The Tower Model

The authors of the Trellis and the Dexter reference models left several issues open in order to cover their inability to accurately predict the future developments of hypertext. Every attempt to state precisely what the features of future hypertext systems would be involves a risk of excluding something that could later turn out to be important. The Trellis model for instance describes the "abstract component level" in a way that makes it sound like there would be no need for containers containing containers (or in more common terminology: composites containing composites). The Dexter model for instance allows undirected (and bidirectional) links, but only between two nodes (called components). Links between more than two nodes are allowed but must be directed (they must have at least one "destination" or "TO" endpoint). Another restriction in the Dexter model is that, while the model allows composites within composites, the hierarchy of composites must be acyclic, thus forbidding so called "Escher effects".

At the fourth ACM Conference on Hypertext, P. De Bra, G.J. Houben and Y. Kornatzky proposed a new, object oriented, extensible data model for hyperdocuments [DBHK92]. This model is again open ended, in that it allows basic elements that are black boxes, it allows values of many data objects to belong to any domain, etc. (The name "Tower Model" was not used by the authors of that paper.)

The model contains basic structural elements, nodes, links and anchors, tower objects and city objects. The tower objects are used to model different descriptions of an object, somewhat like the layers in the Dexter model. Type, storage structure, presentation, etc. are all levels of the tower object. Cities represent sets of views onto (tower) objects.

The model allows every kind of object to be a virtual object (i.e. the result of a function or algorithm). Operators for defining virtual structures are Apply-to-All, Filter, Enumeration and Abstraction (or grouping).

The browsing semantics is described by means of trajectories, a generalization of paths or walks. They are defined using space-time axioms. A separate (and later) paper [ADBHK93] describes browsing in the Tower model using Petri nets. (It is included in the part of this course dealing with browsing semantics in general.)