Page 24 - DCAP408_WEB_PROGRAMMING

Web Programming

Notes a knowledge engineer undertakes to build a knowledge base. As the expense and effort to
acquire knowledge form experts in a specific domain is large, the same is the effort for
discovering and defining explicit links between nodes in hypermedia. This is a very painful,
time consuming process with doubtful completeness, accuracy and consistency of the final
hypermedia “running” system.
1.7.2 Hypermedia — Basic Hypermedia Model

Virtually, all hypermedia systems are founded in basic hypermedia model. Likewise, a large
part of current hypermedia research assumes the underlying existence of this basic model
(Rivlin et al., 1994). Thus, it would be useful if we define the basic hypermedia model. We can
divide basic hypermedia model in two distinguished but interdependent submodels. The
first one is the data submodel. According to this submodel Nodes are interconnected with
directed Links forming the structure of a directed graph (Parunak, 1991). Addition, deletion,
update of nodes, links are valid operations. Process submodel is the second element of basic
hypermedia model. This submodel concerns the information access mechanisms of information
network. Figure 1.5 shows the data and process submodels of basic hypermedia model. This
model is foremost characterised by its generality, flexibility and incompleteness.

Example: E.F. Codd in his ACM Turing award lecture (Codd, 1981) defines a data
model as a combination of a data structure, operations and integrity rules. In this sense,
hypermedia data model is incomplete, since it does not define any constraints (e.g. is valid the
insertion of a link without associate it to a destination node, see Gronbajek et all, 1994) for
determining the consistency of information network. On the other side, process submodel is
very primitive, without using detailed specifications for defining navigation access (e.g, how
a user activates a link).

Figure 1.5

Hypermedia - Process Submodel

Hypermedia concept is not only organised multimedia data interconnected with links.
Navigation, whereby the user moves through the hypermedia network by activating and
following links from one node to another, is another defining feature of hypermedia (Nielsen,
1990B). Navigation is the primary means to access information in hypermedia network,
composing the most essential aspect of basic hypermedia process submodel. The basic
characteristic of navigational access is that users navigate by self motivation without having
any external navigational aid. Some primary navigational functionality like the ability to

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