An introduction to the area of open hypermedia and a description of three influential open hypermedia systems

Overview

Open Hypermedia
- What is Open Hypermedia?
- Microcosm
- Devise Hypermedia
- Chimera
Conclusion

Meyrowitz' Missing Link

Observation: hypermedia has not become a widespread commercial success
1. hypermedia is certainly not available on every desktop computer (1988-89)
2. while there are fairly popular systems such as HyperCard, they are primarily used to create help documents and other specialized hypertexts
3. hypermedia has failed to penetrate as a general structuring tool
Problem: Existing systems are monolithic
- users must abandon their known applications and move wholesale to the hypermedia world
- which they, of course, are a little hesitant about
Observation: Copy/paste was an exotic feature a few years ago found only on experimental Xerox PARC systems
Today (1989) copy/paste is everywhere – users expect it, and it is now a service supported by the operating system
Thus: hypermedia functionality must be made available as a simple operating system service, so that developers can easily integrate it into their applications, making it available to users

What is Open Hypermedia?

The essence is the integration of third party applications:

Any system that is intended to integrate applications will need to be an open hypertext system, in the sense that it will need to be extensible, tailorable and possess some published interface whereby content viewers may communicate with the link service.

(Davis et al.)

Integration of applications

Open hypermedia acts as "glue" between programs not designed to work together

Open hypermedia enables (to a higher or lesser degree) linking into documents not designed for hypermedia

Consequence

Open hypermedia systems store links outside of documents

Open hypermedia systems must be flexible and extensible

Open hypermedia system development is the art of the possible

Overview

Open Hypermedia
- What is Open Hypermedia?
- Microcosm
- Devise Hypermedia
- Chimera
Conclusion

Microcosm

Developed at University of Southampton
- product version distributed by Multicosm ltd. (later became Active Navigation)
Integration of real third-party applications
- minimal entry costs for applications
New link concepts – "Ending the tyranny of the Button"
- 'generic' and 'local' links
Many use experiences
- education
- industry (CAD)
Several platforms
- primarily Windows but also Unix and Mac

Basic challenges (for all open hypermedia systems)

The open hypermedia system (OHS) must or should be able to

launch applications and documents
highlight links ("buttons") or destinations in documents
receive commands from the user working with an applications for
- following links
- creating links
- creating anchors
- ...
and when doing so extract relevant information from the application

Ideally,

the integration should fit naturally within the application
the OHS should stay in the background

So the OHS will be communicating a lot with various, different applications

applications most likely not made to communicate with the OHS...

Microcosm – level of integration

Tailor Made Viewers

specialized application – completely integrated with OHS

Source Code Adaptation

hypermedia primitives (anchors, links) and OHS communication can be added

Object Oriented Re-Use

? just like Tailor Made Viewers (but with inheritance)

Application Interface Level Adaptation

the application can be programmed through an API or a macro language

Shim or Proxy Programs

the application is controlled through a small program that talks to the OHS ("Universal Viewer")

Launch Only Viewers

no communication possible. Can only be used to show whole documents

Microcosm architecture

Very extensible design – new filters can easily be added and removed
Microcosm must launch new or direct old applications as needed

A Microcosm link from a linkbase file

\SourceFile 100.02.24.93.11.39.54
\SourceSelection SPC
\SourceOffset 214
\SourceDocType TEXT
\DestFile 100.02.24.93.12.40.49
\DestSelection manual
\DestOffset 312
\DestDocType TEXT
\Description SPC Definition

This is a fully specified link (a "specific link")
The exact locations of both the source and the destination are known
Anchors located through selection and offsets ("counting")
- an easily broken anchoring mechanism – must be maintained

Link types in Microcosm

Specific link

from an object at a specific location in a source document to an object at a specific location in a destination document

Local Link

from an object at an arbitrary location in a source document to an object at a specific location in a destination document

Generic Link

from an object at an arbitrary location in an arbitrary source document to an object at a specific location in a destination document
("object" is typically a text string)

Text retrieval links – computed destination

search for string match across registered documents
lookup in inverted indexes and computation of "similarity coefficient"

Image links

A Specific Link

\SourceFile 100.02.24.93.11.39.54
\SourceSelection SPC
\SourceOffset 214
\SourceDocType TEXT
\DestFile 100.02.24.93.12.40.49
\DestSelection manual
\DestOffset 312
\DestDocType TEXT
\Description SPC Definition

Specific link

from an object at a specific location in a source document to an object at a specific location in a destination document

A Local Link

\SourceFile 100.02.24.93.11.39.54
\SourceSelection SPC
\SourceDocType TEXT
\DestFile 100.02.24.93.12.40.49
\DestSelection manual
\DestOffset 312
\DestDocType TEXT
\Description SPC Definition

Local Link

from an object at an arbitrary location in a source document to an object at a specific location in a destination document

A Generic Link

\SourceSelection SPC
\SourceDocType TEXT
\DestFile 100.02.24.93.12.40.49
\DestSelection manual
\DestOffset 312
\DestDocType TEXT
\Description SPC Definition

Generic Link

from an object at an arbitrary location in an arbitrary source document to an object at a specific location in a destination document

User Interface Consequences

Generic and local links may match many selections across the document

should all instances be highlighted?
can they be highlighted?

Many applications cannot perform the necessary highlighting (also known as "link decoration")

How will the user know where to click?!

Ending the tyranny of the Button

select text
(try to) follow link
maybe there's a link!
maybe there isn't....

User Interface

Closed applications

launch-only for all applications (at the very least)
universal Viewer (Parasite-program)
follow link from applications when copy to clipboard is supported
destinations may be located through searching and scrolling by "remote controlling" the program

Open applications

use of macro language or similar to extend the user interface
hypermedia functionality available directly in the interface
communication of textual messages about links and anchors

Protocol – simple communication tagged messages

messages are interpreted of one or more filters in the chain

Integration based primarily on Generic Links and Local Links using string match

Application Interface Level Adaptation

AutoCAD integration through macro language integration

Shim or Proxy Program: The Universal Viewer

Calendar integration through Universal Viewer

Microcosm – summary

Very comprehensive experience with integration of third party applications
Nice implementation
Automatic generation of generic links a great advantage
Universal Viewer – smart trick
Excellent scenario at the end of the paper

However...

Very text-oriented – problems with anchoring in graphics and similar
Hypermedia model is very simple: 2-ary links, no composites,..
Lacking multi-user architecture

Overview

Open Hypermedia
- What is Open Hypermedia?
- Microcosm
- Devise Hypermedia
- Chimera
Conclusion

Devise Hypermedia

Industrial prototype developed at DAIMI with EU Esprit support from 1991-95
Developed to support supervision engineers at the Great Belt Bridge project
A successor product called WebNize is available from Hypergenic Ltd. <http://www.hypergenic.com/>
Supports the generality of the Dexter model
Based on a tailorable object-oriented Application Framework
Multi-platform concept
Includes multi-user architecture

The Dexter Hypertext Reference Model

An influential formal hypermedia model developed 1988-90 by a number of leading hypermedia researchers
Purpose
- establish a shared vocabulary among hypermedia researchers
- form a baseline for comparison between hypermedia systems
- enable interchange between different hypermedia systems
Pioneered a clear separation between the layers in a hypermedia system and the interaction between these layers
Identified the link as a first class object
- that is, a thing in itself rather that just an attribute of a text selection
Generalized the composite as a container for other components

The Layered Dexter Architecture

The Dexter Hypermedia Model

Component

The parent class of the other components
Anchors that point into its contents
Presentation information defining how the component should be shown to the user

Atomic Component

The basic container of information – known as 'documents', 'nodes', 'frames', etc. in other systems
Wraps a document of some sort

Link Component

Contains a number (at least 2) link specifiers which point to other components and their anchors
Thus, multi-headed links are possible – even links to links

Composite Component

Can contain other components (also composite components)
Only restriction: It cannot directly or indirectly contain itself

The Dexter Architecture in Action

Devise Hypermedia (DHM)

Implements an object oriented design with:

generic classes for all the Dexter concepts
classes organized in an extensible and tailorable framework for hypermedia development

Immediate benefits from the Dexter model:

Separation between Storage (persistent) and Runtime (transient)
Bi-directional links
Multi-headed (n-ary) links
A basic notion of Composites
An interchange format

Devise Hypermedia Architecture

Storage Component in Devise Hypermedia

The object oriented storage framework

The various media and document types are handled through specialized classes

Requirements of Devise Hypermedia

Openness

DHM should be able to handle the applications and document types used

Collaboration

The engineers must be kept abreast of the bridge's progress and changes to its design
Must support different levels of collaboration from decoupled to tightly coupled (What You See Is What I See)

Should be easily extended

Open hypermedia systems exist in a changing world and must be adaptable

Document Work at the Great Belt Bridge

Devise Hypermedia

DHM in action with Excel, MicroStation, Word, and a couple of browsers open

Collaboration support in Devise Hypermedia

Event notification

creation/modification/deletion of object (nodes, links, etc.)
changes of locks on objects
users subscribe to 'notifications'

Session management

a 'session' is a named collaboration between a group of users during a period
join/leave sessions

Long term transactions

temporary locking of objects to prevent overwrite

Basic challenge: How to prevent changes outside of system's control?

documents handled through document management system (Documentor)

Devise Hypermedia

collaboration architecture

Event Notification in Devise Hypermedia

Integration with third party applications

Whole documents from any application may function as link endpoints (launch-only).
Links to parts of documents require that the applications are open to communication via DDE, AppleEvent, etc.
There should be a macro-programming language
- to extend the user interface
- to communicate with the hypermedia service application
The applications' document format remain unmodified
- built in mechanisms like BookMarks, cell-names, and CAD object ID's are used as anchor values
- positions may used in write-protected documents.
The hypermedia client is customized for new applications using object-oriented specialization as seen in the storage framework
An exception-handling mechanism is used to handle deletions of documents and/or portions of documents containing links.
Integrates well with
- Microsoft Word
- Microsoft Excel
- MicroStation (CAD system)

Devise Hypermedia

DHM		Appliation
	`Present (file) → ← New Link (appl, file, selection) ← Add Endpoint (appl, file, selection) ← Follow Link (appl, file, selection) ← Delete Endpoint (appl, file, selection) Present Endpoint (file, selection) → ← Update (contents, anchorValues, pSpecValues) → Open → ← Close →`

Example Implementation: Excel AppleEvent with the following text data: #XCL#DEVISE1#HYPERMEDIA:DEMO:TEST.XL#NL#1#R1C1:R4C5

Devise Hypermedia

excerpt from the Open HM protocol

newLink: 0
- Usage: create an new link with a "source" anchor/endpoint.
- Example: gnuemacs,0,/users/kgronbak/DHTK/docs/interface.tex,1,3
newLinkDest: 1
- Usage: create an new link with a "destination" anchor.
- Example: gnuemacs,1,/users/kgronbak/DHTK/docs/interface.tex,1,4
newLinkBoth: 2
- Usage: create an new link with a "both" anchor.
- Example: gnuemacs,2,/users/kgronbak/DHTK/docs/interface.tex,1,5
addEndPoint: 100
- Usage: add a "destination" anchor/endpoint to the current link.
- Example: gnuemacs,100,/users/kgronbak/DHTK/docs/interface.tex,1,6
addEndPointSou: 101
- Usage: add a "source" anchor to the current link.
- Example: gnuemacs,101,/users/kgronbak/DHTK/docs/interface.tex,1,7

Variants of Devise Hypermedia

Assessment

Dexter provided a rich framework to work within
Very rich hypermedia structures
Advanced collaborative support

However...

Complicated – challenging to develop for
Relatively few applications integrated compared to Microcosm

Overview

Open Hypermedia
- What is Open Hypermedia?
- Microcosm
- Devise Hypermedia
- Chimera
Conclusion

Chimera: Overview

Open hypermedia system focused on software engineering concerns
Chimera project started as an offshoot of the Chiron-1 User Interface Development System in (roughly) 1990
First demoed in 1993 and first paper appeared at Hypertext 1994 (ECHT'94); followed by six conference papers and four journal papers
Key contributions: views as first class elements of hypermedia data models and work in data scalability
Key contributors
- Kenneth M. Anderson: Principal Designer and Developer
- E. James Whitehead Jr.: Designer and Developer with respect to application integration issues
- Richard N. Taylor: Designer and primary supporter of the work (i.e. Ken and Jim's advisor!)

Chimera: Data Model

Chimera: Data Model in More Detail

Viewer: Editor and/or Browser of Information
Object: Information (typically stored in files; not required, however)
View: The display of an Object by a Viewer
Anchor: Region of Interest on a View
Link: A set of Anchor References (aka Endpoints)
Hyperweb: A set of anchors and links

Chimera: The View

Chimera was first OHS to model views explicitly
- Most systems had the concept implicitly;
Explicit modeling of views enables the creation of view-specific anchors
- Especially useful when a single object can be viewed multiple ways (e.g. Spreadsheets)
It also takes the place of “whole document anchors”; indeed most versions of Chimera allowed links to be a set of anchor and view references

Chimera: Run-Time Architecture

Chimera: Server's Internal Architecture

Chimera: Architecture in More Detail

Architecture supported multiple users working on multiple hyperwebs
- Concurrency control was “pessimistic” on hyperwebs only; Chimera did not have control over individual documents, so needed to rely on operating system support to handle multiple users on same document (not satisfactory)
Process invoker was operating system specific; made use of fork and execve system calls on Unix; never implemented on other platforms
First version of Chimera was implemented in Ada; subsequent versions in Java
Persistent storage for server, changed over the years from flat files to RDBMs to XML files

Chimera: Application Integration

Chimera provided extensive application program interface for use by developers; including a wide range of callbacks that would notify connected applications when webs were open/closed, anchors and links created, etc.
Link creation was handled by the notion of an “active” link
- User would select link in Chimera Server User Interface and “activate” it
- Connected applications then allowed anchors to be added to the active link
  - This allowed applications to not have to deal with link objects; they only had to focus on anchor management
  - This tended to simply application integration efforts
Over lifetime of project, Chimera had many applications either created for it or integrated with it, including
- XEmacs
- vi
- Framemaker
- Microsoft Word
- ArgoUML
- Image Viewer
- Flight simulator
- Sound player
- Video player
- Chimera Shell (cli)
most clients were integrated using wrappers or plug-ins; the rest were built from scratch to be Chimera clients

Chimera Extension Mechanisms

Chimera provided two means for extension and tailoring by developers
- API could be extended via server-side plug-ins
- New link traversal behaviors could be added via server-side plug-ins
  - Examples include link traversals that would open hyperwebs automatically and guided tours

Chimera: In Use

Flight Simulator and FrameMaker integration with Chimera

Chimera: Real-World Use

Chimera was used in a requirements traceability project at Northrop Grumman
Applied to just two subsystems of the avionics flight software for the B-2 Bomber
These two subsystems had over 34,000 pages of source code, documentation and other artifacts
Parsers were written to scan these documents for instances of six link types
These parsers discovered well over 500,000 instances of these links
Modifying Chimera to support this level of scalability involved changes at every level of its architecture

Assessment

History in user interface development led to development of explicit view concept
Focus on software engineering led to large set of integrated clients
Used in real-world situations that led to experience with large-scale open hypermedia structures

However...

Only limited support for cooperation
No explicit support for composites
No explicit support for generic links (or links with computed destinations)

Overview

Open Hypermedia
- What is Open Hypermedia?
- Microcosm
- Devise Hypermedia
- Chimera
Conclusion

A comparison

	Microcosm	Devise Hypermedia	Chimera	World Wide Web
Node	Document	Component	View	HTML Document
Anchor	Selection, offset	Anchor, anchor-id, selection, offset	Anchor and View	"a" tag
Link Marker	None or some highlighting	Highlighted, if possible	Application specific (typically highlighted text or rectangles)	underlined text (controlled by stylesheet)
Link	External (2-ary)	External First Class Object (n-ary)	External First Class Object (n-ary)	Property of "a" tag (unary pointer)
Node Groups	None	Composites	None	None
Link Groups	Linkbases	OO-Linkbases	Hyperweb	None
Dynamic Link Behavior	Yes: Filter-defined	Yes: Can be added through framework	Yes: via link traversal extension mechanism	Some: via Javascript
Collaboration Support	No	Yes	Limited; access control on top of hyperwebs	Some through WebDAV

Summary

Open hypermedia provides users with tools to link their work together

focus on the practice and tools employed by the users

Developing open hypermedia systems is difficult because of third party application integration

applications may not be suitable for integration
different levels of integration in different applications
uneven user experience (major problem: skills do not necessarily transfer from one application to the next)
if the application changes, integration may have to start from scratch

Concomitant with the emerging Web

the Web is the monolithic hypermedia system that succeeded where all others failed
today, (nearly) all applications are integrated with the Web
but we are still lacking most of the sophisticated linking and collaborative features of OHS
on the other hand, we have a wealth of information and resources readily available on the Web
although many Web servers are accessible only on intranets