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Extending an Open Hypermedia System to a Distributed Environment University of Southampton Research Repository ePrints Soton Copyright © and Moral Rights for this thesis are retained by the author and/or other copyright owners. A copy can be downloaded for personal non-commercial research or study, without prior permission or charge. This thesis cannot be reproduced or quoted extensively from without first obtaining permission in writing from the copyright holder/s. The content must not be changed in any way or sold commercially in any format or medium without the formal permission of the copyright holders. When referring to this work, full bibliographic details including the author, title, awarding institution and date of the thesis must be given e.g. AUTHOR (year of submission) "Full thesis title", University of Southampton, name of the University School or Department, PhD Thesis, pagination http://eprints.soton.ac.uk University of Southampton Extending an Open Hypermedia System to a Distributed Environment by Gary John Hill A thesis submitted for the degree of Doctor of Philosophy in the Faculty of Engineering and Applied Science Department of Electronics and Computer Science May, 1994 UNIVERSITY OF SOUTHAMPTON ABSTRACT FACULTY OF ENGINEERING AND APPLIED SCIENCE DEPARTMENT OF ELECTRONICS AND COMPUTER SCIENCE Doctor of Philosophy Extending an Open Hypermedia System to a Distributed Environment by G.J. Hill The hypermedia approach to the structuring information has received a great deal of attention in recent years. Many authoring systems exist to aid in the creation of hypermedia documents, and many commercial applications utilise hypermedia techniques to implement facilities such as online help. However, these 'closed' systems are typically based upon proprietary formats and thus have limited wider applicability. This has led to the creation of so-called 'open' hypermedia systems which are able to offer an underlying, extensible hypermedia service to a wide range of applications. This thesis describes the filter-based architecture of Microcosm, an open hypermedia system developed at the University of Southampton. The functionality of the system is provided in a modular manner, allowing its facilities to be easily adapted to the requirements of the users of the system, and offering a stable platform for the investigation of novel hypermedia functionality. The extension of this model to a distributed environment is then described. The resulting system complements the flexibility of the stand-alone model with a flexible approach to the provision of distributed functionality. This allows a range of network models to be supported, from traditional client/server interaction to cooperation between users. The system is also able to support the integration of other existing distributed information services. Contents List of Figures vi List of Tables ^ Acknowledgements 1 1. Introduction 2 1.1. The Concept of Hypertext 2 1.2. Recent History of Hypertext 3 1.3. The Growth of Electronic Information Delivery 4 1.4. Overview of the Work in this Thesis 5 1.5. Declaration 7 2. The Open Approach to Hypermedia 8 2.1. Why we Need Open Hypermedia Systems 8 2.2. The First Visions 9 2.3. Defining Open Hypermedia Systems 12 2.3.1. Broadening Hypermedia Principles 12 2.3.2. Hypertext Interchange 13 2.3.3. Industrial Strength Hypermedia 14 2.3.4. Seven Issues Revisited 18 2.4. Hypermedia Standardisation 21 2.4.1. Dexter Reference Model 21 2.4.2. HyTime 22 2.4.3. MHEG Standard 23 2.5. Summary 23 3. Existing Open Hypermedia Systems 26 3.1. Introduction 26 3.2. Notecards 26 3.3. Intermedia 27 3.4. Sun's Link Service 28 3.5. Virtual Notebook System 30 3.6. HyperBase 31 3.7. PROXHY 32 3.8. Hyper-G 34 3.9. DeVise Hypermedia 36 3.10. Multicard 37 3.11. Hyperform 39 3.12. Summary 40 4. Distributed Information Systems 45 u 4.1. Introduction 45 4.2. The Dream of a Global Information Network 45 4.3. A New Generation of Distributed Information Servers 47 4.3.1. Wide Area Information Server (WAIS) 48 4.3.2. Gopher 49 4.3.3. Prospero 50 4.3.4. Other Systems 51 4.4. Hypermedia Systems with Restricted Distribution 52 4.4.1. HAM/Neptune 52 4.4.2. Knowledge Management System 53 4.4.3. Intermedia 54 4.4.4. HyperForm 54 4.4.5. Virtual Notebook System 54 4.4.6. Sun's Link Service 55 4.4.7. Summary 56 4.5. Truly Distributed Hypermedia Systems 57 4.5.1. World Wide Web 57 4.5.2. Hyper-G. 60 4.5.3. DHT 61 4.5.4. Summary 62 4.6. Conclusions 63 5. The Microcosm Framework 65 5.1. The Aims of Microcosm 65 5.2. Design History 66 5.3. Development of Microcosm 2.0 69 5.3.1. Loosely Coupled Independent Processes 69 5.3.2. Document Control System 74 5.3.3. Filter Management System 75 5.3.4. Document Viewers and Filters 75 5.3.5. Docuverse 78 5.3.6. System Configuration 79 5.4. Microcosm in Action 81 5.4.1. Open Hypermedia as an Educational Resource 82 5.4.2. Open Hypermedia as the Basis of an Integrated Environment 84 5.5. Summary 86 6. Filters and Filter Management 87 6.1. The Role of the FMS 87 m 6.2. Building a Hypermedia System with Filters 88 6.3. Simple Filter Management 92 6.3.1. Design of the FMS 92 6.3.2. Implementation and Refinement 94 6.3.3. Limitations of the Filter Chain Topology 95 6.4. Summary 104 7. Advanced Filter Management 105 7.1. Introduction 105 7.2. Active Filter Management Methods 106 7.3. Initial Implementation 108 7.3.1. Filter/FMS Interface 108 7.3.2. Message Routing 110 7.3.3. Other Aspects 113 7.4. Analysis of Performance 116 7.5. Summary 122 8. A Flexible Architecture for Distributed Hypermedia Systems 123 8.1. Introduction 123 8.2. Requirements for an Open, Distributed Hypermedia System 124 8.3. Design of the Microcosm Distributed Model 125 8.3.1. Basic Elements of the Distributed Version of Microcosm 125 8.3.2. Flexible Distribution Using Filters 126 8.3.3. Extending the Microcosm Docuverse 129 8.3.4. Providing an Open, Heterogeneous System 130 8.4. Implementation Details 131 8.4.1. Extending the Active FMS 131 8.4.2. Extending the Microcosm Docuverse 140 8.4.3. Improving Performance 142 8.4.4. Possible Improvements 143 8.5. System Testing 144 8.5.1. Performance Issues 144 8.5.2. Heterogeneous Interactions 148 8.6. Summary 149 9. Future Work 150 9.1. Development based on the Active FMS 150 9.1.1. Additional Automation of Filter Configuration 150 9.1.2. Automatic Menus 151 9.1.3. Access to Filter Configuration Details 152 IV 9.1.4. Improved Message Passing 152 9.2. Extensions to the Distributed Microcosm system 153 9.2.1. Refining the Distributed Filter Model 153 9.2.2. Computer Supported Collaborative Work 154 9.2.3. New Network Facilities 154 9.3. Future Open Hypermedia Issues 156 10. Conclusions 158 10.1. The Requirement for Open Hypermedia Systems 158 10.2. The Provision of Distributed Hypermedia Facilities 160 10.3. An Integrated Information Environment 161 A. Distributed Message Formats 163 A.l Introduction 163 A.2. Inter-FMS Messages 163 A.2.1. Locating Microcosm Servers 163 A.2.2. Obtaining Details of Published Filters 163 A.2.3. Connecting to a Remote Filter 164 A.2.4. Transferring Microcosm Messages 165 A.3. Inter-Docuverse Messages 165 Bibliography 167 V List of Figures Figure 1.1: Negroponte's Teething Rings 5 Figure 2.1: The Layers of the Dexter Hypertext Reference Model 22 Figure 3.1: Architecture of Sun's Link Service 29 Figure 3.2: The architecture of the PROXHY model 32 Figure 4.1: The growth of the Internet by network over the past 2.5 years 46 Figure 4.2: General architecture of distributed information services 47 Figure 4.3: Distributed Hypermedia Architecture proposed for Hyper-G 61 Figure 5.1: The Architecture of the Microcosm Version 1.0 Prototype 67 Figure 5.2: Microcosm 2.0 System Model 69 Figure 5.3: A Typical Microcosm Message 71 Figure 5.4: The configuration file architecture used by Microcosm 80 Figure 6.1: Filter Management System configuration dialogue box 88 Figure 6.2: The Microcosm messages understood by basic filter processes 90 Figure 6.3: The initial dual filter chain design of the simple Filter Management System 93 Figure 6.4: The inefficient routing of 'Dispatch' messages through Linkbase filters which do not understand them 97 Figure 6.5: Graph of redundant transfers of Dispatch' messages in the simple, chain-based FMS as the number of link sources increases 98 Figure 6.6: Graph of response time for the chain-based FMS with a simple filter configuration of one Linkbase and one Available Links filter 100 Figure 6.7: Graph of response time for the chain-based FMS when redundant filters are added before the Linkbase and Available Links filters 101 VI Figure 6.8; Graph of response time for the chain-based FMS when redundant filters are added between the Linkbase filter and the Available Links filter...102 Figure 6.9: Graph of response time for the chain-based FMS when multiple Linkbases all locate a certain number of links 103 Figure 7.1: Logical message flow in the passive FMS when a user requests a 'Follow Link' action 107 Figure 7.2: Logical message flow in the active FMS when a 'Follow Link' message is received 108 Figure 7.3: The new filter connection process used by the active FMS 110 Figure 7.4: Conceptual model of relationship between the data structures used by the Active FMS to store filter details 112 Figure 7.5: Configuration dialogue for the active Filter Management System 114 Figure 7.6: The improved user interface of the active Filter Management System.116 Figure 7.7: Comparison of the system response time of the active and passive Filter Management Systems 117 Figure 7.8: Constant system response time for active Filter Management System with redundant filters
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