University of Malta CSA3080: Lecture 7 © Chris Staff 1 of 18 CSA3080: Adaptive Hypertext Systems I Dr. Christopher Staff Department of Computer Science & AI University of Malta Lecture 7: Formal Models of Hypertext Dexter Hypertext Reference Model

University of Malta CSA3080: Lecture 7 © Chris Staff 2 of 18 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?

University of Malta CSA3080: Lecture 7 © Chris Staff 3 of 18 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, Literary Machines, Edition “Hypertext is text which is not constrained to be linear. Hypertext is text which contains links to other texts.” References: –

University of Malta CSA3080: Lecture 7 © Chris Staff 4 of 18 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

University of Malta CSA3080: Lecture 7 © Chris Staff 5 of 18 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 The Dexter Hypertext Reference Model, in Communications of the ACM, 37(2), February, 1994,

University of Malta CSA3080: Lecture 7 © Chris Staff 6 of 18 DHRM Fundamentals DHRM separates the representation of documents (nodes) from the linking of nodes and the navigation through hyperspace [Halasz94]

University of Malta CSA3080: Lecture 7 © Chris Staff 7 of 18 DHRM Fundamentals

University of Malta CSA3080: Lecture 7 © Chris Staff 8 of 18 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

University of Malta CSA3080: Lecture 7 © Chris Staff 9 of 18 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)

University of Malta CSA3080: Lecture 7 © Chris Staff 10 of 18 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:

University of Malta CSA3080: Lecture 7 © Chris Staff 11 of 18 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

University of Malta CSA3080: Lecture 7 © Chris Staff 12 of 18 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

University of Malta CSA3080: Lecture 7 © Chris Staff 13 of 18 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

University of Malta CSA3080: Lecture 7 © Chris Staff 14 of 18 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

University of Malta CSA3080: Lecture 7 © Chris Staff 15 of 18 DHRM Fundamentals [Halasz94]

University of Malta CSA3080: Lecture 7 © Chris Staff 16 of 18 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

University of Malta CSA3080: Lecture 7 © Chris Staff 17 of 18 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!)

University of Malta CSA3080: Lecture 7 © Chris Staff 18 of 18 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…