Microcosm and the WWW: A Distributed Link Service G. J. Hill, L. A. Carr, D. C. De Roure and W. Hall Introduction Open hypertext systems 2 aim to act as an underlying hypermedia ’link service’ (a term first used by Pearl 4 ), rather than provide a closed environment in which to present information. In order to provide such a facility, link information must be managed separately from docu- ments, so that links may be applied to documents in any format. The World Wide Web (WWW) is undoubtedly one of the more successful hypertext systems, but it is a largely closed system, dependent on the use of HTML for the provision of linking facilities. Although links may be created to documents other than those in HTML and image formats, such links are dead ends, and there is no way to follow any further links (for example, links to spreadsheet documents). There is also no way for addi- tional links to be made available by third parties, as all link information is embedded in documents. It is, however, possible to employ an open hypertext ap- proach to the authoring and management of World Wide Web hypertext documents 3 and to provide more flexible facilities. This paper describes how we have provided a link service for the WWW, based upon the model used in the Microcosm open hypertext system 2 . The advantages of an open link service Reduced Authoring and Maintenance Effort The most obvious advantage is the ability to improve au- thoring efficiency, and subsequently to aid in the main- tenance of hypertext documents. Through the use of a wide range of link types 2 , for example generic links and information retrieval facilities, it is possible to rapidly create a useful set of links. In particular, the use of generic links allows common links to be authored only once — wherever the source selection of the link occurs, the link is available, includ- ing any documents subsequently made available. Typi- cally such links might be created on names of people and places, or common terms, to provide access to more de- tailed information. In a closed system, such links need to be created wherever the source term appears in a docu- ment, and new documents also need to be linked into the system manually. This form of linking also reduces maintenance require- ments, as changes to links need only be made to the central link databases, and will immediately be effective wherever the link is available. This can reduce problems frequently encountered in the WWW, such as link fossil- isation and decay 3 . Finally, a separate link database al- lows much more efficient automatic processing and edit- ing of links. Enhanced Reader Experience Rather than provide hypertext facilities as a single ap- plication, a link service aims to provide underlying hy- pertext facilities to the user’s whole environment. In addition, the type of linking described in the previous section allows the user a more flexible approach to link traversal. Rather than rely on those links highlighted by the system, the user is also able to select arbitrary items and query the system for possible links, thus creating a ‘reader-led’ navigation paradigm. Readers may also be able to create their own links, al- lowing them to annotate material and freeing them from a hypertext structure created purely by designated au- thors. If these databases are shared with other users, col- laborative authoring of hypertext resources is enhanced. Alternative Views A powerful mechanism made possible through the use of separate link management is the provision of multiple link databases for the user to select. Thus the user may select link sets that reflect the context in which they wish to investigate a particular set of documents. Another possibility is a separation between information provider and link provider. At present, hypertext mate- rial is usually delivered with links inextricably bound to the associated material. A link service can help to over- come this restriction, by providing the facility to apply completely different link sets to a set of documents, or conversely to apply existing links to new documents not available when the links were originally created. Finally, this facility can also support more efficient man- agement of hypertextual information. If a variety of link structures are to be applied to a particular set of docu- ments, changes to the document set are easier to make if the link information is managed separately. The distributed link service It is clear from the discussion above that the WWW, as it is used at present, is unable to provide many of the desirable features of a hypertext link service. However, the open nature of the framework upon which the WWW is based means that it is possible to extend the hyper- text model of the WWW to support the more advanced features required of a link service. In effect the WWW infrastructure, now widespread and readily available, can be used as the communication framework for a dis- tributed hypertext link service. We have developed the Distributed Link Service (DLS) as such a system 1 . It can work in conjunction with exist- ing WWW resources to support an additional underlying link service, which can provide the features described above. This system is based upon our experiences de- veloping the Microcosm hypertext system. Like Micro- cosm, the DLS utilises a variety of link database pro- cesses to offer flexible hypertext functionality to a wide range of end-user applications. The DLS is composed of two parts: the server facilities which are accessed via a standard WWW server, and the client interface which works in conjunction with the Netscape WWW browser.