Link services badges: A mechanism for announcing available hypermedia functionality

Short scientific article, which I wrote for an invitation-only workshop on "Incorporating Hypertext Functionality into Software Systems", which was organized on 18 September 1994 during ECHT'94, the 3rd ACM European Conference on Hypermedia Technology held in Edinburgh, Scotland, UK. In this short article I argued for the introduction of "link badges" as a standardized means of announcing what kinds of link services a hypermedia system can provide to an external system. The use of these "link badges" is somewhat similar to the way WSDL (Web Services Description Language) is now being used to describe the functionality offered by a set of Web Services.

Providing support for hypertext and hypermedia functionality between existing information systems in an open software environment requires two things: a linking mechanism that allows application-independent, system-level linking between each participating information system, and a notification mechanism that allows each participating information system to specify to what extent it can participate in the linking. We suggest that a software application, which is to be integrated into such an open hypermedia environment, has to wear a "link services badge", as a means of announcing to the hypermedia environment what kinds of link services it can and cannot provide. In this position paper, we discuss where the need for such link services badges arises, what the different levels of link services are that can possibly be provided by existing software applications, and how these link services can be represented, accessed and controlled using link services badges. We conclude this paper by raising a number of open research issues which are of relevance to this workshop.

Link services badges

In the present research efforts directed towards the creation of open hypermedia environments, composed of existing, closed software applications, attention is primarily being devoted to the design of new linking mechanisms that will allow non-hypertext systems to become enhanced with hypertext functionality. In most of these approaches, the basic hypermedia linking mechanism is enlarged into a system-wide facility, under the control of some form of link manager communicating with the different legacy applications [Arents92][Maurer90]. However, this "hyper-retrofitting" of legacy applications fails to address the crucial question of how these different applications announce to the link manager to what extent they can provide the necessary link services. In order to merge seamlessly into an open hypermedia environment, an existing application has to have some means of making public its "ALI" ("Application Linking Interface"), the hypertext equivalent of an API (Application Programming Interface). In the same way as an API makes it possible for an outside application to access the internal data constructs and computation mechanisms of a given application, an ALI would make it possible for an external link manager to access the internal data constructs and navigation mechanisms of the application. Mapping these internal application constructs to external hypertext constructs could be done using bridge laws, as first proposed by Bieber [Bieber91]. We therefore suggest that each legacy application should have associated with it a "link services badge", a compact standardized description of the kinds of link services the application can provide, which it can "wear" to announce to the link manager the details of its ALI. To clarify this with an analogy: linking into/out from legacy information systems in an open hypermedia environment requires that the hypermedia environment has an universal inter-application linking language (such as the one proposed in [Ashman93]), but also that the information systems know how to "speak" this linking language and how to "introduce" themselves (using their link services badge).

Levels of link services

Existing applications, that are to be included in an open hypermedia environment, can provide link services to different degrees. Closed applications are completely sealed off from the operating environment, permeable applications allow some degree of interaction with external applications (typically through a restricted API) and open applications allow full access to their internal constructs (typically through an extensible macro language). The levels of link services that can be provided by these applications differ correspondingly. At the lowest level of link functionality, an application would offer referral-only link services, allowing external applications only to link into internal data constructs, but not itself being capable of linking out towards external applications. At a higher level, an application would offer navigation-enhanced link services, allowing both linking into and out from itself, and providing support for basic navigation constructs such as backtracking, predefined paths and guided tours. And finally, at the highest level, an application would offer process-enabled link services, supporting full linking and navigation functionalities, and providing support for business process-specific constructs such as personal and group annotations, workflow-related navigation, etc. Each level of link services would have associated with it a more complex type of link services badge.

Link services management

A link services badge would consist of a compact, standardized description of the kind of link services the application can provide to the open hypermedia environment. It would contain detailed information on:

Providing this information to the co-ordinating link manager would allow the open hypermedia environment to adhere to the design principles of [Oren90] for standardizing distributed hypermedia functionality, i.e. the hypermedia environment would be able to support all the necessary identity and inspectability mechanisms. A good model for the management of the link services itself is the model used by the OLE (Object Linking and Embedding) 2.0 mechanism for MS-Windows, where every application that is OLE-aware registers the availability of its services in a central repository, so that all other applications can make use of them. In a similar manner, when a new application is introduced in an open hypermedia environment, it would show its link services badge to the link manager, who would then know what kinds of links services it can expect from this new application. In this way, all the legacy applications would offer a uniform ALI to the outside open hypermedia environment, thereby greatly facilitating the implementation of powerful link managers.

Open research issues

This position paper has argued that we need to develop a notification mechanism that will allow existing software applications to announce the level of link services they can provide to an open hypermedia environment. We have proposed introducing the concept of a "link services badge", that will allow an application to announce (to the open hypermedia environment to which it wants to belong) the level of hypermedia support it is capable/willing of providing. A number of important research issues remain open however:


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Written by Hans C. Arents
Published on-line Sep 12, 1994
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