This paper introduces a new definition of method fragment intended to represent MAS development approaches in a more standardized and coherent way, thus facilitating the configuration of situational methods. In order to do that, we take into account three complementary notions: (i) a method fragment description based on SPEM 2.0 elements; (ii) two method fragment perspectives, the internal and the external view, and (iii) four method fragment granularity layers. Moreover, this definition establishes some mechanisms for method fragments' encapsulation and identification. The proposed method fragment definition is illustrated through an example using Tropos.
In order to structure the development and to manage the complexity associated with
Multiagent Systems (MAS), several development methods have been proposed during
the last decade, e.g. Gaia [
Situational Method Engineering is the sub-area of Method Engineering that addresses the controlled, formal and computer-assisted construction of situational methods out of method fragments. Roughly speaking, building a situational method consists of reusing parts of existing methods taking into account a given project situation that encompasses, for example, notions related to the class of the desired application (like traditional and pervasive computing) and project perspectives.
Several approaches concerning the notions of a part of a method and situational
method building have been proposed in the Situational Method Engineering field. For
instance, Brinkkemper and colleagues [
A Method Component [
This paper proposes a definition of method fragment that combines these two notions of part of a method. This definition allows representing MAS development approaches in a more standardized and coherent way. Moreover, it establishes mechanisms for method fragments encapsulation and identification in order to provide a solid base for developing / building MAS situational methods. The paper is organized in five sections. Section 2 presents the proposed definition for MAS method fragment, while Section 3 shows an application of such definition to Tropos. Section 4 presents an overview of the current research concerning situational method engineering applied to MAS field. Finally, Section 5 presents a discussion about the proposed approach for MAS method fragment definition.
The MAS Method Fragment definition proposed in this paper has been mainly
inspired on the Method Fragment notion proposed by Brinkkemper and colleagues
[
Our proposed definition is: “A MAS Method Fragment is a standardized building block based on a coherent part of a MAS development approach”.
The standardization of building blocks considers the notions of (i) identification of
method fragments using well established naming rules in order to convey their desired
semantics; (ii) encapsulation of original work products; (iii) utilization of common
roles for MAS developers to be used as task performers; and (iv) classification of
method fragment based on a semiotic criteria [
In the following subsections, we will describe the main characteristics of the proposed definition for coherence.
In order to have a standardized and common semantics for specifying method
fragment objectives and work products, we have defined a MAS Work Product
Framework mainly based on the MAS components proposed in the Vowel approach
[
Such a work product framework is used to encapsulate original MAS development
work products, explicitly stating their involvement with user requirements or to the
main MAS components. This approach allows enhancing work product flow into a
situational method and making clear the main goal of each work product
independently of their name in the context of the original MAS development
approach. Finally, the method fragment characteristics are specified through the MAS
Semiotic Taxonomy [
The proposed MAS Method Fragment description is based on SPEM 2.0 elements and related associations. To improve readability we use Arial font to concepts proposed in this work and Comic Sans font to describe SPEM elements. Therefore, the main elements used to compose a MAS method fragment description are: Process Pattern, Activity, Phase, Milestone, Iteration, Task Definition, Task Use, Step, Role Definition, Role Use, Work Product Definition, Work Product Use, Category, and Guidance. As proposed by SPEM 2.0, these elements are separated into method content elements (Task Definition, Step, Role Definition, Work Product Definition) and their application in the development process (Process Pattern, Activity, Phase, Milestone, Iteration, Task Use, Role Use, Work Product Use).
A Process Pattern represents building blocks for assembling processes. It describes a reusable cluster of Activities that provides a consistent development approach to common problems. An Activity represents a general unit of work assignable to specific roles, relying on input work products and producing output work products. We have chosen this as the main element of MAS Method Fragment in the Activity Layer. A Phase consists of a significant period in a project, ending with major management checkpoint, as a Milestone that represents a significant event for a development project. We have used Phase and Milestone as main elements of MAS Method Fragment in the Phase Layer. Moreover, Milestone is used to define fragments in the Process Layer. An Iteration is a set of nested Activities that are repeated more than once, allowing the organization of work in repetitive cycles. It has been used to define MAS Method Fragments in the Iteration Layer.
A Task Definition represents an assignable unit of work involving generally a few hours to a few days and usually affecting one or only a small number of work products. A Step describes a meaningful and consistent part of the overall work described for a Task Definition. A Task Use represents a proxy for a Task Definition in the context of one specific Activity. Tasks and Steps constitute the main element of the Activities used to build MAS Method Fragments.
A Role Definition describes a set of related skills, competencies, and responsibilities of an individual or a set of individuals. A Role Use represents a Role Definition in the context of one specific Activity. Roles represent both the development roles originally specified by the AOSE methods and the common MAS role set involved in the MAS Method Fragments definition. Work Product Definition represents pieces of work that are used, modified, and produced by Task Definitions, while a Work Product Use represents a Work Product Definition in the context of a specific Activity. Work Product Definitions represent the artifacts proposed by the MAS development approaches, as models, specifications and diagrams.
A Category represents the classification structure used to group SPEM elements
based on the user’s criteria. It allows defining tree-structures of nested categories used
for browsing MAS Method Fragments based on a semiotic criteria [
The aforementioned elements will be depicted in conjunction of the proposed method fragment views and some standardization notions in Fig. 1. Finally, we have applied an important notion proposed by SPEM - called Variability Elements - to improve tasks and work products defined according to the original MAS development approaches. It allows reaching the completeness and standardization involved in a MAS Method Fragment definition without modifying the original method elements. For example, we can use a Task Variability to complete a task definition introducing performing role and a Work Product Variability to define composite work products, as Agent Model or Requirement Model.
The Internal and External views of MAS Method Fragments depict, respectively, the
whole set of elements that compose them and the main elements that constitute their
interface. They have been inspired on the method component views concept proposed
by Karlsson [
External view Naming Rules
Semiotic Classification
Objectives
&
Work Products (input and output)
Process Pattern Category Work Product
Use
The External View goals are twofold. First, it describes, in a standard way how a method fragment can be used in a situational method configuration by: (i) identifying the method fragment through naming rules, (ii) specifying method fragment objectives in terms of milestones and/or work products, and (iii) describing fragment characteristics as meaning, usage, structure and so on. Second, it describes how a method fragment can contribute to achieve a situational method objective, specifying fragment output work products through a common semantic. The naming rules adopted to identify method fragments are based on simple concepts and will be explained through the method fragments examples in the next section.
These two method fragment perspectives improve method fragment coherence because it allows analyzing method fragments as standard black boxes without loosing the details of the description. Fig. 1 shows the views and associated elements in a diagrammatic perspective. For instance, it depicts Process Pattern as a kind of “logical container” for building MAS Method Fragments, Category to define part of the External View of MAS Method Fragment, called Method Fragment Semiotic Classification and the use of Milestone to represent MAS Method Fragment expected objectives.
The four layers of MAS Method Fragment – activity, iteration, phase and process
have been defined according to Jacobson et al [
The definition of a MAS method fragment in the Activity Layer, for short an
Activity Method Fragment, is based on the notion of Activity proposed by Jacobson et
al [
Fig. 2 depicts the components of an Activity Method Fragment. It is worthy to notice that this figure represents only the main relationships between SPEM elements used to define an Activity Method Fragment. For instance, relationships between Category and Roles, Tasks and Work Products are not depicted. Moreover, we have labeled the relationship arrows with cardinalities that constitute a constraint over SPEM elements definition. For example, in general a Process Pattern can be associate to zero or many Activities, while the Process Pattern used in the context of an Activity Method Fragment must be associated with exactly one Activity.
An Activity Method Fragment is composed of one Process Pattern associated with one Activity and one or more Categories. Such Activity must be associated with at least one Task Use that must produce one or more Work Products as output. However, in the context of an Activity Method Fragment an Activity must not be associated with an Iteration nor to a Phase or Milestone elements, given that these elements are used to define other layers of method fragment. As we can see in Fig. 2, Category and Work Product elements constitute the external view of the Activity Method Fragment. In summary, an Activity Method Fragment must contain one Activity that is composed by at least one Task, one Role and one output Work Product. Moreover, it must be classified by Categories.
The definition of a MAS method fragment in the Phase layer, for short a Phase
Method Fragment, is based on the notion of phase proposed by Jacobson et al [
Category
The definition of a MAS method fragment in the Iteration layer, for short an Iteration Method Fragment, is based in the SPEM homonym element. An Iteration Method Fragment consists of a Process Pattern that involves a set of Activity Method Fragments and/or a set of Phase Method Fragments that are repeated more than once during the process development lifecycle, offering a structuring fragment to organize work in repetitive cycles. Moreover, it must be classified by Categories.
Finally, a MAS method fragment in the Process layer, for short a Process Method Fragment, represents a whole MAS development cycle. It is composed of a Process Pattern classified by Categories that contains several Phase Method Fragments and/or Iteration Method Fragments and ends with a (major) Milestone. The goal of a Process Method Fragment is twofold. First, it depicts the notion of Base Method presented in Section 2, allowing a top-down fashion to configure a MAS situational Method. Second, it allows describing MAS original methods as a MAS Method Fragment ready to be used when an existing MAS method totally matches a given project situation.
The main advantages of having these four method fragment layers are: (i) the reuse of original AOSE methods in several level of granularity; (ii) the representation of MAS development approaches that do not provide a full development method (such as method fragments related to Agent Organizations models), and (iii) the utilization of bottom-up or top-down mechanisms for situational method configuration.
In this section, we use the proposed definition to describe MAS method fragments sourced from Tropos. Tropos proposes a process for building MAS involving the following phases: Early Requirements, Late Requirements, Architectural Design, Detailed Design and Implementation. The goal of the first two phases is to provide a set of functional requirements as well as non-functional requirements for the system to be built, while Architectural Design and Detailed Design phases focus on the system specification. Finally, the Implementation phase transforms the results of the preceding phases using an agent development platform in order to code the MAS.
We have used the Eclipse Process Framework Composer (EPF Composer) [
Fig. 3 depicts the External View of the Process Method Fragment called MMF Tropos Base Method, represented as a Process Pattern (called Capability Pattern in EPF Composer). On the left frame we can see that this fragment is classified in several categories of the MAS Semiotic Taxonomy, e.g. in the social level and iteration degree, it is classified as part of the Low Iteration Fragment Category.
On the right frame we can see that this fragment is composed of four Phase
Method Fragments - MMF Requirement Phase with Tropos, MMF Analysis Phase with
Tropos, MMF Design Phase with Tropos and MMF Implementation Phase with Tropos.
As said before, we propose applying standard naming rules for method fragment
identification. For instance, phase names should convey either the software
development discipline covered by the phase - as Requirement, Analysis, Design,
Implementation, Test - or the phase main development goal - as Inception,
Elaboration, Construction, Transition – as currently used in iterative development
approaches [
Moreover, the MMF Tropos Base Method fragment is composed of a Milestone called Tropos Base Method Milestone - MAS ready to be used that involves three work products: MP Tropos Agent Model, MP Tropos Interaction Model and MP Tropos Requirement Model, where MP stands for MAS work Product. Such work products encapsulate Tropos original work products and provide a standardized and common semantic for describing MAS work products based on the Vowels approach, as proposed in Section 2. For instance, the work product MP Tropos Requirement Model encapsulates the Tropos Actor Diagram and Tropos Goal Diagram, both related to Tropos requirement phase.
As we have seen in Section 2, given that MMF Tropos Base Method is a Process Layer Fragment it can be used into two distinct ways: as standard representation of Tropos or as a Base Method in a top-down configuration mechanism in order to build a MAS situational Method.
Fig. 4 depicts the External View of the method fragment called MMF Requirement Phase with Tropos as well as some elements of its Internal View (highlighted rectangle of the right frame). On the left we can see the fragment classification using the MAS Semiotic Taxonomy. On the right we can see that this fragment is composed of three Activity Method Fragments: MMF Identify Initial Requirement with Tropos, MMF Detail Requirement with Tropos, and MMF Identify Additional Requirement with Tropos. Moreover, it contains the MAS Objectives Described Milestone that involves the work product MP Tropos Requirement Model.
The Internal View of the MMF Detail Requirement with Tropos involves an homonym Activity that contains a Task Use (Task Description in EPF Composer) called MTV Analyze Goals and Plans. The acronym MTV used as task name prefix stands for MAS Task Variability and indicates that we are dealing with a task that extends another one. Such task variability has been defined over the Tropos original task in charge of analyzing goals and plans in order to provide basic MAS roles. In addition, it also changes Tropos original work products by the related MAS work products that forms the External Fragment View (see Fig. 5 for more details).
It is worthy to notice that the fragment MMF Detail Requirement with Tropos is executed twice in the MMF Requirement Phase with Tropos: first after identifying initial requirements (concerning stakeholders identification) and latter after identifying additional requirements (defining system actors).
Finally, Fig. 5 complements the Internal View of the fragment MMF Detail Requirement with Tropos showing its main elements.
As the fragment MMF Detail Requirement with Tropos consists of an Activity Method Fragment, its elements are defined through the task MTV Analyze Goals and Plans. These are the following: (i) the roles System Analyst and MAS Designer, (ii) the mandatory input work product MPV Tropos Actor Diagram, (iii) the output work product MPV Tropos Goal Diagram, and (iv) several steps, as Analyzing goals through means-end analysis and decomposing goals.
These examples of usage show that the proposed definition of MAS Method Fragment offers a more standardized way for representing each phase and activity of Tropos, as well as Tropos own method as a whole. Moreover, it establishes mechanisms for the encapsulation and identification of original Tropos tasks, roles and work products. Finally, it allows reusing method fragments even in the context of its original method.
In AOSE field there are several researches concerning method fragment notion.
Among them we can cite the FIPA method fragment definition [
The preliminary version of the FIPA method fragment definition [
Our MAS method fragment definition is based on SPEM, as shown in Section 2, and it is fully compliant with this FIPA definition since the portion of process is represented by a Task element while Work Product elements describe fragment deliverables as well as fragment inputs. Moreover, we use Guidance elements to describe MAS concepts, to provide fragment guidelines and glossary of terms. Finally, our method fragment definition encompasses an external view that aims to describe the aspects that should be taken into account during method fragment selection and assembling, applying Category elements to classify the fragments based on their context use, their deliverable work products and their development platforms, among other criteria covered by the MAS Semiotic Taxonomy.
The refinement of FIPA definition proposed by Cossentino et al. [
Summing up, at the best of our knowledge, in the AOSE field there is no explicit method fragment definition tailored to facilitate the use of situational configuration mechanisms and to offer a standardized description of a method fragment elements and objectives. The IEEE-FIPA Design Process Documentation and Fragmentation Working Group2 (DPDF WP), recently formed to deal with a definition of method fragment for situational method engineering process, corroborates that these topics constitute open issues in AOSE field.
In this paper, we presented a new definition for MAS method fragment based on Situational Method Engineering and Software Engineering techniques that can be used to represent MAS development approaches in a more standardized and coherent way. It offers two method fragment perspectives - internal and external fragment view - and four method fragment granularity layers: activity, iteration, phase and process. According to the proposed definition, a method fragment is considered coherent when its internal view is described using SPEM 2.0 and their elements and associations follow one of these four method fragment layers definition.
Moreover, we show how this definition can be used to represent method fragments extracted from Tropos, that is a well known MAS method. However, we think that it is not possible to identify a univocal criteria to set the best level of granularity for extracting method fragments, since fragment creation depends more on the experience of the method engineers than on a rigid extraction criteria. We claim that the absence of such a univocal criteria could be mitigated in two ways. First, using a common model, like SPEM, to represent method fragment. Second, establishing a procedure for extracting and storing fragments in a method repository. Such procedure is part of our research future work.
It is worthy to notice that part of the proposed method fragment definition is general enough to be applied to classical software development methods and not only to MAS methods. In fact, the MAS methods distinctive aspects are taken into account mainly through the following notions of our definition: the MAS Work Product Framework based on Vowel approach and some semiotic criteria, as MAS approach (agent /organization centered) and MAS nature (open/closed MAS). Therefore, in general lines, the proposed method fragment definition could be used to represent fragments sourced from classical software development methods after replacing these notions for other more suitable to a giving development paradigm.
We claim that this MAS Method Fragment definition can provide the backbone for defining a method repository for MAS situational method configuration, given that it provides mechanisms for (i) identifying method fragments using a well established 2 http://www.fipa.org/subgroups/DPDF-WG.html naming rules, (ii) conveying method fragment objectives and encapsulating original work products using a common work product framework, and (iii) categorizing method fragment based on a semiotic criteria. Finally, given that we propose distinct method fragment granularity layers, we can build a method repository involving fragments extracted from AOSE methods as well as from other MAS development approaches, such as Agent Organization models. In a top-down situational method configuration fashion, the former could provide a Base Method for the configuration while the latter could contribute with specific Activity Method Fragments in order to improve and complete a original MAS method.
Acknowledgments. This work is a product of the MEDEIA project, supported by FAPESP (Brazil) and CNRS (France). The first and the last two authors are partially supported by CNPq and CAPES (Brazil).