WebStyles and Ontologies: Information Modeling and Software Engineering for the WWW
Telecooperation Group, Johannes Kepler University Linz, A-4040 Linz, Austria
e-mail: ( max | ralf | theo ) @ tk.uni-linz.ac.at
Abstract: The hypertext paradigm is simple yet powerful and has the potential to eventually catalyse an old dream of the software engineering community: to overcome the boundaries between code and data, between dynamic and static information, to have a single common modeling concept. At present, however, the WWW does not offer sufficiently powerful concepts for making this dream come true. We present two closely related approaches which we view as missing links between hypertext today and the universal concept sought for. Considering the Web as a collection of loosely connected sets of rather self-contained webs, the two approaches contribute to the power of webs rather than to the power of the atomic building blocks (nodes, links, anchors). To this end, two-tiered GUTS approach for web typing will be introduced.
We use the term "(a) web" to denote a meaningful, coherent set of nodes and links; webs can be represented as graphs. They may contain other webs hierarchically as "meta-nodes" and comprise links to the "outside world". In the context o f the Workshop, we want to advocate typing concepts for webs which cover both structural and logical aspects. We believe that the re-use of Web Information can be sufficiently improved with such concepts, where re-use may refer to the issue of finding inf ormation by query or navigation or to the issue of incorporation of Web information into augmented hypermedia; by the latter we mean systems which are built on top of hypermedia-based structured information (such as, for instance, hypermedia-based learnin g systems, software engineering environments, decision support systems, and many more).
Typing for the atomic constituents of hypermedia (nodes, links, anchors) has been around for quite some time. Even in HTML, there is a primitive form of such typing in the sense that types of nodes are implicitly defined through the med ia and format (HTML, JPEG, GIF, Quicktime™ etc.), and types of links through the target type (local/remote document, "mailto:", etc.). More advanced hypermedia systems support user-defined types of nodes and links. Usually, this feature is used for modeling the application domain.
In addition to user-defined node and link types – which are present in many hypermedia systems, just not yet "really" in the standard Web – the following typing concepts are useful and can be considered as requirements for a re-use orie nted hypermedia system:
The main benefits which might be drawn from the above-mentioned desired features of web types are listed below; since they are not automatically provided if the above requirements are fulfilled, the following list can regarded as furthe r requirements.
Looking for type concepts for webs, one finds that the Web community has concentrated on augmenting the power of nodes and links rather than the power of webs. Many observations back this claim, some of which are listed below:
While the Web (considered as a particular kind of hypermedia system) is by far the most wide-spread such system, even a de-facto-standard, it is by far not the most advanced system. As a consequence, several of the most interesti ng contributions to the field have been made for other hypermedia systems than the Web. A short excerpt of such contributions – independent of whether they are Web-compliant (cf. [MM97]) or not – i s given below. These contributions relate to fields as diverse as hypermedia authoring/design support, generics and dynamics in hypermedia, database schema approaches, and structural queries.
In summary, there have been considerable achievements in the attempt to provide design support for hypermedia; the most promising ones are based on type concepts of what we call webs in this paper. Most such developments relate to hyper media systems other than WWW. Even worse, the considerable achievements made are contrasted by a rather moderate state of commercially available authoring tools for WWW (with a minor exception being the "site management" support given by systems li ke NetObject Fusion™. The most general and most adequate representation of webs has been found to be a "graph" of nodes and links.
The GUTS approach (generic unified typing system) described here leverages off multi-year research at the hypermedia learning group of the first author. It is based on two principle approaches which realize the abov e-mentioned requirements:
Using learning systems again as an example, their lifecycle may be supported, e.g., via ontologies for instructional analysis, for instructional design, for domain analysis, and so on. As to alternative approaches, there are for instanc e ontologies that express rather traditional instructional concepts and rather advanced ones.
At this point of course, the reader will not easily grasp how the introduction of an ontology might lead to support for a certain lifecycle phase or instructional concept, nor how WebStyle typing actually works. To this end, the authors decided to introduce further details of their concept by way of example, rather than by describing rather abstract "architectures" or "models".
The key to understanding GUTS is its way of representing knowledge. In the teaching context, knowledge means content of courses together with information about the entities involved in the teach ing-learning situation, for example content, courses and learners. The latter kind of knowledge is called meta-information.
Principal mechanisms for knowledge representation and inference as used in GUTS were thoroughly studied the fields of semantic networks and graph grammars (see for example [Sowa91] and [Rozen97], respectively). As advocated earlier, the basic underlying data structure is the Graph. Our extended notion of a Graphs — called WebStyles — compris e a grammar for expressing static (syntactic, structural) and dynamic (semantic, navigational) aspects. The following table may motivate these categories.
WebStyles are based on a work about "generic and dynamic aspects of hypermedia" [Richartz96]. They consist of three parts: generic webs, procedures, and rules. These generic webs are in e ssence graph grammars (cf. [Rozen97]) and consist of nodes and links. A first overview of the symbols used with WebStyles is shown in figure 1.
Figure 1:WebStyle symbols
Transformations: The most notable transformations in WebStyles concern the instantiation of "sequence nodes" (transforming into "chains" of nodes-and-links) and "fan links" (transforming into "bunches" of links originating from the same "source" node); of course, there are more transformations like instantiating a node to a real node. The following figures help to grasp how nodes and links can be transformed.
Transformations of a
a) sequence node
b) fan link
c) a simple web
d) web c) after n transformations
Figure 2: WebStyle transformations
In the left figure, two transformations have been applied to a sequence node. In figure b) a possible transformation of a fan link is shown. In figure c) a web consisting of two types of nodes and two types of links is presented. By app lying the transformations introduced in figures a) and b) to this web and mapping some generic nodes to instantiated nodes, the web shown in d) can be constructed.
To find out which nodes and links are affected by a transformation, the so-called track-algorithm is used. This algorithm defines which nodes and links belong to a track and marks them. In a second step all the marked objects are copied and connected to the original web.
Attributes, procedures, rules: Each WebStyle object has general attributes, like a name, and more specific attributes, like lower bound and upper bound. The bounds for example are used by the transformations and define how many nodes or links can be instantiated. Besides default procedures (like isTraversible which tells if an object may be traversed) user defined procedures and rules may be attached to nodes and links. These procedures and rules may influence the construction of a web even more (e.g. by constraining it) and may influence the navigation in such a web, too.
Further elements of generic nets: Two more types of objects are supported: alternatives and meta-nodes. Alternatives are used by the author of a web to offer a choice from different possibilities during construction. Meta-nodes help to model complex sub-webs and can be used e.g. to build tree-like structures. For more detailed descriptions cf. [Richartz96].
Knowledge representation involves classifying the ‘things’ to be represented, e.g. «Mars» is a «planet», «next» is an «order relation», «is a» is a «genus-species relation». Ideally the classes (concepts, types, the terms inside french quotes «…») are explicitly written down and put in relation with each other. This is called a theory, conceptualization, or, as is fashionable, an ontology. (Ontology as a part of philosophy is the study of being, or, the basic categories of existence. With the indefinite article, the term "an ontology" is often used as a synonym for a taxonomy that classifies the categories or concept types in a knowledge base. [Sowa91], p. 3)
There are ontologies for ‘everything’. For instance, in instructional design, if one wants to use Gagné’s events of instruction [GBW92], he could define an ontology containing «gain attention», «indicate goal», «recall prior knowledge», «present material», «provide learning guidance», etc. Or, to be able to talk in terms of Reigeluth’s elaboration theory [Reigel87], one needs «fact», «concept», «principle», and «procedure».
In these examples we did not consider any relations and formalization of semantics. If one tries to work out these aspects, it soon will be evident that something crucial is missing: How could such an ontology be defined? In which language? Answer: There is a particular ontology built in. GUTS’ representation ontology is rich enough to capture the computational content of new, user defined ontologies. It comprises objects («object», «theory», «abstraction», «type», «rule») and relations («relation», «genus-species», «instance», «composition», «equivalence», «order», «derivation», «functional», «context»).
WebStyles with its representation ontology (the "basic, built-in" ontology that is used to define other ontologies) is a specialized representation language, as is KIF [GF92] with the so-called "frame-ontology". Sowa mentions that "the structure of a knowledge representation language depends critically on its ultimate goal" ([Sowa91] p. 157), and since WebStyles and KIF differ in purpose, their flavor, appearance and computational properties are different. Although WebStyles could be easily mapped to KIF (and back).
In the workshop, we would like to demonstrate the WebStyle editor and discuss the value of ontologies and WebStyles for information and software modeling, for greatly increased WWW re-usability (for knowledge in expert domains, in parti cular), for sophisticated navigation support, for a hypertext "culture" and "common look & feel", for the seamless modeling and implementation of (static) information and (dynamic) software.
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