Dale, Jonathan

DeRoure, David C.

Goose, Stuart

Hall, Wendy

Hill, Gary J.

English

Southampton (e-Prints Soton)

UNIVERSITY OF SOUTHAMPTONDEPARTMENT OF ELECTRONICS AND COMPUTER SCIENCEAN ARCHITECTURE TO SUPPORTAN OPEN DISTRIBUTED HYPERMEDIA SYSTEMStuart Goose, Jonathan Dale, Gary J. Hill,David C. DeRoure and Wendy HallUniversity of SouthamptonECS Research Journal November 19951An Architecture to Support an Open Distributed Hypermedia SystemStuart Goose, Jonathan Dale, Gary J. Hill, David C. DeRoure and Wendy HallMultimedia Research GroupDepartment of Electronics and Computer ScienceUniversity of SouthamptonUK{sg93r, jd94r, gjh, dder, wh}@ecs.soton.ac.uk1. IntroductionIt is now widely accepted that hypertext functionality can perform a useful role as an underlyingcomponent, or “link service” of an information system. This type of system, which is closer to theoriginal perception of hypertext envisioned by Nelson [5], has been described by various authors [6,1],and the applicability of such an approach illustrated [4].The majority of hypermedia systems have failed to address the needs of information sharing andinterchange on a global scale. As a consequence, users of current systems have been conditioned tooperate in isolation, thus creating an archipelago of information islands.Contemporary systems, such as the World Wide Web (WWW), have illustrated how crucial globalaccess to information is to the Internet community. For many, the WWW also served as an introductionto the concept of hypermedia; demonstrating its potential in providing an integrated and unifyingparadigm for information organisation. As demands upon the WWW grew, it became apparent that thesimplicities in its hypertext model were proving inadequate as the requirements moved from pureinformation access to incorporate information management, discovery and navigation. The WWWcommunity are now looking towards open hypermedia strategies to help solve these problems.In general, Microcosm [2] meets the requirements of an open hypermedia system, but is weak in itssupport for distributed operation, although Hill [3] describes initial experimentation with a networkedversion of Microcosm. This article outlines significant advances on Hill’s contribution through theprovision of a heterogeneous and distributed framework, above which a new open hypermedia systemhas been layered. The new system retains the Microcosm philosophy but offers greater scaleability,improved process management and more efficient communication. We anticipate that users will benefitfrom the superior model for open hypermedia that this widely distributed version of Microcosmprovides.2. A Distributed Hypermedia ModelAs has been mentioned previously, one of the major limitations of the WWW is the simplicity of itshypertext model. To take into account distributed information exchange and management, a newhypermedia model is required to encompass the additional functionality. This model must allow usersto share data easily, whilst maintaining the structure of the information in such a way that differentinterpretations can be imposed upon it in different contexts.To facilitate such a model, the concept of a hypermedia application (figure 1) has been introduced. Thisprovides mechanisms for encapsulating closely related documents and limiting the scope of a resourcebase to a defined and manageable entity. A hypermedia application, then, is the binding of a suite ofprocesses (with their associated configuration), a collection of documents and an arbitrary amount oflink data. Due to their modular nature, applications exhibit the properties of transitivity and may, inturn, comprise many other applications.A session is a collection of hypermedia applications. By allowing users to build multiple sessions, theycan import hypermedia applications from remote and disparate sources to form larger logical2information spaces that are possibly related to their interests or work areas.Additionally, to accommodate user-level access control, an application has ownership and a set of accesspermissions. Application owners can publish them, allowing other users to access them according totheir permission fields. The binding between applications and their sessions are as loose or as tight asthe permissions fields specify.3. Distributed CommunicationWilkins and Heath [7] propose a scaleable message passing solution, called the Direct CommunicationModel (DCM), above which Microcosm could be layered. It provides several mechanisms by which peerentities executing on a single machine may communicate with one another. The new model adapts theDCM and enhances it to function effectively within a distributed heterogeneous multi-userenvironment.For direct communication to take place one process must be able to uniquely identify or address theprocess it wishes to communicate with. This rudimentary requirement gave rise to a novel processaddressing scheme using a familiar metaphor borrowed from computer filing systems.The process addressing scheme forms the foundation upon which the support for message passingbetween processes executing on different machines is based. The first major benefit of the addressingscheme is that it can be customised by the architects of the host system to reflect the context in which itoperates. This means that a process can send a message identifying the intended recipients using termsthat have meaning within that context. The second key benefit is the wildcard attribute, which aids aprocess in disseminating a message to a wider audience. By placing a wildcard in one or more positionswithin the destination process address, a message can be targeted at a specific community of processes.For example, one host system may require all process registrations to conform to the following customaddressing template: /ProcName/UniqueProcId/Document/ServiceUsing this template, two link database processes could each register two service entries in the followingstyle: /linkbase/152.78.64.64.13/lion.raw/follow.link/linkbase/152.78.64.64.13/lion.raw/create.link/linkbase/152.78.64.48.11/tiger.raw/follow.link/linkbase/152.78.64.48.11/tiger.raw/create.linkA viewer process could then direct a message to all link database processes that service follow linkrequests by quoting the following destination process address, where asterisk represents a wildcard: /linkbase/*/*/follow.link4. Architecture of the Heterogeneous Communication ModelThe router (figure 2) acts as a bureau where processes, using the addressing scheme as the vehicle, candynamically advertise and withdraw their services and also post messages to other registered serviceproviders. A process manager is also started with each session to govern load balancing across thenetwork and the distributed invocation of processes. Via its user interface, processes can be remotelyconfigured and managed.Figure 1 : Application Encapsulation and Information Exchange between UsersUser 2Session 2Application5Application6User 1Session 1Application1                Session 2Application2                             Session 1Application4Application33For the router and participating processes to dynamically configure their network connections, asupporting piece of infrastructure proved necessary. A single daemon process to serve the local networkdomain1, providing the sole fixed point of contact, helped achieve this. A crucial characteristic of a scaleable architecture is that the addition of processes to the system hasminimal impact upon efficiency. Another vital scaleability issue specific to the communicationarchitecture is that the number of intermediate steps travelled by a message remains constant regardlessof the number of processes present within the system. Both of these properties are exhibited by theHCM; the latter can be appreciated in figure 2, as all of the processes satellite the router, resulting in amessage only ever travelling two steps to its destination.Interactions between user sessions are also illustrated in figure 2. Here, user 1 in domain 1 initiates anaction causing the daemon of domain 2 to be contacted, requesting a connection between their routerand the router belonging to user 2. This currently means that the daemon in domain 1 is aware of thepresence of the daemon in domain 2. Once a virtual connection has been established, negotiationbetween the two routers occurs. The process registration entries declared as published (that is, servicesthat are available to other users) are then exchanged between the two routers. This activity alleviatessuperfluous message passing between routers. The scaleable framework for providing widelydistributed, peer-to-peer communication can now be exploited.5. Microcosm: The Next Generation PrototypeThe Microcosm TNG prototype developed using the HCM has taken the core components of Microcosmand modelled them as discrete, distributed processes. These processes (linkbases, viewers, availablelinks, linkers, docuverses, etc.) are described in terms of the messages that they send and receive and theprocessing that they perform. Processes are considered to be peer entities due to the fact that they cancommunicate with other processes asynchronously. The distribution of both processes and messages ishandled by the HCM subsystem. In this way, the naming of a process is kept independent to the locationof the process.Resources are distributed through the application entity to which they belong. Once a user has authoredan application, they can publish it to the world at large. The Microcosm TNG system provides amechanism for allowing remote applications to be transparently included within a user’s session. Also,since an application can comprise both data and processes, this inclusion not only increases thefunctionality of the user’s session, but also increases their available dataspace; all actions (for example,link queries) will be automatically forwarded to all of the user’s local and remote applications.These concepts are illustrated in figure 3. In this diagram the two users, Jonathan and Stuart, both havesessions running in their respective domains. In a strict local session, all link queries and link creationsonly apply to the applications that are bound to a particular session (for Jonathan, this is the “Tigers”1 A domain is an administrative term that is used to describe a set of logical machines where HCM processes can execute.Figure 2 : Widely Distributed Multi-User EnvironmentDomain 1User 1Session 1RouterProcess2Process1DaemonDomain 2User 2DaemonSession 2RouterProcess3Process4ProcessManagerProcessManager4application). Jonathan and Stuart can create their applications in isolation, importing documents andmedia, and making links between them.However, imagine that Stuart wishes to create a new application, called “Cats”, which is composed ofboth the “Tigers” and “Lions” applications. This new application exists in his local session and he makesa connection to the “Tigers” application through Jonathan’s domain address (which he obtainedpreviously through an agent, for example). Once connected, Stuart has access to both applications andhe can create links between them and import new documents as appropriate.Upon completion, the “Cats” application can then be published for other people to connect to and usedin a similar fashion as indicated previously. It is important to note that when users access thisapplication, connection to the “Tigers” and “Lions” applications and hypertext links are handledtransparently.This prototype has demonstrated that distributing an open hypertext system above the HCM showsgreat promise for the future. The implementation has demonstrated wide-scale distribution of users,data and processing, open hypertext functionality and integration capabilities for co-operative workingenvironments. The next phase of development will reinforce and extend the system described here toproduce an industrial-strength, distributed open hypertext system.6. Future WorkThe HCM was designed to accommodate collaboration and as such broadcasts notification events uponusers beginning and terminating sessions. This information was subsequently used to build anawareness utility, showing users within the domain. But current research examining the potential ofMicrocosm to support advanced CSCW features will undoubtedly impact upon the future developmentof the Microcosm TNG system.When disclosing resources for remote users to peruse, security becomes a genuine consideration.Authentication mechanisms are also required if users are to be charged for the time/resources they use.Preliminary work in this area has been conducted but further effort is required to develop these ideas.Wilkins et al. [8] discuss how communities of co-operating intelligent agents can greatly assist with avariety of tasks within Microcosm. The role of agents within an open hypertext system can be sub-divided into three categories:• Resource location and discovery.• Maintaining information integrity.• Navigation assistance.It has been recognised that the HCM and Microcosm TNG provide an ideal framework for furtherexperimentation with agent technology and the Internet.Figure 3 : Instances of the Microcosm TNG Prototype Communicating across a Widely Distributed EnvironmentDomain 2DaemonStuart’s Session"Lions"ApplicationLinkbaseRouterLinker AvailLinksDocu-verseDomain 1DaemonJonathan’s Session"Tigers"ApplicationRouterLinker AvailLinksDocu-verseLinkbaseViewer"Cats"Appli-cationDocu-verseLinkbaseProcessManagerProcessManager57. ConclusionsThe client/server model, the predominant architecture among systems using the Internet, can restrictthe development of truly distributed systems. This paradigm makes it very difficult for currentinformation services to take the initiative in delivering fresh information to the user. Furthermore, someprocesses need to act as both client and server depending upon whom they are interacting with. Theopen model of the HCM allows systems to benefit from a more flexible peer-to-peer based alternative.The rapid development of the HCM-based hypertext system described in this paper illustrates the easewith which the HCM allows distributed systems to be built, and as such enables Microcosm TNG toprovide a degree of flexibility not found in other distributed hypertext systems. As one would expect,collaboration between users is promoted together with strong support for the sharing of information. Inparticular, the ability to encapsulate a group of distributed processes with their respective resources andpublish them as a single entity provides a welcome degree of abstraction. The hypertext applicationconceals configuration details and provides a scaleable mechanism for building and composing newapplications.References[1] DAVIS, H. C., HALL, W., HEATH, I., HILL, G. J. and WILKINS, R. J., Towards an IntegratedInformation Environment with Open Hypermedia Systems. In: Lucarella, D., Nanard, J., Nanard,M. and Paolini, P., Eds., ECHT ’92, Proceedings of the Fourth ACM Conference on Hypertext,Milan, Italy (November), ACM Press, pages 181-190, 1992.[2] HILL, G. J., WILKINS, R. J. and HALL, W., Open and Reconfigurable Hypermedia Systems: A FilterBase Model, Hypermedia, 5(2), pages 103-118, 1993.[3] HILL, G. J. and HALL, W., Extending the Microcosm Model to a Distributed Environment. In:ECHT ‘94 Proceedings, Edinburgh, Scotland (September), ACM Press, pages 32-40, 1994.[4] MALCOLM, K. C., POLTROCK, S. E. and SCHULER, D., Industrial Strength Hypermedia:Requirements for a Large Engineering Enterprise. 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An Architecture to Support an Open Distributed Hypermedia System

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