Contents in Current Page:

1. Dexter - Introduction
2. Dexter - Storage layer
3. Dexter - Within component layer
4. Dexter - Run time layer
5. Dexter - Anchoring interface
6. Dexter - Presentation specifications layer.
7. Dexter - Some thoughts

Dexter - Introduction

Dexter Hypertext Reference Model is an attempt to capture both formally and  informally, the important abstractions found in a wide range of existing and future Hypermedia systems. The goal of the model is to provide a principle basis for  comparing Hypermedia systems. The model is divided into three layers. The Storage layer describes the network of nodes and links. The run-time layer describes mechanisms supporting the user interaction with the hypermedia. The  within-component layer covers the content and structures within hypermedia nodes. Between this three layers there are two interface mechanisms: the anchoring  mechanism which acts as an interface between storage layer and within-component layer, and the presentation specification mechanism which acts as an interface between storage and run-time layers. The model as currently stated is far more powerful than any existing hypermedia system. N-ary links and composite components are intended to accommodate the design of future hypermedia  systems. No existing system includes both n-ary links and composite components. The model has yet to be compared in detail to the hypertext systems it is designed to represent. A necessary step is the development of several hypertext systems using the constructs from the Dexter model. This hypertext systems should be chosen to represent a broad spectrum of designs, application domains,  implementation platforms.

Dexter - Storage layer

* Describes a "database" of data-containing components interconnected by relational links. * Focuses in the mechanisms by which link and non-link  components are "glued" together to form hypermedia networks. * Does not differentiate between text components, graphics components etc. All components  are equal and they behave just as containers of data. * No attempt to describe this data or the structure of the data (e.g. the structure of a document ). Data and the structure are described in the within-componenet layer. Specifications * Storage layer describes the structure of a Hypertext as a finite set of components together with two functions: the accessor and resolver functions. These two basic function are jointly responsible for retrieving components. * The fundamental entity and basic unit of addressability is the component. * A component is an atom, a link, or a composite entity made from other components. * Composite components are relative rare in current hypermedia systems (exception Augment hypertext system). * Every component has a globally unique identity which is captured by its unique  identifier (UID). * Apart the base component there is an component information entity associated with each component. The component information entity  describes the properties of the component rather than its data. * Accessor function maps IUD into components  ( Accessor(x) : UIDs -» COMPONENTs ). * UID  as a basic addressing mechanism may be to restrictive (for supporting implicit linking). * Component specifications can be used in conjunction with resolver  function. Resolver function is responsible for resolving a component specification into a component UID.  ( Resolver(x) : component specification -» UIDs ).

 Dexter - Within component layer

* Concerns with the contents and structure within the components of hypermedia systems. * This layer is purpose depended not elaborated with Dexter model. The  range of possible content-structure that can be included in a component is not restricted.

Dexter - Run time layer 

 * Provides the functionality for access, view, manipulate the hypermedia network. * It provides a mechanism for instantiation of a component. * At any given moment  the user of a Hypertext can be viewing or editing any number of component instantiations. * It has a history sequence, a mapping between instantiations and  components, an instantiator function, a realizer function, run time resolver function.

Dexter - Anchoring interface

 * Interface mechanism between 1 and 2. * Mechanism for addressing ( referring to ) locations or items within the content of an individual component. * An Anchor has  two parts, an anchor ID and an anchor value. The anchor value specifies some location, item, substructure of a component. Every component can include N anchor entities. Anchor is an indirect addressing mechanism within the component's data or structure. * A specifier is an entity consists of Component specifications,  Anchor ID, a DIRECTION attribute and a PRESENTATION specification. A specifier specifies a component and an anchor within this component. * A link is a sequence of two or more specifiers. This provides the capability for arbitrary arity links. Links in Dexter model can be directional or bi-directional.

Dexter - Presentation specifications layer. 

* Interface between 1 and 3. * Provides the mechanism for encoding presentation  specifications into storage layer. In consequence the way in which a component is presented to the user can be a function not only depended in the specific hypermedia tool.

Dexter - Some thoughts

Move Up and Down The Tree

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1. Hypermedia Reference models