A FRAMEWORK FOR THE ANALYSIS AND COMPARISON OF
HYPERMEDIA DESIGN METHODS
Susana Montero, Paloma D´ıaz and Ignacio Aedo Laboratorio DEI. Dpto. de Inform´atica
Universidad Carlos III de Madrid
Avda. de la Universidad 30. 28911 Legan´es, Spain [email protected], [email protected], [email protected]
http://www.dei.inf.uc3m.es
ABSTRACT
Due to increasing size and complexity of hyperme-dia and web applications, there is growing concern by how to develop quality, reusable and maintainable hypermedia systems. In fact, a series of well-defined design methods have been proposed to be used during the design process. In this paper, we propose a framework of reference based on software and hypermedia engineering fields in order to perform a constructive analysis of the most outstanding me-thods to date such as HDM, RMM, OOHDM, Autoweb, WebML, UWE and OO-H method.
With this survey, we attempt to help developers to select which method is the most suitable for their needs. Moreover, from this analysis some lacks in the hypermedia development process are revealed.
KEY WORDS
hypermedia, design methods, software engineering
1
Introduction
Hypermedia systems, and specially web applications, have been extremely demanded in different areas like commerce, education, health, and libraries, both to provide a hyperme-dia interface for existing information systems and to create new hypermedia applications. But all of this has been made in a very short period of time what has led most developers to skip the conceptual design phase and directly go to the implementation stage, producing applications of poor qual-ity, usability and maintainability. The majority of these im-plementations have been made with software tools such as NetObjects’ Fusion product or DreamWeaver which sup-port an automated implementation by contents and an easy set up [1], but do not pay attention to intrinsic features of hypermedia systems such as sophisticated navigational structures, interactive behaviours and multimedia compo-sitions. Moreover, other feature which should be taken into account is security, since hypermedia applications are ac-cessed by different users with different purposes [2].
Design methods, and hypermedia methods in partic-ular, face up to the design of large and dynamic systems
through a number of stages and products which make pos-sible to approach modeling in a systematic and integrated way. As a result, systems will have a better quality, usabil-ity, maintainability and reusability.
Several methods have been proposed including HDM, RMM, OOHDM, Autoweb, WebML, UWE and OO-H method. So that it is complicated to decide which one is the most suitable for a specific development. In this paper, we propose a framework of requirements to cover the mod-eling space of design methods for hypermedia systems. On the basis of this framework, the methods previously men-tioned have been analysed.
The rest of the paper is organized in the following way: in section 2 we propose a set of requirements to anal-yse or compare hypermedia design methods derived from the literature and from the experience of our group in the design of this kind of systems [3, 2, 4]. Section 3 presents the analysed methods as well as those requirements that are covered by such methods. In section 4, some research ar-eas are opened from the requirements that have not been taken into account. Finally, in section 5 some conclusions derived from this work are drawn.
2
A framework for hypermedia design
me-thods
To analyse or compare different design methods, it is neces-sary to establish a framework of reference. In our case, this framework is determined by a set of requirements gathered from both the software and hypermedia engineering fields. The experience gained in years of research in the soft-ware field can help to improve hypermedia development as it is suggested by Lowe and Webby [5]. In fact, most of hy-permedia methods borrow the notation of best-known data models, like the E/R (Entity-Relationship) model or OMT (Object Modelling Technique), in order to represent their system structure.
However, these approaches cannot be used directly to model hypermedia systems since their development is much more incremental and iterative than in the rest of software systems [5] and, moreover, as Nanard and Nanard point out, software engineering lacks elements and
mecha-nisms to model aesthetic and cognitive aspects which are a basic concern in hypermedia design [6].
2.1
Requirements derived from software
en-gineering
We will borrow some features of the design process from the software engineering area, but will not consider issues such as project planning or project management. Accord-ing to [7, 8], any design software method must:
S1. Describe a formal process to guide the development
of a software application. That is, a method should
describe what phases of the development are, how these phases are integrated into the overall develop-ment process and what artifacts are created. Typical process models for the software life cycle, including waterfall, prototyping, incremental development and spiral model [9] are not suitable for hypermedia since these kinds of applications pay more attention to or-ganisation of information to promote their access [5] rather than to information processing or management, as it is in the case of software applications.
S2. Contain a model to describe the real world and
transfer it to a physical system. A model is a form
of abstraction which allows us to represent the essence of a design. Several models belonging to the software engineering field, like Entity-Relationship model or UML techniques (Unified Modeling Language), could be used for that, as in [10], but they do not provide mechanisms to represent cognitive (e.g. navigation aid structures, see H2) and aesthetic (e.g. contents alignment and synchronisation, see H3) aspects. S3. Provide the designer with artifacts to specify the
system requirements. Such requirements are
cate-gorized into: functional, i.e., what functions the sys-tem is expected to support; non-functional, i.e., how the system will work in terms of accessibility, accu-racy, efficiency, integrity and so on; and usability, i.e, how the interaction process will be designed. With regard to hypermedia applications, functional require-ments include browsing capabilities, personalization, security constraints and interactive behaviours (see H2, H3, H4, H5 and H6 in subsection 2.2). Non-functional requirements include content and link vali-dation, node size, cohesion and consistency of the in-formation [11]. Finally, usability requirements are ori-ented towards improving the interaction between the user and the hypermedia application [12] and they in-clude aspects like aesthetic, readability, consistency, self-evidence or predictability [13].
S4. Include validation rules for each design product. The method has to provide mechanisms to test the conceptual correctness of each design product and its
completeness. For example, a hypermedia link has al-ways a source and a target, usually referred to as an-chors. A validation rule will check if for each source anchor embedded into a piece of content there is also defined a target anchor resolving to a node or content. S5. Maintain relations of integrity among the different
design products. In order to provide an integrated
framework, the method should support the traceabil-ity, both backwards and towards, of those design en-tities which are described in different design prod-ucts, which can belong or not to the same develop-ment phase. For example, an integrity rule can check if for each node defined in a conceptual phase there is a set of presentation specifications defined during a detailed design phase (v.g., background colour, size and so on).
S6. Support design reuse. Design is a creative process that cannot be learned from a book but from the de-signer’s experience and existing systems. On the one hand, design patterns [14] describe problems that oc-cur repeatedly and provide the core of the solution in order to use it many times and in different contexts and they can be combined with any design method. Hypermedia patterns are usually concerned with inter-face and navigation problems [15]. On the other hand, design components can reuse pieces of previously de-veloped design as in [16].
S7. Count on software support tools to help in systems
development process. In the development process of
hypermedia applications a variety of users with differ-ent levels of knowledge and skills have to interact, in-cluding authors, designers, artists and engineers [17]. A method should integrate a toolset to be used by all these kinds of users. Such a toolset should facilitate modeling and design of the system in a graphical way, automate the documentation generation, validate the design products and generate code for the specified models.
2.2
Requirements derived from hypermedia
technology
Hypermedia applications differ from the rest of software ones due to the use of a non-linear structure whose infor-mation may be a combination of different types of media such as text, graphics, audio and video, and even it can hold interactive behaviours (e.g., applets embedded into web pages). Moreover, the user can navigate freely through such a structure selecting links.
For these reasons, it is necessary to stress the fol-lowing requirements took from hypermedia engineering [18, 19].
H1. Allow to describe the problem domain in terms of
hypermedia components. These components are
an information holder able to contain a number of in-formation pieces termed contents. A link is a connec-tion among two or more nodes or contents. A link is defined between two set of anchors [4]: the source and the target, referring each anchor to a node or piece of content. Treating these elements as independent de-sign entities allows to separate information from com-position, navigation and presentation design and, in addition, it makes possible to provide multiple views of the same data.
H2. Provide tasks to model the navigation structure. One of the most important tasks in hypermedia mod-eling is the design of the browsing which comprise nodes and links and, in fact, all hypermedia methods support the navigation design. Hypermedia methods should also provide elements to model navigation aids such as visual maps, active indexes, guided tours, marks, footprints and backtracking mechanisms [20]. Moreover, navigation modeling has to deal not only with declarative but also with event-based specifica-tions (dynamic links).
H3. Organize and harmonize multimedia contents. An-other basic feature of hypermedia concerns presenta-tion issues, since nodes include multimedia contents that need to be organized and harmonized in different dimensions such as time and space. In fact, the way and rhythm used to deliver the contents to the user de-termines to a great extent the hyperdocument useful-ness. The method should allow us to describe the con-ceptual properties of the contents and their relation-ships as well as to decide where to place the contents on the screen (layout planning) and over the time, as for example in SMIL, where tags to synchronize con-tents are provided [21].
H4. Model the different types of users. Since most hy-permedia applications are accessed by different kinds of users [11], the method should include mechanisms to deal with the conceptual design of users, that will be used to design adaptative applications or with security purposes (see next requirement).
H5. Provide conceptual tools to formalize security
poli-cies. Nowadays hypermedia systems are used in multi-user environment where different types of users with different privileges access the hyperdocument [2]. Thus, the method should allow designers to model security issues such as which contents should be de-livered to which users, who can modify the structure of the hyperdocument, who can personalize items or which constraints have to applied when creating or modifying a link.
H6. Allow to describe the interactive behaviour of
hy-permedia systems. Hyhy-permedia applications include
complex functions such as dynamic generation of nodes, adaptive presentation and navigation, access to
external applications (e.g., databases or existing infor-mation systems) or interactive contents (e.g., exercises in a hypermedia educational environment). Therefore, these dynamic features have to be taken into account in a conceptual way and not be relegated till the im-plementation phases. With this purpose, event-base specifications are required, so that designer will be able to determine how the system will behave when some events occur [4].
H7. Support a bottom-up design. Some web applica-tions are designed starting from a mock-up of their interface and creating the conceptual design in a fur-ther stage. Moreover, top-down and bottom-up cycles should be supported to deal with the abilities of differ-ent developmdiffer-ent teams as stated in [6].
H8. Make possible the evaluation of the system
util-ity. Hypermedia applications are distinguished by the
complexity of their interfaces and aesthetic and cogni-tive aspects of their contents. Therefore, it is needed a user-centred development where the evaluation plays a basic role to analyse whether the design solutions fulfil the user expectations and needs. Prototyping could help to improve the utility and usability of the application [12].
3
Methods under consideration
Recently, a huge quantity of development methods for hy-permedia systems, and specially for web systems, are ap-pearing. This reveals a web systems success as well as the necessity of controlling the modeling process. We have analysed a set of the most outstanding methods. For each of them a brief introduction and their most excellent fea-tures are presented. Moreover, those requirements that are completely accounted for will appear in brackets (Si or Hi).
Hypermedia Design Model - HDM
HDM [22] is based on techniques and models from database and hypertext field. Five dimensions are identified [13]: content, structure, presentation, dynamics and interaction (H6). To describe the system, they adapt the E/R model (S2) to introduce hypermedia concepts such as nodes, con-tents, anchors and links (H1, H2). Contents (slot) can be organized (collections), in addition to being synchro-nized and aligned by a set of dynamic relationships (H3). A toolset has been developed for web prototyping devel-opment and execution (S7). Moreover bottom-up design (H7) and XML are supported. HDM-Edit is the compo-nent that supports the HDM model, design patterns, reuse of design components (S6) and documentation generation. A set of evaluation criteria (H8) are applied to applications designed with HDM [13].
RMM [23] is the unique hypermedia method that owns a complete software development cycle, from feasibility analysis to testing and evaluation of the system (S1). More-over, it supports top-down and bottom-up modeling (H7) alike. This method is based on a model (S2), RMDM (Re-lationship Management Data Model) which is inspired in the E/R model and HDM. The method starts by represent-ing the information structure via an E/R diagram. In the next step, attributes of entities are organized (H3) in m-sliceswhich can be aggregated and nested. This allows different views of the information. Links and theirs an-chors are also included (H1, H2). A Case Tool, RMCase has been developed to support the RMM stages (S7). To evaluate the application (H8), they suggest using the tech-nique proposed by Garzotto [13].
Object-Oriented Hypermedia Design Method - OOHDM
OOHDM [24] is a four-activity process that is performed in an iterative and incremental way (S1). The starting point is the Conceptual Design, based on OMT (S2) to model the application domain. The following is the Navigation De-sign (H2) that allows us to build different navigation views (Navigation Context) for the same conceptual model. In such navigation views anchors (H1) and behaviours (H6) can be defined. The third activity is the Abstract Interface Design that specifies the interface aspect. In this activity, contents (Abstract Data View) can be organized (H3). In the same way as in the preceding activity, different in-terfaces for the same navigation model can be built. The last two activities can be used for personalization purposes by means of the building of a specific view for each user profile. The system users can be modeled into the concep-tual schema like another type of object (H4). An impor-tant point is design reuse by means of design patterns (S6). In the last activity, Implementation, a series of steps guide the mapping of the design into an implementation environ-ment. In relation to CASE tools, the OOHDM-Web envi-ronment is under construction (S7).
Autoweb
Autoweb [25] proposes a top-down method based on three phases: the conceptual design, the database generation and the implementation of the Web application (S1). All these phases are supported by a tool suite so-called the AutoWeb System (S7) which automatically generates a Web appli-cation. The conceptual design is formalized by the HDM-lite model (S2), an evolution of HDM [22] for Web. It al-lows us to describe the structure, the navigation and the presentation of the system (H1, H2). In contrast to the rest of methods, Autoweb allows us to define the contents (component-type) and structure of a node (entity-type) in the Structure Schema and to place them within the vi-sualization area in the Presentation Schema (H3). To have different views of the system, various Presentation Schemas have to be generated.
Web Modeling Language - WebML
WebML [26] specifies data-intensive Web applications by four views: Structural, Hypertext (H2), Presentation and Personalization (S1). An iterative design process guides the development process from the requirements collection to the design customization. In relation to hypermedia ele-ments, contents (units) of a node can be specified by the Composition Model. These contents can be organized in self-contained regions of the screen (page). In addition, it can be specified if regions will be displayed together or just one of them. Some navigation patterns are taken into account. In contrast to the others methods, WebML owns a collection of rules to build correct logical and physical hy-pertext (S4). WebML includes the notion of group (set of users) and user (individual) which are modeled as a special entity in the Structural Model allowing the personalization of the hyperdocument (H4). The interactive behaviour is supported byoperation unitswhich are linked to other units and can be predefined or built by the designer (H6). The whole design process is supported by a CASE envi-ronment called ToriiSoft (S7). Besides, WebML supports XML syntax to get a platform-independent design.
The object-oriented-hypermedia (OO-H) method
The OO-H method [27] is UML-compliant and allows us to model both statics and dynamics (H6) of the system (S2). These two views are extended with the Navigational Ac-cess Diagram (NAD) (H2) and the Abstract Presentation Di-agram (APD) in order to capture web features. The designer has to construct as many NADs as user types when infor-mation personalization is required. The two strong points of this method are its CASE tool (S7) and a Design Patterns catalog (S6). Prototypes can be generated by means of its CASE tool.
UML-based Web Engineering methodology - UWE
The UWE [28] is an object-oriented, iterative and incmental approach whose core modeling activities are the re-quirements analysis (S3), conceptual, navigation and pre-sentation design (S1). Task models and statecharts of Web scenarios are included to model the dynamics aspects of the application (H6). The UML (S2) and a set of stereo-types defined for the modeling of navigation (H2) and pre-sentation aspects (H3) of Web applications are used for the artifacts representation. The design models are trans-formed into XML representations. In relation to hyper-media elements, nodes are represented by classes, links by stereotyped association and navigation aids by stereo-typed classes (i.e.<<index>>) (H1). Furthermore, semi-automatic generation of web applications from design model is produced by ArgoUML tool (S7).
4
Outstanding requirements: some research
open areas
During the analysis of the aforementioned methods, some requirements of our framework are not addressed by them.
Requirements for design methods of hypermedia systems HDM RMM OOHDM Autoweb WebML OO-H UWE
S1. modeling process P C C C C P C
S2. based on a model C C P C P C C
S3. functional, no-functional and usability requirements P P P P P P P S4. validation rules for each product N N N N P N N S5. integrity relationships among phases N N N N N N N
S6. design reuse C N C N P C N
S7. software support tools C P P C C C P H1. hypermedia components C C C C C C P H2. navigation structure C C C C C C C H3. organization and harmonization of multimedia contents C P P P P N P
H4. user modeling N N C N C N N
H5. formalization of security policies N N N N N N N H6. description of interactive behaviour C N C N C C C
H7. bottom-up design C C N N N N N
H8. evaluation of system utility C C N N N N N
Table 1. Comparison between requirements and methods
Next, we summarize the most important aspects that have been discussed in the previous sections in the table 1. The left part enumerates the software and hypermedia re-quirements and on the right side their rate of performance is shown for each method. Three kinds of notations have been used: C, if the method fulfills the whole of require-ment;P, if the method takes into account the requirement partially; andNwhen the requirement is not taken account by the method.
From this table, some weak points of the analysed me-thods are outlined below:
• Non-functional and usability characteristics. These
aspects could improve the quality and efficiency of the system.
• Validation and integrity rules. These rules could
help determine the correctness and the completeness of design and the integrity of the designed elements. • Design reuse. Navigation patterns are usually used,
but none relates to the interface and the structure of the system nor design components. They would allow us to reuse designers’ experience.
• Content modeling. Most of the methods do not model
contents in a separate way and do not take into ac-count their multimedia nature, therefore different data views, synchronizations and alignments among con-tents cannot be specified.
• User modeling. System users are not taken into
ac-count like another type of elements into modeling pro-cess so the personalization of the system is made by means of ”cut-paste” technique.
• Security modeling. Security policies could be
ap-plied as access control mechanisms to different users. • Bottom-up design. A design is not allowed from
pro-totype of the application interface.
• Evaluation stage. This stage could help improve the
utility and usability of the application and make pos-sible a development more incremental and iterative.
5
Conclusions
It is difficult to find a hypermedia design method that is fit for the development of any hypermedia system due to rapid change in this technology and the combination of different techniques and fields (database, object oriented, hyperdia, web, ...) which have to be applied. However, the me-thods presented here are a proof of the increasing concern about covering modeling process of hypermedia systems in a more complete way, making an effort to establish the steps and techniques which have to be carried out in order to get a scalable, maintainable and usable application.
With this survey, we want to reveal both the bene-fits and limitations of design methods in order to provide some descriptions that can be relevant for the designers, re-searchers and even users who are considering developing hypermedia applications using design methods.
Furthermore, we have been developing a hypermedia method called Ariadne [18] which applies an iterative pro-cess based on the evaluation of design solutions with po-tential users to determine their utility and usability. The method provides mechanisms to define time- and space-based constraints among contents, to model the users struc-ture and security policies, and to guarantee the correctness and completeness of the design by means of validation and integrity rules. Finally, for a greater acceptance and feed-back from designers and developers with the aim of refin-ing the modelrefin-ing process, we are workrefin-ing on a support soft-ware tool for automatic generation of applications.
Acknowledgements
This work is supported by ”Direcci´on General de Investi-gaci´on del Ministerio de Ciencia y Tecnolog´ıa” (TIC2000-0402).”
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