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
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
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)
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. 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. 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
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
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].
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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