ACM Computing Surveys
31(4), December 1999,
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The Evolution of Hypertext Link Services
Multimedia Research Group Web: http://mmrg.ecs.soton.ac.uk/
Highfield Lane, Southampton, SO17 1BJ, UK
1 Justification for Hypertext ServicesEarly visions of hypertext environments took the view that all information would be made available to everyone through a particular system. Bush's Memex [Bush 1945] was such a system, but it was parasitic on the existing network of research libraries; Nelson's Xanadu [Nelson 1987] by contrast proposed a whole new computer network of ``xanalogical storage'' sites with associated franchise opportunities. Other hypertext environments, while not assuming such a level of ubiquity, were still characterised by the need to import data into the control of the system, irrespective of the fact that information was originated, modified and consumed outside of any specific environment, using many tools and programs, for many purposes, perhaps on many different machines and computer architectures.
The recognition of the diverse nature of information environments drove other communities to develop standards such as SGML for open information interchange (subsequently adopted by the Web in simplified form) allowing documents and data to be interchanged between different applications and work processes. By contrast, the database community produced SQL, a standard for interoperation, so that instead of transferring data between different programs, applications could query any database which held the required information.
Early on in the history of the hypertext research community, there were calls to similarly extend the capabilities of hypertext systems to allow interoperability across a range of software components [Meyrowitz 1989]. Sun's Link Service [Pearl 1989] was the first practical implementation of this idea: part of a commercial software development environment, the link service was composed of a server providing link deployment features together with a library that could be used to turn any application into a client of the service. Several text editors, a file browser and a project scheduling tool were shipped as `link aware', together with various third-party CASE tools.
2 Open Hypermedia SystemsThe term `open hypermedia' came to be associated with the provision of a hypertext service which enabled client applications to create, edit and activate links which were managed in separate link databases; it contrasted with the monolithic approach to hypertext systems in which the functionality of both data and link management was provided within a single indivisible application.
Microcosm [Fountain 1990] provided a similar link service solution from the perspective of a different application domain. Developed out of a need to be able to provide a hypertext environment for the presentation of existing archival data (video disks, sound and image libraries as well as textual documents), its aim was to provide scholarly and teaching access for those data sets. It did not have a client/server architecture, but instead distributed the various hypertext functions into a chain of processes which communicated by message passing [Davis 1992]. This enabled the behaviour of the system to be modified easily at run time, adding different link database processes or swapping history management and tours facilities in and out depending on the system behaviour required by the user (which in turn may be a function of their technical or subject sophistication). Document viewers and editors were simply specific instances of hypertext processes in the chain, and similarly communicated by message passing.
Multicard [Rizk 1992] provided another example of this separation, with an extensible protocol (M2000) communicating requests between specially adapted document editors and a hypertext back end. Document editors were not required to implement the full protocol: a minimum of Open/Close Node was deemed sufficient in order to access each document from a networked storage database.
An Architectural Basis for Open Hypermedia ServicesDatabase systems came to be seen as an ideal basis on which to build higher level hypertext functionality: for example HyperBase [Schütt 1990] on a relational foundation and HB1 [Schnase 1993a] on a semantic platform.
A more abstract but highly influential attempt to provide a standard decomposition of hypertext facilities resulted in the Dexter Hypertext Reference Model [Halasz 1990]. It established how the architecture of a hypertext system could be organised into separate layers concerned with component storage, run-time and document content issues. Some of the persistent ideas which emerged from the Dexter separation of concerns were the classification of hypertext objects into nodes, groups, anchors and links and the identification of the run time role of session management.
Particular hypertext features were deliberately excluded from the model, including a model of the contents of each node or document in the hypertext and the presentation and behavioural aspects of the components of a link. The runtime layer focussed on the dynamic interactions between the user and the various components of the system, most importantly the meaning of ``following a link'' in terms of identifying a target link anchor, mapping it onto a link component in the storage layer, determining the resulting anchor components and presenting them in the context of their stored nodes.
Despite providing a layered model, Dexter did not necessarily result in implementations providing identifiably separate services [Nürnberg 1997]; the Flag Taxonomy [Østerbye 1996] directly addressed this issue, modelling the decomposition of roles into interdependent services. The Open Hypermedia Protocol (OHP) [Davis 1996] was a collaborative development by the open hypermedia community to implement a universal protocol between these services, effectively allowing mix-and-match access to the storage and runtime layers of a range of hypermedia systems. Layered systems based on the Dexter model started off in a single computer environment, but over a period of several years were reworked to address issues of the network. Devise HyperMedia [Grønbæk 1993] used an object-oriented schema and an OODB to implement Dexter's storage layer; the authors subsequently proposed extensions to the Dexter model to accommodate open hypermedia systems [Grønbæk 1996] and ultimately technology for deploying these Dexter services in a Web environment [Grønbæk 1997] [Grønbæk 1999]. HyperDisco established an environment for tool integration firstly on a single machine [Wiil 1996] and later across the Internet [Wiil 1997]. Chimera [Anderson 1994] also established a service model, not directly designed on Dexter, but which was later successfully used to apply link services to the Web [Anderson 1997].
A significant outcome of this work has been the demonstration of three independent hypertext systems interoperating in various roles across a network [Reich 1999]. Providing such hypertext linking across application boundaries is a principle goal of open hypermedia interoperability, and offers great hope for the usefulness of data stored in legacy systems.
3 Open Hypermedia Linking ServicesThe role of links is varied (structural, navigational, spatial etc.) and the kind of linking which can be used within an open environment may be of crucial importance [DeRose 1989]. XLink, the Web proposed standard for hypertext linking [DeRose 1999], allows databases of links to be maintained separately from the documents, with each of the links is fixed to specific regions of specific documents. Hyper-G [Andrews 1995a] provided a solution for the problem of broken links in the Web's open environment in the form of a safe but restricted environment with guaranteed consistency for link editing. By contrast, the Distributed Link Service consisted of an unenclosed environment providing a navigational overlay to Web pages based on within-node text analysis to identify implicit link opportunities such as key phrases, personal names and bibliographic citations [Carr 1998].
To recall the opening theme, hypertext services must provide `more than' services to diverse sets of text and multimedia data. Hypertext links describe various kinds of relationships, not all of which can be easily modelled by `hard', point-to-point hypertext links anchored at fixed document offsets. Instead they may be computed or inferred based on attributes of the document [Parunak 1991] , features of its contents [Hirata 1997], characteristics of the runtime environment [Stotts 1989] or traits of the community which uses the documents [Hill 1997].
Computations which process knowledge and meta-knowledge, data and metadata are proposed for providing the 'more than' services of the Web [Berners-Lee 1997]. Metadata as well as analysis of the data contents themselves can be used as the basis for deriving linkable relationships. Initially used to encode keywords and publication cataloguing information, the Web standard RDF [Lassila 1999] can also make general statements about documents which a suitable future service can use to infer many kinds of link relationships (e.g. two documents are related because their metadata declares that they describe the same research project).
Link services emerged as embedded parts of a monolithic system but have become value-added services in a heterogenous and unco-ordinated environment. This has opened up enormous challenges of robustness and scalability, not the least of which is dealing with disappearing documents and broken links.
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