Web design tools and approaches owe much to the debate on hypermedia mod- eling and design, semistructured data modeling, and hypermedia development
tools proposed prior to the advent of the Web. In the following sections, we re- view some of the most important contri- butions.
14.1 Modeling Notation
Historically, most of the modeling nota- tion adopted by current Web develop- ment methodologies stems from the evo- lution and hybridization of previous conceptual models for database and hy- permedia design.
The common ancestors of many subse- quent proposals are the entity relation- ship model [Chen 1976], in the database field, and the Dexter model [Halasz and Schwarz 1994] in the hypermedia area.
The Dexter model originates from the effort to provide a uniform terminol- ogy for representing the different hyper- text structuring primitives offered by hypertext construction systems; the core of the model is the ordered repre- sentation of a hypertextual application at three levels: storage, within-compo-
nent, and runtime levels. The storage
level describes the network of nodes and links of the hypertext, without details on the inner structure and node con- tent, which is the focus of the within- component layer. The runtime level deals with the dynamics and presenta- tion of the hypertext.
The modeling concepts available at the storage level are very basic: compo-
nents describe pieces of information
that constitute the hypertext, and can be either atomic or composite. Links are a special kind of component used to represent navigable paths. Although not conceived for hypertext design, the Dex- ter model advocates many concepts, such as the distinctions among the structure, navigation, and presentation of a hypertext, whose influence has been long-lasting.
Several subsequent contributions arose from criticism of the Dexter model, and added more complex forms of hypertext organization and more powerful navigation primitives [Gar- zotto et al. 1993]; time and multimedia
synchronization features [Hardman et al. 1994]; and formal semantics of navi- gation and a structured design process [Isakowitz et al. 1995]. Among these evolutions, HDM [Garzotto et al. 1993] and RMM [Isakowitz et al. 1995] have been particularly influential in the de- sign of hypermedia applications.
HDM and its variants [Schwabe et al. 1992; Garzotto et al. 1993; Garzotto et al. 1991; Garzotto et al. 1993] shifted the focus from hypertext data models as a means to capture the structuring primitives of hypertext systems, to hy- pertext models as a means for capturing the semantics of a hypermedia applica- tion domain. HDM integrates features of the entity relationship model and the Dexter model, to obtain a notation for expressing the main abstractions of a hypermedia application, their internal structure and navigation, and applica- tion-wide navigation requirements. Web structure is expressed by means of enti- ties, substructured into a tree of compo- nents. Navigation can be internal to entities (along part-of links), cross-en- tity (along generalized links), or non- contextual (using access indexes, called collections [Garzotto et al. 1994]).
RMM (Relationship Management
Methodology) [Isakowitz et al. 1995] evolves HDM by embedding its hyper- media design concepts into a structured methodology, splitting the development process into seven distinct steps and giving guidelines for the tasks. RMM’s data model (called RMDM) structures domain entities into slices, and orga- nizes navigation within and across enti- ties using associative relationships and structural links.
Most of the recent proposals for Web modeling are built on top of the entity relationship model and hypermedia de- sign models (notably HDM and RMM), adapted to the specificity of the Web context: Araneus and WebArchitect draw from RMM and the entity-rela- tionship model, Autoweb and W3I3 have proposed a Web-specific evolution of concepts first proposed by HDM.
to Web modeling comes from research on the representation and querying of semistructured data, i.e., data with par- tial or missing schema. The proposed data models, thoroughly reviewed in Florescu et al. [1998], express Web con- tent by means of relations, labeled graphs, hypertrees, and logic. Araneus and Strudel are examples of Web con- tent management systems based on semistructured data models and query languages.
14.2 Processes
Web development processes evolved in parallel with Web design notation, and have the same hybrid origin from the information system and hypermedia fields.
In hypermedia, the evolution from the creative definition of content to the con- tent-independent organization of the structure of a hypermedia application is attributed to the work on HDM [Gar- zotto et al. 1993], which stresses the difference between authoring in the large, i.e., designing general structure and navigation, and authoring in the small, i.e., deciding the layout and syn- chronization aspects of specific compo- nent types.
HDM, however, did not prescribe a formal development lifecycle, which was first advocated by RMM [Isakowitz et al. 1995], where the following seven ac- tivities, i.e., entity relationship design,
slice design, navigation design, conver- sion protocol design, interface design, behavior design, and implementation and testing are proposed. The first three
activities provide a conceptualization of the hypermedia application domain in terms of entities, substructured into slices, and navigable relationships. Con- version protocol design is a technical activity that defines the transforma- tions to be used for mapping the concep- tual schema into implementation struc- tures. In addition to defining the development lifecycle, RMM also gives guidelines for slice and navigation de-
sign, the two tasks most particular to hypermedia design.
OOHDM [Schwabe and Rossi 1995] takes inspiration from object-oriented modeling and simplifies the RMM life- cycle to only four steps: domain analy-
sis, navigation design, abstract interface design, and implementation. In domain
design, classical object-oriented tech- niques are used, instead of the entity relationship model. Navigation design adds specific classes (e.g., node, link, index) to represent different forms of navigation. The same is done for pre- sentation, which is described by means of classes (e.g., button, text field) added during interface design. Implementa- tion then fleshes out the classes identi- fied during design with code in the im- plementation language of choice.
In the Web context, most methodolog- ical proposals concentrate on visual de- sign and usability criteria [Sano 1996], much as the hypermedia field authoring guidelines were the first concern of de- velopment. The requirement of applica- tion scale-up drives the most recent con- tributions, like Araneus, Autoweb, Web Architect, and W3I3, which organize the development process into activities drawn both from the above mentioned hypermedia methodologies and from traditional object-oriented and database design methods.
14.3 Other Design Tools
Besides the projects described in Sec- tion 13, other research efforts have con- tributed to the development of proto- types of hypermedia and Web design tools.
RMC [Diaz et al. 1995] gives CASE support to the design and implementa- tion phases of the RMM methodology. It consists of a diagram editor, which as- sists the input of RMDM schemas, and a code generator, which outputs HTML pages from data stored in an internal repository. Pages are produced offline in a precompiled fashion. RMC is not based on a database architecture, and thus does not provide facilities for scal-
ing-up and updating the information base.
HSDL [Kesseler 1995] has an archi- tecture similar to RMC, but starts from a HDM-like design model, augmented to support a fine-grain personalization of navigation and presentation semantics. HSDL objects are annotated by means of programs in a scripting language, called expanders, which drive the pro- duction of HTML pages. Expanders can be attached both to schema elements and to instances, to provide uniformity and exceptional handling at the same time. Expander programming, although powerful, is a low-level task that resem- bles the programming of page templates in Web-DBPL integrators. The differ- ence is that HSDL, like RMC, does not rely on database technology to store the application content, but has an internal repository.