Location: Hypertext Review / Implementation Issues / Dynamic Hypertext Site Map

5 Dynamic Hypertext

Halasz had identified dynamic or virtual structures, computation, and extensibility/tailorability as some of the issues to be addressed by next generation hypertext systems [Halasz, 1988b]. Most current generation hypertext systems implement a static and explicit model of hypertext – nodes, links, and link markers must be declared explicitly and be fully enumerated during creation time as opposed to being declared dynamically and generated upon demand [Bieber, 1993b].

Information systems such as Decision Support Systems (DSS) and Expert Systems require a dynamic implementation of hypertext, one that relies primarily on virtual structures and computation in order to generate a hypertext network in real time [Bieber, 1991]. Bieber developed a DSS shell that supported multiple DSS applications through a hypertext user interface. The user interface containing the hypertext engine provided DSS applications with hypertext functionality such as navigation, virtual structures, computation, and tailored presentation. Since many of the components making up the DSS are generated in real time as a result of user interaction, it was difficult to pre-define all nodes, links, and link markers at creation time.

Bridge laws were developed to determine the appropriate links and link markers automatically and embed them in the interactive application [Bieber, 1991]. These link markers provided access to reports, operations (DSS commands), and other components of DSS applications. Bridge laws are translation routines provided by the application to the hypertext interface. They map the elements defined in the application’s original non-hypertext data or knowledge base to entities in the hypertext engine. They do not alter the application’s data or knowledge bases.

A user interface control subsystem was developed to maintain global information about user profile, user-defined links, comments, application keywords and application supplied bridge laws. It was responsible for interpreting contents coming from the application (using bridge laws) to provide virtual link markers by highlighting the objects. It tailored different views of the application based on different sets of filters.

Stotts and Furuta describe virtual structures as dynamic adaptation of hypertext structure [Stotts & Furuta, 1991]. It involves collecting information from user interaction with a hypertext system, making inferences and decisions based on this information and creating appropriate physical changes in the document at appropriate times. Adaptation can occur at two levels – behavior of the document (timing of sequences, providing automated help, presenting collections in parallel or in sequence etc) and structure of the document (the way the nodes are linked).

According to Stotts and Furuta, a hypertext document can be considered to have two layers – a fixed underlying information structure that is created by the hypertext author and a flexible structure that is superimposed on the former and is tuned to each user’s requirements. The flexible layer can be generated dynamically. The manner in which information is organized and presented can be altered without actually changing the information relation contained in the original links. This is similar to Bieber’s concept of bridge laws which simply map an application’s non-hypertext data to a hypertext interface without changing the underlying data. Thus, a document can change to adapt the needs and preferences of individual users, the author’s original structure being retained. Such a dynamic adaptation technique has been implemented in the Petri-net based Trellis system developed by Stotts and Furuta.