This paper discusses experiences and lessons learned from the design of an open hypermedia system, one that integrates applications and data not "owned " by the hypermedia. The Dexter Hypertext Reference Model [8] was used as the basis for the design. Though our experiences were generally positive, we found the model constraining in certain ways and underdeveloped in others. For instance, Dexter argues against dangling links, but we found several situations where permitting and supporting dangling links was advisable. In Dexter, the data objects making up a component's contents are encapsulated in the component; in practice, references to objects stored apart from the hypermedia structure should be allowed. We elaborate Dexter's notion of composite component to include composites that "contain " other components and composites with structured contents, among others. The paper also includes a critique of Dexter's notion of link directionality, proposes a distinction between marked and unmarked anchors, and discusses anchoring within a composite.

Computing support for large engineering enterprises provides an example of the need for hypermedia-based collaborative computing systems composed of a large number of distributed heterogeneous tools. These computing environments place complex requirements on the underlying hypermedia platform. To support integration of independently written tools for these environments, hypermedia platforms must address several important issues such as scalability, openness, distribution, heterogeneity, interoperability, extensibility and computation. This paper describes the HyperDisco approach to open hypermedia systems. HyperDisco provides an extensible object-oriented hypermedia platform supporting intertool linking, computation, concurrency control, notification control, version control, access control, query and search, and various other features. The present work has two main objectives: 1) to provide a platform to integrate existing and future distributed heterogeneous tools and data formats an...

This paper presents a taxonomy for open hypermedia systems. The purpose of the Flag 1 taxonomy is manifold: #1# to provide a framework to classify and concisely describe individual systems, #2# to characterize what an open hypermedia system is, #3# to provide a framework for comparing di#erent systems in a system independent way, and #4# to provide an overview of the design space of open hypermedia systems.

Today's networked software engineering environment is characterized by a multitude of autonomous, heterogeneous information repositories, a variety of incompatible user interfaces, diverse, unconventional data types, including text, graphics, and possibly video and sound, rapid change, both in structure and content, and multiple ways of viewing relationships among the same information items. Existing information storage mechanisms fail to combine diverse data types/models, complex objects and storage structures, personal views and organizations of shared objects, access to distributed, heterogeneous repositories, and ease of evolution. This paper examines these issues and describes a Distributed Hypertext architecture that provides transparent access to autonomous, heterogeneous software object repositories, resulting in both a powerful organizational tool and a simple yet effective integration mechanism. Keywords: Hypertext, Distributed Systems, Heterogeneous Distributed Object Manage...

This paper presents an approach for providing hypermedia services in this heterogeneous setting. Central notions of the approach include the following: anchors are established with respect to interactive views of objects, rather than the objects themselves; composable, n-ary links can be established between anchors on different views of objects which may be stored in distinct object bases; viewers may be implemented in different programming languages; and, hypermedia services are provided to multiple, concurrently active, viewers. The paper describes the approach, supporting architecture, and lessons learned. Related work in the areas of supporting heterogeneity and hypermedia data modeling is discussed. The system has been employed in a variety of contexts including research, development, and education

Hypermedia is often viewed as either a paradigm for human-computer interaction or information organization. Human-computer interaction provides a view of hypermedia that involves the creation, manipulation, and access of information through a "point-and-click" navigation mechanism. Information organization provides a view of hypermedia that involves the storage of information as a set of data and metadata objects, where metadata objects capture structural relationships among information objects. This paper describes a third view of hypermedia --- hypermedia as a computing paradigm. In this paper, we explore the implications of pushing hypermedia beyond its traditional role in human-computer interaction and information organization into the computer's core operating environment. We believe the resulting hypermedia operating systems provide a new paradigm for computing --- one in which human-computer interaction, information storage and retrieval, programming, and control are integrated ...

"Linking by inking" is a new interface for reader-directed link construction that bridges reading and browsing activities. We are developing linking by inking in XLibris, a hypertext system based on the paper document metaphor. Readers use a pen computer to annotate page images with free-form ink, much as they would on paper, and the computer constructs hypertext links based on the ink marks. This paper proposes two kinds of readerdirected links: automatic and manual. Automatic links are created in response to readers' annotations. The system extracts the text near free-form ink marks, uses these terms to construct queries, executes queries against a collection of documents, and unobtrusively displays links to related documents in the margin or as "further reading lists." We also present a design for manual (ad hoc) linking: circling an ink symbol generates a multi-way link to other instances of the same symbol.

In this paper, we consider the evolution of hypermedia system architectures from the monolithic systems of the 1980's 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 terms 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.

This paper provides an architectural framework for modeling third-party application integrations with open hypermedia systems, which collects and extends the integration experience of the open hypermedia community. The framework is used to characterize applications prior to integration, and describe the qualities of a complete integration. Elements of the architectural model are artists, which are used to manipulate anchors, links, and native application objects; communicators, which manage information flow to and from the open hypermedia system; and containers which group the other elements. Prior integration experience is collected in a standard way using the model. Guidance in selecting the final integration architecture is provided by this prior integration experience, in conjunction with the degree of difficulty of an integration, which is related to the integration architecture.

The 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.