ACM Computing Surveys 31(4), December 1999, http://www.acm.org/surveys/Formatting.html. Copyright © 1999 by the Association for Computing Machinery, Inc. See the permissions statement below.

Hypermedia Research Directions:
An Infrastructure Perspective

Uffe K. Wiil, Peter J. Nürnberg

Aalborg University Esbjerg    Web: http://www.aue.auc.dk/
Department of Computer Science     Web: http://www.cs.aue.auc.dk/
Niels Bohrs Vej 8, 6700 Esbjerg, Denmark
Email: ukwiil@cs.aue.auc.dk, pnuern@daimi.au.dk
Web: http://www.cs.aue.auc.dk/~kock, http://www.cs.aue.auc.dk/~pnuern/

John J. Leggett

Texas A&M University     Web: http://www.tamu.edu/
Department of Computer Science     Web: www.cs.tamu.edu/
College Station, TX 77843-3112, USA
Email: leggett@csdl.tamu.edu
Web: http://www.csdl.tamu.edu/~leggett


Abstract: This paper offers a perspective on the directions in which hypermedia infrastructure research will move in the next several years. The perspective is based on the authors' experiences and insights from a decade of active participation in this research area. After a review of hypermedia infrastructure research, the paper focuses on two particular threads of such research named "multiple open services" and "structural computing". We believe that these threads show much promise for the future.

Categories and Subject Descriptors: H.5.4 [Information Interfaces and Presentation]: Hypertext/Hypermedia - Architectures

General Terms: Design, Experimentation, Standardization

Additional Key Words and Phrases: open hypermedia system (OHS), component-based open hypermedia system (CB-OHS), multiple open services, structural computing, hypermedia operating system (HMOS)



1 Introduction

From its inception, hypermedia has been less about adding new functionality to computer systems (e.g., the ability to link objects together) and more about changing the way computer systems work to reflect more accurately the way in which people work. Bush [Bush 1945] noted that people use associative recall to remember information, and thus proposed that our tools should mimic this functionality. Engelbart [Engelbart 1962] spoke in terms of building machines that allowed "automated external symbol manipulation" on our terms as human users, not in ways dictated to us by computers. Nelson [Nelson 1967] spoke of hypermedia environments as freeing us from the single world-view paradigm of early computers.

Hypermedia structuring principles have been applied to a wide variety of application domains such as argumentation support [Conklin 1987b], [McCall 1990], digital libraries [Dewan 1995], [Schnase 1994], engineering enterprises [Grønbæk 1994a], [Malcolm 1991], information analysis [Marshall 1995], [Shipman 1995], classification [Parunak 1991], [Parunak 1993], and collaborative work [Streitz 1992], [Wiil 1992a]. The diverse and complex requirements of these domains have resulted in extensive research into hypermedia infrastructure. This paper offers our perspective on the directions in which hypermedia infrastructure research will move in the next several years. This perspective is based on our experiences and insights from a decade of active participation in this research area. After a historical review of hypermedia infrastructure research (Section 2), the paper focuses on two particular threads of such research named "multiple open services" and "structural computing". We believe that these threads show much promise for the future. The basic ideas and characteristics of multiple open services and structural computing research are described in Section 3 and Section 4, respectively. Section 5 provides our conclusions.

2 Hypermedia Infrastructure History

Hypermedia infrastructure has evolved tremendously since the first monolithic system architectures were developed in the 1960s. Current hypermedia infrastructure is more open and modular with general and extensible system components that provide well-defined services. These services are becoming increasingly standardized within the research community, as the past decade of work has led to a great deal of consensus on the form of at least the most common of them [Wiil 1997b], [Wiil 1998], [Wiil 1999a]. Figure 1 depicts four different stages of development in the evolution of hypermedia infrastructure. Figure 2 presents a historical view of some of the more prominent hypermedia infrastructure research directions and their mutual relationships.

Figure 1

Figure 1. Different stages of development in the evolution of hypermedia infrastructure (inspired by similar figures in [Nürnberg 1997a], [Nürnberg 1998] and [Wiil 1999b]). A shaded box indicates the closed part(s) of the infrastructure - implemented by a single (type of) process.

The earliest hypermedia systems were monolithic [Akscyn 1988] [Engelbart 1962] [Halasz 1987] (Figure 1a). A monolithic system includes all system services in a single process (application, link service and store). In 1987, system developers began to publish results on new types of hypermedia systems: link server systems (LSS) and hyperbase management systems (HBMS). Both of these types of systems are based on client-server architectures (Figure 1b). A client-server system operates with a single server process that includes link service and store functionality. An open set of applications can access the server functionality. Meyrowitz [Meyrowitz 1989] and Pearl [Pearl 1989] first discussed research on LSS. Later, several LSS emerged, including PROXHY [Kacmar 1991], Microcosm [Davis 1992], [Davis 1994], [Fountain 1990], [Hall 1996], Multicard [Rizk 1992], Chimera [Anderson 1994], Hyper-G [Maurer 1996], and KHS [Rittberger 1994]. Research on HBMS was first reported by Campbell and Goodman [Campbell 1987]. Later, five HBMS research groups were active: GMD-IPSI [Bapat 1996], [Schütt 1990], [Schütt1993], Aarhus University [Grønbæk 1994a], [Grønbæk 1994b], University of North Carolina [Shackelford 1993], [Smith 1991], Texas A&M University [Leggett 1994], [Nürnberg 1996], [Schnase 1992], [Schnase 1993a], and Aalborg University [Wiil 1992b], [Wiil 1997a], [Wiil 1993]. In 1994, people from the HBMS and LSS areas joined to form the Open Hypermedia System (OHS) research thread [Wiil 1994], [Wiil 1996a], [Wiil 1997b], [Wiil 1998], [Wiil 1999a]. OHS mostly focus on providing link services to an open set of applications (Figure 1c). The enormous success of the World Wide Web (WWW) [Berners-Lee 1994] has resulted in a separate research thread focusing on integration of OHS with WWW browsers such as Netscape and MS Internet Explorer. Since 1995, many OHS have been integrated with the WWW to provide "external linking" as opposed to the "embedded linking" style available in the unaugmented WWW [Anderson 1997], [Carr 1995], [Grønbæk 1997].

Figure 2

Figure 2. A historical view of hypermedia infrastructure research (updated from Figure 1 in [Wiil 1997a]).

Research on component-based open hypermedia systems (CB-OHS) grew out of the early OHS research. Two different types of CB-OHS (Figure 1d) exist. Multiple open services work was first discussed in 1995 in connection with HyperDisco [Wiil 1995], which provides different types of link, integration, distribution, and collaboration services [Wiil 1996], [Wiil 1997a]. Structural computing work was first discussed in 1996 in the initial HOSS paper [Nürnberg 1996]. In 1997, the HOSS project reported first results of provision of different types of hypermedia structure services (i.e., linking, spatial and taxonomic) [Nürnberg 1997a], [Nürnberg 1997b] backed by a "hypermedia operating system" (HMOS).

3 Multiple Open Services

A multiple open services environment is a CB-OHS that consists of two types of components. Structure-aware stores provide core hypermedia services (i.e., objects, attributes, behaviors and relations), and core collaboration services (i.e., transactions, concurrency control, notification control, access control, and version control) to an open set of middleware components [Bernstein 1996], [Wiil 1999b]. Each middleware component provides a specific set of services to an open set of participating applications such as hypermedia structure, collaboration (CSCW), storage, information retrieval (IR), integration and interoperability, and versioning services.

Middleware services can be divided into "infrastructure" and "application" services. Infrastructure middleware services encompass those that deal with the operation of the overall multiple open services environment, and thus are used only indirectly by applications (e.g., storage, location, interoperability, heterogeneity, and distribution). Application middleware services are directly used by applications (e.g., integration, hypermedia structure, CSCW, IR, and versioning services).

A multiple open service environment has three characteristics that allow it to provide generally applicable middleware services.

Firstly, middleware services are hosted in an open architectural framework. An open architecture consists of an open set of inter-operating components. Each component provides a set of well-defined services through a well-defined interface. An open architecture is based on a "plug and play" metaphor for changes to the architecture allowing: insertion of new components with new services; replacement of existing components by new; and, removal of existing components.

Secondly, the provided middleware services must themselves be open. An open service is available to an open set of applications in the computing environment. Services are provided by computing entities or components in the computing environment that are accessible by essentially all applications (e.g., middleware or operating system components). An open service is orthogonal to other services used by participating applications (e.g., storage and display services). Applications must able to use an open service without altering the existing use of services by the application. It is general enough to be useful across applications. The service is operational both internally in the application and across to other applications of the same type or other types (e.g., "cut, copy and paste" services). An open service provides different levels of its services. Applications need not integrate advanced levels of a service if only a basic level is desired.

Finally, the middleware services are available on most major computing platforms such as Suns, PCs and Macintoshes, for most major operating systems such as Unix, Windows, and MacOS, and to applications written in many different programming languages.

4 Structural Computing

Structural computing describes the view that structural abstractions should constitute the fundamental building block of CB-OHS. Instead of speaking only in terms of structure-aware stores, structural computing systems describe entirely structure-aware OS, or HMOS. Such HMOS take to the logical extreme the trend toward making structure awareness increasingly pervasive. In HMOS, structure is all-pervasive, and replaces the data-based abstractions of traditional systems. Instead of entities (applications and middleware) mapping structural abstractions into the data abstractions of traditional operating systems and infrastructures (which is done idiosyncratically), HMOS entities map data into the provided structural abstractions, which can be done in a well-defined manner.

Since structural computing CB-OHS backends provide structural abstractions to their clients, all middleware services are seen as structure services. For the services "traditionally" described in the hypermedia field, such as navigational hypermedia services (i.e., building associations among data) [Bush 1945], [Nelson 1967] or spatial hypermedia services (i.e., using flexible, dynamic structuring mechanisms to organize data) [Halasz 1987], [Marshall 1995], this structure-intensive view has clear and immediate benefits. The hypermedia field has long accepted that the design and implementation of such a hypermedia service can be accomplished more quickly and efficiently on a structure-aware backend [Wiil 1999b]. Additionally, provision of such hypermedia services within a common framework, such as that provided by CB-OHS, allows for intra- and inter-domain interoperability. Since structure representation is supported at such a low level in the system, all services in an environment can use and understand the use of such facilities.

Structural computing posits additionally, however, that the design and implementation of arbitrary services accrue these same benefits on such an infrastructure. Clearly, no service is made more difficult to design or implement in a structural computing CB-OHS. Those services currently based on traditional data abstractions can be ported to a structural computing CB-OHS by simply ignoring the structure representation mechanisms supported by the HMOS. However, we assert that a wide variety of such data-oriented services can in fact be usefully reconceptualized as structure services. For example, consider the memory management service prefetching. Prefetching algorithms use the notion of semantic locality to predict which pages are most likely to be referenced in the near future. Semantic locality can only be inferred indirectly in traditional systems by measures such as virtual address space proximity. In a structural computing CB-OHS, with its structure-aware HMOS, the prefetching service is a structure service, and is therefore guaranteed to be able to understand structure among objects represented in pages, which are generally a direct measure of semantic relatedness (and thus locality) [Nürnberg 1996].

5 Conclusion

Early hypermedia work assumed that building hypermedia systems was a matter of building the correct interface to computers. However, our field has moved toward the view that hypermedia interfaces require hypermedia infrastructure. CB-OHS are the latest attempt to supply such infrastructures. There are two important directions in this work. A multiple open services environment focuses on what services should be supplied by such an infrastructure; structural computing focuses on how such services should be conceived.

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