The hypermedia tield has long realized the need for firstclass structural abstractions.However, we have failed to generalize the concept of ubiquitous structure management to problem domains other than navigation of information spaces. In this paper, we argue for the recognition of such a generalization, called structural computing, in which we assert the primacy of structure over data. We provide examples of four problem domains that are more naturally modeled with structure than data. We argue that support for structural computing must come in the form of new models, operating systems, and programming languages. We also assert that the experience gained by hypermedia researchers over the last decade may be naturally extended to form the basis of the new field of structural computing, and furthermore, the broadening of the applicability of our work is necessary for the continued vitality of our research community. KEYWORDS:

Early hypertext systems were monolithic and closed, but newer systems tend to be open, distributed, and support collaboration. While this development has resulted in increased openness and flexibility, integrating or adapting various different tools, such as content editors, viewers or even other link servers has remained a tedious task. Many developers were implementing essentially similar components, simply for the benefit of having their own platform on which to experiment with hypertexts.The open hypermedia community is addressing this issue of interoperability between open hypermedia systems. The goal of the community is to provide an open hypermedia framework that can be used by application developers outside the community to construct more powerful hypermedia-aware applications. The design and evolution of this framework is presented along with the requirements that drove its development. The framework has matured to the point where it has supported the creation of a number of research prototypes. These prototypes are described and evaluated with respect to their use of the framework. KEYWORDS:Open hypermedia protocol navigational interface (OHP-Nav), component-based open hypermedia system (CB-OHS), reference architecture, data model, interoperability, open hypermedia systems working group (OHSWG), structural computing.

INTRODUCTIONThe historical development of hypermedia systems can be characterized as a series of successive abstractions of functionality away from the "core" hypermedia server, often resulting in a new open layer in the hypermedia environment architecture. Recently, this trend of abstraction has been applied to the hypermedia server itself, replacing the notion of a single, closed hypermedia server with an open layer of structure servers. This newest development brings with it a new set of challenges and research issues for open hypermedia researchers. In this paper, we discuss these issues, review some of our collective applicable experience with contemporary open hypermedia systems and other work, and point out some of the more pressing and intriguing open questions that we feel are facing open hypermedia researchers today. We also examine the "split" in the current hypermedia research community between "system" and "domain" researchers and the still-present need for interoperability among systems, and discuss why any attempt to address the issues we discuss in this paper must account for these observations. Hypermedia systems have developed over the past several decades from the monolithic systems of Bush and Engelbart to the open hypermedia systems of today. Each development in hypermedia system design has been characterized by increased openness, which in turn led to greater applicability and flexibility. Recently, open hypermedia researchers have started to address opening the hypermedia server itself, discussing hypermedia environments that consist of an open layer of structure servers with their associated servers and clients. This "component-based" hypermedia middleware approach allows us to support a much wider range ol structural abstractions than has been possible with the closed link server layers that are typical in contemporary systems.With these new hypermedia component-based systems comes a new set of research issues. Although we have little direct experience with many of these new issues, we do have a wealth of related experience upon which we can draw to form initial understandings. In this paper, we present an agenda containing some of the new issues that confront us as we consider the next generation of component-based open hypermedia systems (CB-OHS's). We also present a framework that allows us to generate and analyze these issues systematically.KEYWORDS: Open hypermedia system (OHS), componentbased open hypermedia system (CB-OHS), structural computing, hypermedia middleware, hyperbase, hypermedia operating system, hypermedia domain research Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, to republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee.HyperText 98 Pittsburgh PA USA Copyright ACM 1...

The areas of application of hypermedia technology, combined with the capabilities that hypermedia provides for manipulating structure, create an environment in which version control is very important. A hypermedia version control framework has been designed to specifically address the version control problem in open hypermedia environments. One of the primary distinctions of the framework is the partitioning of hypermedia version control functionality into intrinsic and application-specific categories. The version control framework has been used as a model for the design of version control services for a hyperbase management system that provides complete version support for both data and structural entities. In addition to serving as a version control model for open hypermedia environments, the framework offers a clarifying and unifying context in which to examine the issues of version control in hypermedia.

KEYWORDSOpen hypermedia system (OHS), component-based open hypermedia system (CB-OHS), hypermedia middleware, distributed system services, distributed system architectures ABSTRACT In this paper, we consider the evolution of hypermedia system architectmes from the monolithic systems of the 198(ys to the middleware-oriented component-based open systems of today. We look at the various problems that users and system designers encountered with systems at various stages of this development, focusing particularly on problems that were solved or caused by moving toward a more middleware-oriented approach. Although we cast our discussion in tetms of specific hypermedia systems, we believe that many of our observations on the advantages and challenges of our current middleware approach may be helpful to those in other areas of system research.