university of malta csa3080: lecture 7 © 2003- chris staff 1 of 18 [email protected] csa3080:...

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CSA3080: Lecture 7© 2003- Chris Staff

CSA3080:Adaptive Hypertext Systems I

Dr. Christopher StaffDepartment of Computer Science & AI

University of Malta

Lecture 7:Formal Models of HypertextDexter Hypertext Reference Model



Aims and Objectives

• Adaptive Hypertext Systems are built using hypertext navigation support and information is inter-linked, hypertext style

• Most AHSs are deployed over the Web, but the Web isn’t a particularly good example of a hypertext…

• So what are the properties and characteristics of “good” hypertexts?



Aims and Objectives of Hypertext

• ‘Well, by “hypertext” I mean non-sequential writing--text that branches and allows choices to the reader, best read on an interactive screen’ Ted Nelson, 1987. Literary Machines, Edition 87.1.

• “Hypertext is text which is not constrained to be linear. Hypertext is text which contains links to other texts.” http://www.w3.org/WhatIs.html

• References:– http://www.google.com/search?q=define:Hypertext



Hypertext 1988 and beyond

• The WWW was first launched in 1991, but only gained popularity in 1993

• The Hypertext community came to a head in 1990 to iron out many inconsistencies and incompatibilities in terminology

• Many models of hypertext were also proposed, based on petri-nets, sets, etc.

• The most popular model is based on graph theory



Dexter Hypertext Reference Model

• Why a formal model?– “The goal of the model is to provide a principled basis

for comparing systems as well as for developing interchange and interoperability standards” [Halasz94]

• DHRM has been implemented as Amsterdam, CMIFed, AHAM, DeVise/WebVise, RHYTHM (Bologna)…

• References:– Halasz, F. and Schwartz, M. 1994. The Dexter Hypertext Reference Model, in

Communications of the ACM, 37(2), February, 1994, 30-39.



DHRM Fundamentals

• DHRM separates the representation of documents (nodes) from the linking of nodes and the navigation through hyperspace

[Halasz94]



DHRM Fundamentals



DHRM Fundamentals

• Components– DHRM components are the equivalent of nodes, and

are represented in the Storage Layer

– Nodes were called frames, cards, documents, and articles

– Even today, on the Web a node is referred to as a document or more commonly, a page

– DHRM doesn’t really care about what happens within a component, only how the hypertext network interfaces with the component



DHRM Fundamentals

• Anchors– References to locations or items within

documents– Components can be composites, hierarchical

combinations of atomic components (DAG)– Anchors can be the source or destination of

links– Anchors can be entire components, or spans

(segments of a component)



DHRM Fundamentals

• More about anchors…– Anchors have two parts

• Anchor ID (used by Storage Layer)• Anchor Value (used by Within-Component Layer)

– The anchor value can be a region within a component, and the value is sensible only to the application responsible for editing/accessing the component

– Anchors are unique: <componentID, anchorID>



DHRM Fundamentals

• Links– Links are represented in the Storage Layer– Specify a source anchor, a destination anchor,

and a direction that specifies how the link can be traversed

– Links can also be the destinations for other links

– Links, therefore, are totally separate from the components that contain them



DHRM Fundamentals

• Presentation Layer– A hypertext isn’t much good if you cannot manipulate

it and navigate through it

– From the Presentation Layer users can access, view, and manipulate the hypertext



DHRM Fundamentals

• All components (including links) have presentation specifications

• The Presentation Layer can also impose presentation specifications on the accessed links and components to capture user preferences, for instance



Referring to components

• Components have unique identifiers (UIDs) and component specifications

• Component specifications are essential, because a user may be able to describe a component without knowing its UID

• Components may be identified from their description using a resolver function, and then retrieved using an accessor function



DHRM Fundamentals

[Halasz94]



DHRM Fundamentals

• More about links– Links are first class objects– Links are created by combining a component

specification, anchor ID, direction, and presentation information into a specifier

– Direction can be FROM, TO, BIDIRECT, NONE

– A link is a sequence of two or more specifiers, at least one of which must be TO or BIDIRECT



Conclusion

• Interesting features of DHRM– Links are separate from documents containing them– Anybody can be an author (link creator)– Search capability is built into hypertext model (resolver

function)– Presentation specifications can change behaviour of

component when displayed– Links “know” their origin and destination– Components can be composite– Dangling links are not allowed (supposedly!)



Conclusion

• DHRM was defined in 1990• Most existing hypertext systems were small

scale, catering for individuals and small workgroups

• The Internet (using TCP/IP) had existed for 7 years

• The WWW did not yet exist…

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