Peter Fröhlich, Nicola Henze, and Wolfgang Nejdl
Institut für Rechnergestützte Wissensverarbeitung
University of Hannover
Lange Laube 3
30159 Hannover, Germany
phone: +49 511 762 9714
fax: +49 511 762 9712
email: {froehlich, henze, nejdl}@kbs.uni-hannover.de
Semantic modeling is an important activity in database and software engineering, several formalisms exist in these areas. The acceptance of semantic models for hypermedia documents has been hindered so far by their lack of navigational constructs and extensibility. In this paper we describe a framework for hypermedia modeling, which contains provisions for domain modeling, navigational modeling, visualization modeling, and user modeling. It follows a strict formal definition based on a meta database, and guarantees the consistency and extensibility of large hypermedia documents maintained by several authors.
Hypermedia-Documents consist of artifacts of different types (texts, pictures, sounds, etc.) interrelated by a link structure for navigation. Especially in cases where several authors develop such a document, systematic metadata management is needed to guarantee consistency, maintainability and extendibility. The following types of metadata have to be managed:
Specialized frameworks for hypermedia design have been developed. The Hypertext Design Method (HDM) [3] provides an object model for hierarchically structured entities. In contrast to Entity Relationship Modeling relations between entities are intended as navigation links. The RMM methodology [4] defines a diagrammatic language which extends Entity-Relationship-modeling by navigational concepts like links, groups and indexes. OOHDM [12] on the other hand uses a variant of the OMT language for domain modeling which is complemented by nodes providing views on domain objects and links allowing to navigate among nodes. These frameworks support only a predefined restricted set of navigational constructs whose semantics are not formally defined. The visualization and adaptation of hypermedia content is not modeled in any of the existing frameworks. Thus, central semantic aspects of the hypermedia application are left to the implementation which has a negative impact on extendibility, reuse and documentation.
Database support is also very limited in current hypermedia modeling frameworks. The OOHDM method does not explicitly support the construction of a database, while RMM supports database use on a low level (indices).
The current paper describes a meta-modeling approach to hypermedia design which is rigorously database-oriented and supports maintenance and reuse. Our methodology defines a declarative, extendible modeling language, which combines the four types of metadata mentioned above in one metadata model. The modeling language itself as well as the metadata model are implemented using the deductive object-oriented database manager ConceptBase [7]. To illustrate our language we show examples from the meta model of our software engineering lecture.
We have used Meta Modeling to implement a modeling language capable of representing all aspects of a hypermedia application. The meta model describes modeling concepts and their semantics. The application of meta modeling has several advantages over the design of a fixed modeling language. First, by using the Telos Language [10] for expressing our meta model, its declarative semantics is well-defined because the semantics of the Telos language is defined by logical axioms. In contrast to that, the abovementioned modeling languages are only specified by natural language descriptions. Additional effort would be required to capture their semantics formally. Second, our Telos meta model is interpreted by the deductive database manager ConceptBase [7] which implements a dialect of the Telos language. Thus, meta modeling saves the effort for implementing the modeling language if an implementation exists for the meta language. Third, using a meta model allows us to extend and modify the modeling language easily. This is an important characteristic for rapidly evolving domains like hypermedia. Our hypermedia modeling architecture consists of 4 layers, as shown in figure 1:
The layers of our model roughly correspond to the layers of the ISO IRDS standard ([5], [6]). However, IRDS is based on relational database technology, while we use an object-oriented deductive language. Furthermore, our model is designed for a specific application (hypermedia modeling) and is thus less generic. Our mandatory data model concepts whose presence can be assumed by client applications underline this application orientation. In the next section we will introduce the concepts of our modeling language and demonstrate their usage with an example from our software engineering lecture.
Figure 2 shows the basic domain modeling concepts of our framework. The domain model is expressed by Domain Classes and Relationships. Domain classes are characterized by their Attributes. The domain classes for an application can be organized in a class hierarchy with single inheritance relations among classes. We support binary relationships between domain classes, which are classified by their cardinality (1:1, 1:n, m:n). In addition we allow to specify a key for the n-side of a 1:n relationship. As shown in figure 3 our notation for these modeling concepts is an extension of Entity Relationship Diagrams. Figure 4 provides a small example, taken from our software engineering book: A Software Process consists of several Phases, thus there is a 1:n relationship between Software Process and Phase. This relationship is indexed by the key attribute PhaseNr (Phase Number). As we will see in section 3.2, this key attribute specifies the order in which the Phases are presented. At the bottom of figure 4 we see the Waterfall Model as an instance of Software Process which is related to its phases. Our domain modeling technique is similar to RMM but extends it by inheritance and m:n-Relationships. The possibility to specify a key for the n-side of a 1:n-relationship is also an extension.
Navigation in a hypermedia document must be supported in two ways: Data Model Navigation is the systematic navigation based on the domain model, which is supported by general hypermedia modeling frameworks like RMM or OOHDM. This type of navigation allows selective reading and quick access to information by following hyper links. Sequential Navigation on the other hand presents pages belonging to different domain classes types in a sequence. This type of navigation is needed to read the document or parts of it through in order to gain a first overview on its contents.
The relationships defined in the domain model are the basis for systematic semantics-based navigation in the hypermedia document. As in RMM, this type of navigation is defined on the schema level, i.e. for every navigational construct, it is defined, which relationship it supports. Database queries are used to obtain the actual hyperlinks. This enhances the extendibility of the document since the link structure automatically adapts to the database content.
The navigational concepts which support data model navigation are shown on the right side of figure 5. Links support bidirectional navigation based on 1:1-relationships. Links are displayed by inserting a hyperlink to the corresponding domain object in each page. Following the RMM method, we provide three concepts for navigating a 1:n-relationship: An Index resembles a table of contents: It consists of a list of links to related objects. If an index key is specified for the relation, it determines the order of the links. In the example from figure 4 links to the phases Requirements, Design, Implementation and Integration are inserted in the Waterfall Model page. Since Requirements has the smallest key the corresponding link appears first. A Guided Tour is an alternative representation of an 1:n-relationship. Here, a link to the first phase is inserted in the Waterfall Model page. Each phase page contains a link to its successor (the phase with the next larger key). The last phase page links back to the Waterfall Model page. An Index Guided Tour combines the previous concepts: Each page has links to the next in a sequence and also to the main page (the Waterfall Model page in our example). We extend RMM's navigational constructs by a Cross Reference Index concept, which allows to navigate a m:n-relationship.
We stress that data model-based navigation allows to access information more selectively compared to a conventional book. However, the possibility to read parts of the documents or the whole document sequentially provides important guidance especially for the beginner. Therefore we define the concept of a Reading Sequence. A reading sequence consists of a sequence of pages connected by hyper links, whose graphical representation differs from the data model links shown above (reading sequence links are shown as 3D buttons at the top of the page while data model links are shown as usual blue hyperlinks at the end of the page). Our framework allows the automatic construction such reading sequences from the domain model structure.
Up to now we have modeled the concepts of the application domain and their relationships (domain model) as well as the possibilities to navigate through these topics (navigation model). The next step is specify how information is presented (see figure 6). In our system, topics are presented by WWW Pages. A WWW Page itself consist of a sequence of fragments containing MIME Objects. MIME [1, 9] (Multipurpose Internet Mail Extension) is a standard for sending different types of content (Text, Pictures, Sound, Video) over the Internet. MIME is supported by most EMail and WWW clients. The MIME type hierarchy is part of our modeling language. Each MIME Object belongs to a MIME Type, which tells the hypermedia server how to insert it into the WWW Page and tells the hypermedia client how to display it (see section 4). The connection between the Visualization Model and the Domain Model is that every WWW Page, i.e. collection of MIME Objects implements a domain object. To see the practical impact of these definitions, consider another software engineering example shown in figure 7. The domain object Use Case is an instance of the domain class Method. It is implemented by the WWW Page Use Case Page. This page consists of 3 fragments: The first contains a document of type TEXT/HTML providing an overview of Use Cases (use_case_overview.txt), the second contains a picture of type IMAGE/GIF, which gives an example of a use case diagram and the third is another document of type TEXT/HTML describing the impact of use cases for the design phase.
We use a simple user model to adapt both page content and navigation structure of hypermedia documents. This model consists of two parts:
The adaptation of content based on this information is achieved by constraints on the visibility of fragments. In ConceptBase constraints are expressed in a variant of typed first-order logic. This allows to express complex conditions declaratively. Consider a page on interface design, which contains as an example an abstract data type for sets in C++. The fragment containing the example shall only be displayed if the user has good knowledge in C++ (rated on a scale from 1=no knowledge to 5=perfect knowledge) and has read the page on abstract data types. This is expressed as
forall u/User (this visibleTo u) => ((u hasRead ADT_Page) and
(exists rating/Integer (u cppKnowledge rating) and rating > 3))
This constraint states that the fragment (this) is only visible to users u, who have read the page called ADT_Page and whose value of the cppKnowledge attribute is greater than 3. The adaptation of navigation works in a similar fashion. The navigational objects (links, indexes, ...) are augmented with constraints on their visible instances. Our current adaptation model can be seen as a database-oriented reconstruction of Kay and Kummerfield's customization technique [8], which controls the visibility of texts by preprocessor statements. We consider our modeling technique more declarative, because visibility conditions and the internal structure of the pages are explicitly represented as part of the data model.
In the previous sections we have outlined how all data relevant to the hypermedia document is stored in ConceptBase following a declarative meta model. Figure 8 shows the basic system architecture, which is based on standard components complemented by a server-side applet and the ConceptBase database manager. After logging into the system by activating the login applet a user-specific starting page is displayed. The user navigates the document by activating links. These links however do not represent static HTML pages but start CGI scripts instead. Whenever a hyper link is activated the name of the corresponding domain object plus the name of the user are passed to a server-side applet. The applet queries the data base for the fragments of the page representing the domain object and for the domain object's navigational possibilities. From this information it constructs a user specific page. It does so by combining the visible fragments in a HTML page. Then it updates the user model by marking this page as read and uploads the page on the server. The page is then displayed to the user in his WWW client.
In this paper we have proposed a four level architecture for semantic modeling of hypermedia documents, based on a strict, database oriented formalism. In contrast to previous approaches our modeling framework takes into account all aspects of a hypermedia application, i.e. domain model, navigational model, visualization model, and user model. Our modeling language is implemented in the meta modeling formalism Telos which provides a clean semantics and flexibility to adapt the language to the changing requirements of an innovative application domain.
This document was generated using the LaTeX2HTML translator Version 96.1-h (September 30, 1996) Copyright © 1993, 1994, 1995, 1996, Nikos Drakos, Computer Based Learning Unit, University of Leeds.
The command line arguments were:
latex2html -split 0 -no_navigation metadata.tex.
The translation was initiated by Peter Froehlich on Sat Jul 19 11:56:40 MET DST 1997
