department of computer science and engineering From Web to Workplace chapters 4, 18 Martin Čadík Czech Technical University in Prague, Czech Republic

May 13, 2003 (2) department of computer science and engineering Content ch. 4: Locations, Placements, Interconnections ch. 4: Locations, Placements, Interconnections –embedded locators –Dexter group’s proposal – external links –new approach supporting locations, placements, interconnections ch. 18: Open Hypermedia and the WWW ch. 18: Open Hypermedia and the WWW –examples of existing systems –open hypermedia service for the web using Dexter-based architecture

May 13, 2003 (3) department of computer science and engineering Locations, Placements, Interconnections open hypermedia goal: interconnecting of third- party applications open hypermedia goal: interconnecting of third- party applications locator: address or pointer that identifies points or regions of material 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 placement: data structure that associate access to hypermedia structures with a position or span of a material interconnecting: manner to create traversable structures interconnecting: manner to create traversable structures

May 13, 2003 (4) department of computer science and engineering Links as embedded locators (WWW) all information about the link is stored in the placement and its source end all information about the link is stored in the placement and its source end less overhead for closed, monolithic systems (Augment, KMS, HyperCard), than f.e. separate link database less overhead for closed, monolithic systems (Augment, KMS, HyperCard), than f.e. separate link database BUT not for an open hypermedia system BUT not for an open hypermedia system go to

May 13, 2003 (5) department of computer science and engineering External Links (Dexter group) links interconnect block objects that manage the link's endpoint connections links interconnect block objects that manage the link's endpoint connections =>links are stored and accessed separately from the content of the linked components =>links are stored and accessed separately from the content of the linked components Link object Anchor object

May 13, 2003 (6) department of computer science and engineering Pros and Cons Embedded locators Embedded locators –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 External links 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

May 13, 2003 (7) department of computer science and engineering Anchoring (Dexter model) anchor: identifier + anchor value (place or region) anchor: identifier + anchor value (place or region) reference objects for links, encapsulating location information reference objects for links, encapsulating location information can refer to: segments of sound or video areas of pictures rows in relational database can refer to: segments of sound or video areas of pictures rows in relational database

May 13, 2003 (8) department of computer science and engineering refSpecs and locSpecs Dexter's group: identifying/accessing component resolver: identifying a component Dexter's group: identifying/accessing component resolver: identifying a component takes a specification, returns unique identifier takes a specification, returns unique identifier accessor: accessing a component accessor: accessing a component retrieves the component with the given identifier retrieves the component with the given identifier New forms of specification: refSpecs, locSpecs New forms of specification: refSpecs, locSpecs Motivation: 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)

May 13, 2003 (9) department of computer science and engineering refSpecs and locSpecs (cont.) target: computability also for anchors, composites => two new entities: target: computability also for anchors, composites => two new entities: locSpecs – location specifications locSpecs – location specifications –“contextual links” –URL like + attributes refSpecs – reference specifications refSpecs – reference specifications –“independent links” –uniform means of packaging a locSpec for use within hypermedia system –foundation of hypermedia structuring

May 13, 2003 (10) department of computer science and engineering Open Hypermedia and the World Wide Web web: significant limitations (dynamic link creation and sharing..) => augmenting the web with Dexter-based hypermedia services web: significant limitations (dynamic link creation and sharing..) => augmenting the web with Dexter-based hypermedia services existing systems with hypermedia support: existing systems with hypermedia support: –Walden paths, Footsteps, ComMentor, –Hyperwave, Distributed Link Service (DLS) – support for nonembedded links to web pages

May 13, 2003 (11) department of computer science and engineering HyperWave & DLS HyperWave (Hyper-G) HyperWave (Hyper-G) –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) 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 => platform- dependence, no collaboration support

May 13, 2003 (12) department of computer science and engineering Augmenting the web with hypermedia Dexter-based hypermedia service Dexter-based hypermedia service Architectural integration Architectural integration –applets (Java) – to gain access to external links WWW server Applet with hypermedia user interface WWW browser HTTPOLE Application Layer Communication Layer CGI scripts Runtime Layer Storage classes Hypermedia service process Application interfaces Storage Layer (conceptual) Storage Layer (physical) Hypermedia Database server HSP HDB

May 13, 2003 (13) department of computer science and engineering Augmenting the web with hypermedia Application layer integration Application layer integration –platform independent extension to a web browser => Java applet with several threaded lightweight processes => Java applet with several threaded lightweight processes + JavaScript – browser calls the applet when following links + JavaScript – browser calls the applet when following links –CGI scripts – communication between the applet and HSP or direct TCP/IP communication or RMI… or direct TCP/IP communication or RMI… Client-side challenges: Client-side challenges: –present new links as naturally as possible => links resemble ordinary links, use different color => links resemble ordinary links, use different color –maintain control over the link presentation

May 13, 2003 (14) department of computer science and engineering Collaborative work on web materials User should be able to: 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…) Users registered with a shared HSP Users registered with a shared HSP HTML editor with open hypermedia applet HTML editor with open hypermedia applet

May 13, 2003 (15) department of computer science and engineering From Web to Workplace Thank you for your attention