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863Proc. 10h Australasian Conference

on Information Systems, 1999

Modelling Frameworks:

The Essential Link Between Models and Methodologies

P. M. Steele

The Peninsula School of Computing and Information Technology,Monash University, Melbourne, Australia

Email: Phillip.Steele@infotech.monash.edu.au

Abstract

The representation of abstractions through models is an over-arching theme in system

development methodologies. In order to express all the subtleties of complex systems,

methodologies suggest the use of several models, with each representing some aspects

of the universe of discourse. Unfortunately, there is no universal agreement about

which aspects of the existing and/or the new information system should be modelled

and the notations that should be used to model them. This paper proposes a

generalised three dimensional modelling framework that can be used to make sense of

the wide variety of models and modelling notations used within system development

methodologies.

Keywords

IS development methodologies, IS development methods and tools, models, modellinglanguages

MODELS AND MODELLING IN METHODOLOGIES

Avison & Fitzgerald (1995), define an information systems development methodology as acollection ofprocedures, techniques, tools and documentation aids that will help systemsdevelopers in their efforts to implement a new information system. They suggest that amethodology consists of phases, themselves consisting of sub-phases, which will guide the

systems developers in their choice of the techniques that might be appropriate at each stageof the project, and also help them to plan, manage, control and evaluate information systems

projects.

Although the use of IS development methodologies is widespread, much diversity exists inthe concepts, methods, beliefs, values and normative principles upon which methodologiesare based (Jayaratna,1994). In order to make sense of this diversity, it has been suggestedthat the paradigms on which information systems development methodologies are based may

be defined along two dimensions: a subjectivist-objectivist dimension and an order-conflictdimension (Hirschheim, et al, 1995). It is argued that a given methodology may be

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thought of as having elements that are positioned somewhere in the two-dimensional spacedefined by the intersection of these dimensions.

In the first dimension, the objectivist view adheres to scientific method that is considered to

apply equally well to the natural and social universe. The subjectivist view rejects scientificmethod as being appropriate for understanding the social universe and seeks to understandthe social world by analyzing the subjective experience of individuals. In the seconddimension, the ordered view emphasizes a social world characterized by order, integration,consensus and functional co-ordination. The confliction view stresses change, conflict,disintegration and coercion. The four quadrants established in the two-dimensional spaceare labeledfunctionalism (objective-order), social relativism (subjective-order), radical

structuralism (objective-conflict); and neohumanism (subjective-conflict).

The information system development methodologies considered in this paper, are those withthe majority of their elements based in the functionalism quadrant, although they do havesome elements that could be placed in other quadrants. These functionalist methodologiesare selected due to their maturity, popularity and widespread acceptance, their proven abilityto be used effectively in a wide range of development situations including large scalecomplex projects, and the relatively high level of complexity and formality in their diagrams,notations and models. These mainstream system development and software engineeringmethodologies include Structured Analysis and Design (DeMarco 1979, Gane & Sarson1979, Yourdon 1989), Information Engineering(Brathwaite 1992, Martin 1990, TexasInstruments 1990) the Unified Software Development Process (Jacobson et al, 1999),

Fusion (Coleman et al 1994), and Open (Firesmith et al, 1998).

The development of a computer based information system is a complex activity. The varietyof issues to consider and deal with is large, with often-conflicting demands and expectations

being made on system developers by a range of stakeholders. Shaw (1981), provides auseful insight into how we deal with complexity.

We (humans) have developed an exceptionally powerful technique for dealingwith complexity. We abstract from it. Unable to master the entirety of a complexobject, we choose to ignore its inessential details, dealing instead with the

generalized, ideal model of the object.

Abstraction is the process of formulating general concepts by choosing to ignore conceptsthat are considered unimportant, at least temporarily, by an observer. The representation ofthese concepts forms a model. A model is an abstraction or simplification of some part of the

present or proposed information system (Avison & Fitzgerald, 1995). A model is asimplified, stylized representation of a system, abstracting the essence of the systems

problem studied. (Wijers, 1991). A good model captures the crucial aspects of a problemand omits the others (Rumbaugh et al, 1991). A model is a general description of an existingor potential domain, where it denotes either a simplified part of the real, perceivable world,

an intellectual construction, or a mixture of both (Frank, 1997).

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The representation of abstractions through models is an over-arching theme in functionalistinformation system development methodologies. In order to express all the subtleties ofcomplex systems, it is necessary to use multiple views (models), with each view (model)representing a subset of the aspects of interest (Booch, 1994, page 172).

To assist in understanding the diversity of models used in methodologies, some form ofclassification of modelling techniques is useful. Brinkkemper (1990) defines a modellingtechnique as a modelling procedure and a corresponding notation to carry out a certain typeof modelling activity. The modelling procedure defines the steps or tasks involved indeveloping the model, while the notation is used to record the details of the model itself.

Brinkkemper (1990) identifies three categories of modelling techniques formal,structuredand informal. Structured modelling techniques are those for which the syntaxis well defined, but the semantics is less well defined. Used in this sense, structuredmodelling techniques are used not only by methodologies such as Structured Analysisand Design, but by any methodology using modelling techniques with a well defined syntax,

but a less well defined semantics. The semantics of these techniques is often shown byexamples without a formal mapping to an underlying mathematical formalism. Most of themodelling techniques used by functionalist ISDMs fall into this category.

The structured modelling techniques used by functionalist ISDMs vary considerably basedon the underlying modelling paradigm on which they are based. Structured analysis anddesign methodologies employ modelling techniques such as data flow diagrams,minispecs, structure charts, etc (DeMarco 1979, Gane & Sarson 1979, Yourdon 1989).The structured modelling techniques listed use modelling elements including process, data

flow, data store, data element, external agent, etc.

Information engineeringmethodologies employ structured modelling techniques such asentity relationship diagrams, activity hierarchy diagrams, activity dependency

diagrams, dialog flow diagrams and action diagrams (Brathwaite 1992, Martin 1990,Texas Instruments 1990). These structured modelling techniques use modelling elementssuch as entity, attribute, relationship, function, process, elementary process,

procedure, dialog flow, etc.

Object-orientedmethodologies employ structured modelling techniques such as use-casediagrams, state diagrams, class diagrams, sequence diagrams, activity diagrams,deployment diagrams, etc. (Booch 1999, Coleman et al 1994, Firesmith et al, 1998,Frank 1997, Jacobson et al 1994). These structured modelling techniques employ modellingelements such as actor, use case, scenario, class, object, relationship, attribute,operation, message, etc.

Clearly, each functionalist system development methodology uses its own unique set ofstructured modelling techniques and modelling elements, even though many similarities mayexist between them based on a common underlying modelling paradigm. However, it is not

uncommon for one methodology to use one structured modelling technique for a variety ofpurposes at different stages of the system development life cycle. The role and purpose of

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models within methodologies forms part of the methodology's modelling framework that isdefined implicitly or explicitly within the methodology.

MODELLING FRAMEWORKS

A number of researchers have defined modelling frameworks in an effort to explicitlydefine how models and modelling techniques should be used within system developmentmethodologies.

Modelling frameworks define the models to be used and their role(s) within themethodology (Booch 1994, DeMarco 1979, Essink 1986, Frank, 1997, Jacobson 1992,Olle et al 1991, Sowa & Zachman 1992).

Essink (1986) defines a two-dimensional modelling framework, based on four layers in

one dimension representing four major modelling foci and eightperspectives in the other,representing the aspects that are to be modeled within each layer. The four modelling fociare the object system model (OSM) that defines the real world activities and organizational

processes, the conceptual information systems model (CIM) that defines the functions ofthe IS and how these are to be achieved, the data system model (DSM) thatembodies thelogical implementation decisions but remains target platform independent, and theimplementation model (IMP) that is a complete definition of the information system to beimplemented.

The second dimension of the Essink framework consists of eight perspectives that are

applicable to each of the four model layers identified. These perspectives enable themodeler to focus on a given aspect of each layer separately, thereby providing a basis forsimplifying and structuring the modelling activities within each layer. These eight perspectivesare the goal structure perspective, the environmental interaction perspective, the

functional perspective, thestructural perspective, the process perspective, the systemdynamics perspective, the equipment used perspective and the organizational and

social perspective.

Some of the structured modelling techniques listed in the previous section and the modellingabstractions that they employ are used within the Essink modelling framework. For

example, entity relationship models may be used to model the structural perspective of theOSM, CIM, DSM and IMP layers. Within a given layer, one model may represent morethan one perspective. For example, while entity relationship models represent only the

structuralperspective, data flow models represent aspects from thefunctional, structuralandprocess perspectives.

Sowa and Zachman (1992) propose a two-dimensional modelling framework forestablishing context for the modelling activities within ISDMs. One dimension of thisframework is based on the different stakeholder perspectives ofplanner, owner, designer,builderand subcontractor, while the second dimension consists of a broad classification of

the perspectives that are important to each stakeholder. These are identified as what

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(data), how (function), where (network), when (time), who (people) and why(motivation).

Frank (1997) proposes the MEMO Multi Perspective Enterprise Modelling framework thatincorporates object-oriented modelling approaches. This two dimensional framework has

three perspectives on the enterprise: strategic, organizational, and information system.Four modelling foci are established:process, structure, resource and goals.

Other methodology modelling frameworks are common in the literature. The old physical,old logical, new logical, and new physical modelling layers proposed by DeMarco(1979), as part of structured analysis and design, represents a widely used modellingframework providing context for the use of models and modelling techniques. Olle et al(1991), propose a modelling framework consisting of two dimensions. The first dimension is

based on the life-cycle stages of information systems planning, business analysis, systemdesign and construction design. The second dimension consists ofdata oriented, processorientedand behavior-orientedabstractions that define what should be modeled in eachdimension.

While each of the above approaches provides a well-defined context for understanding theuse of models within a given methodology, each framework is different. A clear need existsfor a generic framework to make sense of how models and modelling techniques may beused within methodologies generally.

A GENERIC METHODOLOGY MODELLING FRAMEWORK

In response to this need, the author has developed a generic three-dimensional modellingframework for making sense of models and modelling techniques within methodologies. Thisis shown in Figure 1.

Figure 1: Three-dimensional Methodology Modelling Framework

The first dimension of the generic framework is called the modelling elements dimension ofthe methodology. The term semantic space has been used also to describe this aspect(Steele, 1995). It consists of the union of all the modelling elements or modelling

abstractions used by all of the structured modelling techniques of a functionalist systemdevelopment methodology. Following the work of Rumbaugh et al (1999) a modelling

Modeling Elements

ModelingContexts

ModelingTechniques

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abstraction ormodelling element is defined as an abstraction drawn from the system beingmodeled. Modelling elements may be primitive or composite in nature. Compositemodelling elements consist of other modelling elements.

The second dimension of the generic framework is called the structured modellingtechniques dimension of the methodology. This dimension consists of the union of all thestructured modelling techniques (Brinkkemper, 1990) used by a methodology.

The third dimension of the generic modelling framework is called the modelling contexts ofthe methodology. It consists of the union of all the modelling contexts of a methodology. Amodelling context is a modellinguniverse of discourse, or area of modelling focus (Halpin,1995). Methodology modelling contexts are often not explicitly defined by a methodology,

but may be implicitly embedded in the methodology task structure or the purpose of thedeliverables produced.

Table 1(a) shows the relationship between a subset of the modelling elements andstructured modelling techniques of the Rational Unified Process Methodology whichuses the Rational UML modelling techniques, while Table 1(b) shows the relationship

between a subset of thestructured modelling techniques and modelling contexts of thesame methodology (Krutchen, 1999, p42).

Table 1(a) Table 1(b)

Rational Unified Process Rational Unified Process

Modelling Techniques /Elements Modelling Techniques /Contexts

Tables 1 (a) and (b) reveal a number of issues to help make sense of how the RationalUnified Process methodology uses structured modelling techniques, modelling elements andmodelling contexts. Table 1 (a) shows that the class diagram modelling technique uses themodelling elements class,package and relationship (and others not shown). TheSequence Diagram modelling technique uses the modelling elements objectand message.The asterisk indicates that the class name of a class defined in a class diagram may appearon the sequence diagram, thereby being indirectly represented in the diagram.

Table 1(a) shows that one modelling element may be used in many structured modelling

techniques and one modelling technique may use many modelling elements. Semanticconnections are established between modelling techniques where they use a common

Modeling Technique

Modeling Element

Class

Package

Relationship

Object

Message

y

y

y

Class

Diagram

y*

y

y

Sequence

Diagram

Modeling Technique

Modeling Context

Management Set

Requirements Set

Design Set

Implementation Set

Deployment set

y

y

y

y

y

Use Case

Diagram

y

y

Class

Diagram

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modelling element. Where this occurs, the common modelling element(s) may have differentgraphical and/or textual representations in each of the modelling techniques in which they

appear. For example, a class appears as a named rectangle in a class diagram but asunderlined text in a sequence diagram.

Table 1(b) shows how the UML modelling techniques used by the Rational SoftwareProcess are used in different modelling contexts. Management set artifacts that mayinclude use case diagrams assist in the management of a project (Kruchten, 1999, p42).However, one or more use case diagrams may also form part of the requirements setartifacts that specify the definition of the software system. In a similar way, use casediagrams may appear as design set artifacts used to define the behavior of the actualsystem

The proposed framework may be easily mapped to any functionalist developmentmethodology to show how its modelling contexts, modelling techniques and modellingelements are inter-related. For example, the OMT methodology has modelling contexts ofanalysis, system design, object design and implementation (Rumbaugh et al, 1991, p5).The OMT modelling techniques consist of the object model, the dynamic model and the

functional model, which may be defined in each modelling context. The object modelrepresents the class and object structure of the information system using modelling elementssuch as class, object and relationship. The dynamic model represents the temporal

behavior of objects using modelling elements including object state and state transition.

Thefunctional modelrepresents transformation functions associated with the execution ofoperations that are features of objects.(Rumbaugh et al, 1991, p6). The class and objectmodelling elements defined in the object model are the same class and object modellingelements whose temporal behavior is defined in the dynamic model, and whosetransformational logic is defined in thefunctionalmodel.

The proposed framework may also be used to explain how methodologies use the samemodelling technique in different modelling contexts. Where this occurs, although the modelsinvolved use identical modelling elements, these modelling elements may be representingdifferent things in different universes of discourse. For example in the Booch methodology

(Booch, 1994), a class diagram (model) may be used during analysis to represent domainclasses and during design to represent design and implementation classes.

The generic three-dimensional modelling framework proposed in this paper provides auseful basis for understanding how the models and model elements used in a methodologyare related to each other. It recognizes that a given modelling technique may be used inseveral different modelling contexts within the one methodology, and that the resultingmodels are different, even though they are based on common modelling elements and have

been developed following a common modelling procedure.

The different modelling contexts embodied in a methodology are frequently highlighted bydistinguishing between a model that is developed using a given modelling technique and themethodology product in which it is embedded. For example, in the Information

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Engineering Methodology (Texas Instruments, 1990), the entity relationship modellingtechnique may be used to create different ER models contained in different methodology

products. TheAnalysis Data Modeland the Implementation Data Modelare differentmethodology products defined using the same modelling elements, modelling language andmodelling procedure. However, they are developed as part of different methodology

processes and become part of different methodology products that establish differentmethodology modelling contexts.

The modelling contexts, modelling techniques and modelling elements forming part of aninformation systems development methodology define the modelling worldview orWeltanschauung of the methodology (Jayaratna, 1994). Although a modelling contextcannot be considered complete in a mathematical sense, it is complete and meaningfulin a

methodological sense. A modelling context provides a meaningful sub-model representingsome part of the existing and/or proposed information system. Each model used within agiven modelling context serves as a sub-model of that context. All the models (consisting ofmodelling elements) and the modelling contexts of which they form part, constitute what themethodology designer believes is sufficient knowledge to allow the methodology user toanalyze, design and implement computerised information systems using that methodology.

CONCLUSION

This paper has explored the use of modelling in system development methodologies. It has

focused on the use of structured modelling techniques as these approaches are usedextensively in functionalist information systems development and software engineeringmethodologies.

The need for models to embody the philosophy, values and assumptions on whichmethodologies are based has been recognized and a number of modelling frameworkshave been explored that focus methodology modelling activities to incorporate these.However, each of the frameworks considered reflects a specific view about the aspects thatshould be modelled, the notations that should be used to document those aspects and thecontexts that are appropriate for the modelling activity. A more general framework that

does not make specific assumptions about these issues has been developed and applied to asubset of the Rational Unified Process methodology.

A generic three-dimensional methodology modelling framework has been defined whichattempts to make sense of how functionalist system development methodologies use models,modelling elements and modelling techniques. It recognizes that the aim, goals and purposeof using one or more modelling techniques within a methodology must be considered. Thisaspect is addressed through the modelling context dimension of the framework. Thesecond dimension of the framework incorporates the modelling techniques used by themethodology. This captures the modelling procedures and notations that are used to jointly

represent all the semantics required in each modelling context. The final dimension includesthe modelling elements used by the methodology. This dimension represents thefundamental building blocks or modelling abstractions that are used by the modelling

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techniques used by the methodology. The modelling elements dimension provides a basis onwhich to integrate and inter-relate the models and modelling techniques used within one or

more methodology modelling contexts.

The proposed framework has proven to be useful in a number of ways. It has been usedsuccessfully as a basis for teaching undergraduate computing students about the relationship

between models in system development methodologies and the relationship between modelsand methodologies. It provides an easily understandable basis on which to explain how agiven modelling technique may be used in a variety of contexts within a methodology andhow different modelling notations may be semantically connected with each other byrepresenting at least one common modelling element or related modelling elements. Theframework has also been used successfully to assist in the comparison of system

development methodologies. It enables a clear separation to be drawn between themodelling elements used by a methodology, the modelling techniques that represent thesemodelling elements and the modelling contexts that define the purpose of using modellingtechniques within methodologies.

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COPYRIGHT

Phillip Steele 1999. The authors assign to ACIS and educational and non-profit institutionsa non-exclusive licence to use this document for personal use and in courses of instructionprovided that the article is used in full and this copyright statement is reproduced. Theauthors also grant a non-exclusive licence to ACIS to publish this document in full in theConference Papers and Proceedings. Those documents may be published on the WorldWide Web, CD-ROM, in printed form, and on mirror sites on the World Wide Web. Anyother usage is prohibited without the express permission of the authors.