Transcript open hypermedia system

From Web to Workplace
chapters 4, 18
Martin Čadík
Czech Technical University in Prague, Czech Republic
department of computer science
and engineering
Content
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ch. 4: Locations, Placements, Interconnections
– embedded locators
– Dexter group’s proposal – external links
– new approach supporting locations, placements, interconnections
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ch. 18: Open Hypermedia and the WWW
– examples of existing systems
– open hypermedia service for the web using Dexter-based
architecture
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Locations, Placements, Interconnections
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open hypermedia goal: interconnecting of thirdparty applications
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locator: address or pointer that identifies points or
regions of material
placement: data structure that associate access
to hypermedia structures with a position or span of
a material
interconnecting: manner to create traversable
structures
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Links as embedded locators (WWW)
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all information about the link is stored in the
placement and its source end
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less overhead for closed, monolithic systems
(Augment, KMS, HyperCard), than f.e. separate
link database
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BUT not for an open hypermedia system
go to
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External Links (Dexter group)
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links interconnect block objects that manage the
link's endpoint connections
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=>links are stored and accessed separately from
the content of the linked components
Anchor object
Link object
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Pros and Cons
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Embedded locators
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External links
– more complicated (specialized link
server)
+ user can maintain its own links
+ links can be traced (database
query)
+ links can be named and organised
+ links can have more than one target
– only owner can create links
– links to specific parts of
document can be created only if
target tags exist
– impossible to see which
documents point to another
documents
– only one set of links from a
given documents (different
users can't have different links)
– unidirectional
+ simplicity
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Anchoring (Dexter model)
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anchor: identifier + anchor value (place or region)
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reference objects for links, encapsulating location
information
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can refer to:
segments of sound or video
areas of pictures
rows in relational database
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refSpecs and locSpecs
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Dexter's group: identifying/accessing component
resolver: identifying a component
takes a specification, returns unique identifier
accessor: accessing a component
retrieves the component with the given identifier
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New forms of specification: refSpecs, locSpecs
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Motivation:
– use of directionality to model embeddedness
– inconsistent treatment of entities that refer in the Dexter model
(support for computation only with link endpoints)
– uniform means of specification, portability (URL like)
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refSpecs and locSpecs (cont.)
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target: computability also for anchors, composites
=> two new entities:
locSpecs – location specifications
– “contextual links”
– URL like + attributes
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refSpecs – reference specifications
– “independent links”
– uniform means of packaging a locSpec for use within hypermedia
system
– foundation of hypermedia structuring
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Open Hypermedia and the World Wide Web
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web: significant limitations (dynamic link creation
and sharing..)
=> augmenting the web with Dexter-based
hypermedia services
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existing systems with hypermedia support:
– Walden paths, Footsteps, ComMentor,
– Hyperwave, Distributed Link Service (DLS) – support for
nonembedded links to web pages
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HyperWave & DLS
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HyperWave (Hyper-G)
–
–
–
–
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hierarchical navigation, linking, attribute/content search
external hypermedia structures stored in a database
proprietary SGML document format HTF
weakness: special-purpose browsers, special document format, no
collaboration support
Distributed Link Service (DLS)
–
–
–
–
based on Microcosm’s open hypermedia system
generic, local and specific links stored in link bases – called filters
available through a pop-up menu in the window title bar
weakness: low-level patching of a browser => platformdependence, no collaboration support
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Augmenting the web with hypermedia
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Dexter-based hypermedia
service
Architectural integration
WWW
server
HTTP Applet with
OLE
hypermedia
user interface
WWW
browser
Application Layer
CGI
scripts
– applets (Java) – to gain access
to external links
Communication Layer
Application
interfaces
Runtime Layer
Storage Layer
(conceptual)
Storage Layer
(physical)
Hypermedia
service
process
HSP
Storage
classes
Hypermedia
Database
server
HDB
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Augmenting the web with hypermedia
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Application layer integration
– platform independent extension to a web browser
=> Java applet with several threaded lightweight processes
+ JavaScript – browser calls the applet when following links
– CGI scripts – communication between the applet and HSP
or direct TCP/IP communication or RMI…
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Client-side challenges:
– present new links as naturally as possible
=> links resemble ordinary links, use different color
– maintain control over the link presentation
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Collaborative work on web materials
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User should be able to:
–
–
–
–
create links to and from documents without write access
follow both ordinary and DHM-based links
structure web documents, use guided tours
obtain CSCW support (lock exchange, awareness notification…)
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Users registered with a shared HSP
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HTML editor with open hypermedia applet
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From Web to Workplace
Thank you
for your attention
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