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Structuring data
The unstructured and tangled nature of links in many WWW systems is a source of frustration to users and
administrators alike. As van Dam said, people ``got linkitis" [van Dam, 1988]. There is certainly no significant data
model for WWW, and in many ways first generation systems may rightly be likened to novice programs that used too
many GO TO statements. A case for developing hypermedia systems without links is made in [Maurer et al., 1994].
Second generation systems such as Hyper-G [Andrews et al., 1995a] allow system administrators to implement systems
which reflect the data's inherent structure. Hyper-G [Maurer, 1995b] organises data objects into clusters [Kappe et al.,
1993], that are grouped into collections, which in turn may be grouped together in a pseudo-hierarchical manner - as
described in the next section. Hyper-G thus imposes structure on top of a flat file database. Most significantly, it
provides users with an alternative to link browsing. Users navigate up and down hierarchies and thus can follow the
inherent structures.
In the article ``Hyper-G Organises the Web" the author states, ``It [Hyper-G] can organise the mass of unstructured data
and unmanageable hyperlinks" [Flohr, 1995].
The HM-Data Model
The HM-Data Model, developed at Graz Institute of Technology, addresses each of the following problems that are
usually associated with first generation systems:
1. Loss of orientation encountered by users.
2. Tedious editing of links where, as we have mentioned in Section 2, deleting documents can result in dangling
links.
3. Loss of visible semantic structure when sets of nodes are combined with other sets [Andrews et al., 1995b].
The HM-Data model supports the structured browsing of hypermedia databases [Maurer et al., 1993a].
The databases consist of a set of structured collections called (or just collections for short) [Maurer
et al., 1995]. An encapsulates both a chunk of multimedia data (content), and a particular internal
structure. The internal structure consists of a ``pseudo-hierarchical" group of other collections. We use the term
``pseudo-hierarchical" because in fact the structure is a directed acyclic graph of parent and children nodes, where a
child may belong to more than one parent. This means that any document can belong to multiple collections - without
duplication of content.
The HM Data Model provides a number of predefined subclasses of (see Figure 1) as follows:
1. An Envelope where all members are inter-linked
2. A Folder which is essentially an ordered list
3. A Menu which is a simple hierarchical structure
4. Freelinks where members may be arbitrarily connected