Issue 8: Open hyperdocument and Open hyperlinking architecture

• Design of open hypermedia systems has been discussed by the hypertext community for some time
• Sun's link service
• Englebart's open hyperdocument model
• Hyperlinking: ability to create and traverse associations between any arbitrary information elements.
• Few hypermedia systems can claim a universal link service: the ability to support hyperlinking between any two objects, no matter what the content model, application interface, or computing environment
• This ability requires the system to support the delivery of any document type
• e.g. CORBA, Distributed COM: system services for distributed objects will enlarge the extent of open hypermedia systems
• Distributed object systems combine a general object model with location transparent access to any object. Any application or service can be integrated into a distributed computing environment by defining language-independent interfaces.
• Use content-based addressing -> related to content-based retrieval

• Distributed hypermedia systems form an information structure which useful computation can be performed for users including search, dynamic document creation, event generation, generalization, and discovery
• Efficient performance depends on the available computing and network resources throughout the system
• Integration of the hypermedia systems with distributed computing environments provides a flexible foundation from which such computation can be performed
• Mobile code refers to the ability to relocate applications or applets for execution at any server or client in a distributed system.
• e.g. Java applets
• Mobile obeject systems could be implemented using distributed object systems
• There are a number of related issues in the design of Mobile obeject systems
• Scalability
• Security
• Management
• Services
• Computation Flow
• Integration of mobile code and distributed objects with the WWW by the joint W3C/OMG workshop

• Ability to represent and deliver hypermedia documents which include rich interactive time-based multimedia presentation
• Integrating a multimedia document model with a hypermedia systems increase the representational power of the system and the generality of the system design.
• Multimedia Models: Amsterdam Hypermedia Model (AHM), HyTime

• SGML-based metalanguage for designing multimedia document models.
• Main modeling abstractions: hyperlinks, location addressing (anchoring), N-dimensional spaces called finite coordinate spaces, transformations between spaces (projection and rendering), time/space composition
• Has HyQ--a document query language
• HyQ can be used to form dynamic links and to define the content of a ducument by a search for document property or attribute
• Address some of Halasz issues--search and query functionality, virtual structures over node collections, and computation over hypermedia networks.
• Provides many different addressing techniques that can be used to form an anchor
• Addressing attribute: name space, data, node, property, bibliographic

• Today's distributed hypermedia systems have simplistic use of multimedia data types because of limited semantics of document models and lack of underlying systems support for delivery of multimedia information
• Once suitable multimedia system services are available, it will be possible to support real-time scalable delivery of hypermedia content
-->more efficient and responsive delivery ->reduce cost of resource management
• Resource management in distributed multimedia systems involve resource reservation based on the application's required quality of service
• The system provides an orchestration service which manages the resource allocation and monitoring in a distributed environment.
• Unit of systems resource allocation is called a session and created per-media basis.
• In hypermedia systems, resource requirements depend on characteristics of the document being presented.
• User's navigation through a collection of documents may cause frequent changes to the resource requirements.
• Concept of a hypermedia session (HM) which extends the concept of a continuous media (CM) session used in various models of CM scheduler
• Hypermedia session is an aggregation of a number of continuous and discrete media sessions for a given hyperdocument.
• Hypermedia session can be used by an application to describe the presentation and delivery requirements of a hypermedia document
• Hypermedia application constructs the description of the hypermedia session which it wants the orchestration layer to perform resource management for.
• Orchestration layer uses this session description to request initial allocation of session resources form the disk server and other resource managers

Figure: Hypermedia document and meta-information database for orchestration layer

Issue 11: Security

• Transaction security
• Safety of mobile objects
• The requirements for security in distributed hypermedia systems are varied because of the wide range of activities for which the system might be used
• Confidentiality
• Intellectual property rights
• Authentication
• Privacy
• Access rights
• Transaction integrity
• Secure communication
• Management
• e.g. OMG's CORBAServices defines a security model for both intraORB and interORB environment. The specification includes a security reference model, a security architecture, security interface for applications and system designers