i.sup.*] metamodel have been proposed. For instance, in [1] where a methodology for COTS selection is proposed.

7 Conclusion

We have presented an overview of the Tropos metamodel. Like other software development methodologies, Tropos supports a variety of models that need to be analyzed for syntactic and semantic consistency. The metamodel serves as a basis for checking for syntactic consistency. Making it richer, could also help in supporting some forms of semantic consistency currently conducted through a series of tools offered within the Tropos software development environment.

(1) Plans in Tropos correspond to tasks in i*.

(2) http://www.omg.org/technology/documents/ modeling_spec_catalog.htm#MOF

(3) http://www.eclipse.org/emf/

(4) http://www.eclipse.org/gef/

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Angelo Susi, Anna Perini and John Mylopoulos

ITC-irst, Via Sommarive, 18, I-38050 Trento-Povo, Italy

E-mail: susi@itc.it, perini@itc.it, jm@cs.toronto.edu

Paolo Giorgini

Department of Information and Communication Technology

University of Trento, via Sommarive 14, I-38050 Trento-Povo, Italy

E-mail: paolo.giorgini@dit.unitn.it
Table 1: Comparison of the meta-models of four Agent-Oriented
methodologies.

Agent Structure ADELFE GAIA

Agent Cooperative Agent Agent Type
Role Not explicit Role in a organization
Goal Not explicit Not explicit
Plan Not explicit Activity of Role

Agent Interaction

Comm. & Protocol Agent Communication Communication associated
 Agent Interaction to a role and protocols
 Protocols associated
 associated to a communication
 to communication

A. Organization

Structure & Rules Cooperation rules OrganizationStructure,
 Organization,
 OrganizationalRule

A. Development

Modeler Open-Tool --
Analysis tooos Open-Tool --
Code Generation -- --

Agent Structure PASSI Tropos

Agent Agent Actor
Role Role in a scenario Specialization of Actor
Goal Not explicit Goal and goal relation-
 ships
Plan Ontology of Action Plan and plan relation-
 ships

Agent Interaction

Comm. & Protocol Communication associated Not in the current meta-
 to a role and model. AUML interaction
 Messages as diagram UML sequence
 components diagram messages
 of communication for communication acts

A. Organization

Structure & Rules Not explicit Strategic Dependency,
 Ownership, Delegation
 and Trust
 Organizational patterns

A. Development

Modeler PASSI Toolkit TAOM, OME, DW-Tool,
 ST-Tool
Analysis tooos PASSI Toolkit GR-Tool, DW-Tool,
 ST-Tool, T-Tool
Code Generation PASSI Toolkit SKwyRL
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