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Hypermedia Information Systems Research Laboratory, Electronic Enterprise Engineering Program, New Jersey Institute of Technology, University Heights, Newark, New Jersey 07102, U.S.A. email: bieber@cis.njit.edu
Department of Computer Science, Università di Bologna, Mura Anteo Zamboni, 7, I-40127 Bologna, Italy. email: fabio@cs.unibo.it
HELEN ASHMAN
Information Technology Division, Defence Science and Technology Organisation, P.O. Box 1500, Salisbury, S.A. 5108, Australia. email: helen.ashman@dsto.defence.gov.au
E-Papyrus, Inc., and Graduate School of Management, Rutgers University, Newark, NJ 07102, U.S.A. email: bala@pegasus.rutgers.edu
Department of Information Processing Science, University of Oulu, Linnanmaa, FIN-90570 Oulu, Finland, email: hok@rieska.oulu.fi
World Wide Web authors must cope in a hypermedia environment analogous to second-generation computing languages, building and managing most hypermedia links using simple anchors and single-step navigation. Following this analogy, sophisticated application environments on the World Wide Web will require third- and fourth-generation hypermedia features. Implementing third- and fourth-generation hypermedia involves designing both high- level hypermedia features and the high-level authoring environments system developers build for authors to specify them. We present a set of high-level hypermedia features including typed nodes and links, link attributes, structure-based query, transclusions, warm and hot links, private and public links, hypermedia access permissions, computed personalized links, external link databases, link update mechanisms, overviews, trails, guided tours, backtracking, and history-based navigation. We ground our discussion in the hypermedia research literature, and illustrate each feature both from existing implementations and a running scenario. We also give some direction for implementing these on the World Wide Web and in other information systems.
© 1997 Academic Press Limited
We envision sophisticated application environments on the WWW, which seamlessly integrate fourth-generation hypermedia tools with the application's other functionalities. These could include environments on the Web that would support group decision making and collaborative work (Hiltz & Turoff, 1993; Jessup & Valacich, 1993; Nunamaker, Dennis, Valacich, Vogel & George, 1991; Shackelford, Smith & Smith, 1993; Streitz, Haake, Hannemann, Lemke, Schuler, Schütt & Thüring, 1992), authoring and guiding users through multitask processes (Noll & Scacchi, 1996; Scacchi, 1989; Tanik, Yeh, Bieber, Kurfess, Liu, McHugh, Rana, Rossak & Ng, 1996), educational and distance learning environments (Leidner & Jarvenpaa, 1995; Hiltz, 1993; Benyon, Stone & Woodroffe, 1997; Rana & Bieber, 1997), personal and group organizing tools (Marshall & Shipman, 1995), and enterprise coordination (Engelbart, 1990). While each of these environments has its own core set of functionality, the same types of hypermedia features, such as annotation, overviews, paths and guided tours, would serve users in all of them.
While many people think of hypermedia in terms of the World Wide Web, hypermedia research has been ongoing since the early 1960s when Engelbart developed NLS, a multi-user, distributed hypertext system (Engelbart & English, 1968). Since then, the hypermedia research community has been developing a wealth of features, systems, guidelines, frameworks and theory focusing on structuring, presenting and accessing interrelated information. We encourage builders of Web technology to incorporate high-level hypermedia features, so everyday authors and readers can utilize them. Similarly, we encourage developers of non-Web based systems to consider augmenting their applications with these features. We present several high-level hypermedia constructs which the Web could provide now. We describe each, give examples from research or commercial systems (on and off the Web), and note implementation considerations for each. We describe features in terms of a 3rd and 4th generation authoring environment and illustrate them within a running scenario. In Section 2 we describe the concept of hypermedia. In Section 3 we introduce our scenario and present our set of hypermedia functionalities. Section 4 concludes with some thoughts on how the hypermedia field can learn from the Web community, just as the Web community can learn from the hypermedia field. Our hope is that this paper will launch a major thrust to research and develop both high-level hypermedia features and the high-level authoring environments necessary for application developers, as well as everyday authors and readers, to use them. Furthermore, we hope this effort inspires researchers in other fields to put forth their own visions of sophisticated application environments, high-level features and corresponding high-level authoring environments for the World Wide Web.
A word on our terminology, which comes from the hypermedia research literature. First, hypermedia researchers view the terms hypertext and hypermedia as synonymous and use them interchangeably. Hypermedia nominally applies hypertext concepts to multiple media. Second, we separate users into two categories, readers and authors, in order to highlight how each interacts differently with a hypermedia system. We distinguish authors from the system developers who develop browsing software and other hypermedia environments. Readers traverse links during the act of browsing. Third, we distinguish the underlying link anchor from the manifested link marker displayed on the screen. Link markers visibly indicate a link's presence. Link anchors contain parameters and other internal information, which users do not see. Finally, nodes represent the documents or primary content containers (Halasz and Schwartz, 1994) in a hypermedia system.
In a larger sense, hypermedia increases comprehension (Thüring, Hannemann & Haake, 1995). Through the process of structuring information as an associative network, authors often come to understand that information better. Comprehension also increases through the enriched context that comes from sophisticated navigation support and supplemental relationships. For example, hypermedia encourages authors to provide multiple relationships around a piece of information, which readers can access directly. Thus, for readers, freedom of access within an associative structure enhanced with contextual support provides a rich environment for understanding the information they find.
Conklin (1987) identifies two major dangers of free-formed hypermedia access within an associative network: disorientation and cognitive overhead. Conklin defines disorientation as "the tendency to lose one's sense of location and direction in a nonlinear document," using the expression "lost in space" to describe it. He defines cognitive overhead as "the additional effort and concentration necessary to maintain several tasks or trails at one time." Cognitive overhead refers to the reader's ability to follow links related indirectly to the current reading task (on a purposeful tangent or detour, or by accident), as well as the need to follow several interconnected paths to visit as much of the associative network as necessary. Cognitive theory reminds us of the overhead that comes from needing to choose among multiple links, especially for novices not familiar enough with a domain to decide among these easily (Wurman, 1989).
While many hypermedia researchers believe that disorientation and cognitive overhead are problems inherent in hypermedia, proper implementations of advanced hypermedia features will alleviate these problems, as well as provide readers with a rich information environment. In a recent survey of World Wide Web usage (Pitkow & Kehoe, 1995), only 6.54% of the 23,000 respondents reported problems with "getting lost in hypertext." While this survey was not a balanced, randomly-selected sample of Web users [1], it does suggest that basic Web browser features such as bookmarks and hot lists, backtracking and history lists, and the forward, backward and home buttons provide an adequate set of navigational functions for a large group of experienced Web users. Indeed, the simple backtracking function afforded by Mosaic's back button was shown by Catledge and Pitkow (1995) to be one of the most frequently-used navigation actions. Tauscher and Greenberg (1997) actually conclude that "the only history mechanism used extensively is the Back button."
It appears that experienced readers apparently learn to get by with the available functionality. However the use of Web technology is in part determined by its capabilities. Readers learn to make do with the available tools instead of demanding better tools, perhaps because they are not aware of how better tools might help them. Also, Web browsers deal traditionally with conceptually quite unstructured information, such as prose text, images and videos. Conceptually structured data can only be represented by flattening it into (usually) text, such as the way CGI scripts retrieve relational data from a database and present it in flat text format. Web browsers have no inherent way of presenting the structure and interrelationships of data of any sort. For example, there is no way to visualize even the simple interrelationships of Web documents, such as "Where can I go from here? [2]" or "Which documents point to this document?" The reader has no idea of the position of a given document within the corpora unless an author explicitly embeds such details. Yet such information is very important, as indicated by the predilection of Web page authors to provide tables of contents, explicitly stating document interrelationships.
In summary, Web users may not need features more complex than those listed above, but this paper aims to illustrate that their productivity could be improved greatly by incorporating a range of 3rd and 4th generation hypermedia features to enhance Web applications. Thus, while we do discuss many of the advanced hypermedia features in terms of how they reduce cognitive overhead and disorientation, we motivate each fully by showing how it enhances their Web usage.
WWW applications are not the only target of this paper. Indeed, we claim that even users of non-Web applications could benefit from hypermedia features. Despite the rush to place information on the WWW, many legacy systems and other applications will keep their own interfaces, and will not be moved onto the Internet or an intranet any time soon. Yet their users still would benefit from hypermedia functionality (Davis, Knight & Hall, 1994; Bieber & Isakowitz, 1995) although they are reluctant to abandon their current non-hypermedia oriented systems in favor of hypermedia features. Thus, the myriad of today's personal, scientific and business applications, which were not designed specifically as hypermedia-oriented and do not appear on the Web, should be augmented with hypermedia features. The hypermedia functionality approach focuses on incorporating hypermedia features into software systems so as to provide their users with an associative way of accessing, analyzing and organizing information (Ashman, Balasubramanian, Bieber & Oinas-Kukkonen, 1996, 1997; Oinas-Kukkonen, 1997a).
In summary, the benefits of adding hypermedia functionality to information system applications are that hypermedia provides a contextual, navigational access for viewing the information and that it represents knowledge in a form relatively close to the cognitive organizational structures that people use, thus supporting human understanding.
An insurance company plans to market and sell all insurance-related products (auto, life, homeowners, etc.) to the general public through the WWW. Existing clients will be able to access their accounts on-line. The system also will deliver product-related information to sales agents. The amount of information to be delivered to these three audiences varies based on access privileges.
For example, information to the general public may include descriptions of products, their suitability for people in various circumstances, real-life anecdotes from clients and premiums for different kinds of coverage. Existing clients receive customized promotional information (for example, clients who already own auto insurance policies will find out that they are eligible for discounts on homeowner policies). Clients also have access to account statements. On the other hand, agents will have access to all the information available to the public and the clients; as well as details about commissions and incentives for selling different products; instructions, guidance and tips on selling policies and targeting clients (and potential clients); and internal policies and procedures.
Information can take the form of on-line brochures, instruction kits, application forms and short synopses, and will include interrelationships. Assuming that all this information is produced by different marketing departments for different products, three different flavors for the three different audiences (clients, sales agents and the general public) have to be published for each product. Also, all product marketing information has to be approved by appropriate legal authorities within the company. In addition, the system will provide views. Agents will be able to see exactly what a particular client would see and what the general public would see. Versions of the material presented on-line have to be preserved and remain accessible for regulatory reasons.
Interesting questions from a hypermedia functionality viewpoint include:
Third- and fourth-generation hypermedia encompasses both high-level hypermedia features and the high-level authoring environments system developers build for authors to specify them. Some of these features deal with node and link structure while others concern navigation tools. Many have both structural and navigation aspects. We also differentiate how authors might utilize several features in a third-generation authoring tool versus in a fourth-generation authoring environment.
Feature | Section |
---|---|
typed nodes and links | 3.1 |
link attributes and structure-based query | 3.2 |
transclusions, warm links and hot links | 3.3 |
annotation and public or private links | 3.4 |
computed personalized links | 3.5 |
external link databases and link update mechanisms | 3.6 |
global and local overviews | 3.7 |
trails and guided tours | 3.8 |
backtracking and history-based navigation | 3.9 |
other features | 3.10 |
The gIBIS system (graphical Issue Based Information System) illustrates node and link typing well (Conklin & Begeman, 1989). gIBIS supports argumentation dialogues among a team of software designers, including transcriptions of design decisions and design rationale. gIBIS' conceptual model focuses on the articulation of the key issues in the design process. Each issue can have many positions, where a position is a statement or assertion that resolves it. Each position, in turn, may have one or more arguments that either support it or object to it. The corresponding hypertext model consists of three node types: "issues," "positions" and "arguments." gIBIS' eight link types represent the interrelations among the nodes [3]. Table 2 lists the link types and which node types they interrelate. Together the types create and maintain a semantics for the hypertext network designed specifically to support its domain of argumentation dialogues. QuestMap is a commercialized version of the gIBIS prototype supporting on-line planning and decision making (Corporate Memory Systems, Inc., 1996). QuestMap's interface features a semantic map linking questions, ideas and information about a project. It provides an electronic shared "whiteboard" which teams can use during or between meetings.
Node Type (Source) | Permissible Link Type(s) | Node Type (Destination) |
issue | generalizes, specializes, replaces, questions, is_suggested_by | issue |
issue | is_suggested_by, questions | position |
issue | is_suggested_by, questions | argument |
position | responds_to | issue |
argument | objects_to, supports | position |
In our insurance example, semantic types could distinguish a "request for policy quote form" from a "policy application form." Each node would appear as a different kind of input template. Semantic types could differentiate different types of documents which come through the regular mail, which are imaged and linked to the appropriate portions of the system. These include requests for new policies, claims, accident descriptions, police reports, complaints against agents, complaints against the insurance company, client testimonials, change of address requests, and so on. Semantic types also can express a weight, context or urgency, such as "legal opinion" versus "agent communication."
Advanced Web authoring environments could take advantage of semantic types to declare such templates, i.e., skeleton pieces of applications which authors could declare and instantiate (Catlin, Garrett & Launhardt, 1991; Kaindl & Snaprud, 1991; Malcolm, Poltrock & Schuler, 1991). Templates can combine nodes and links that go together. For example, when a user wishes to comment upon an existing node in gIBIS or QuestMap discussions, the system presents the set of legal link types from that node and the set of legal node types for each link type. Choosing one displays an empty template form to fill in, tailored to that node type. Bieber et al. (1997) briefly describe a Delphi methodology (Turoff & Hiltz, 1995) group consensus system in which authors use hypermedia templates to construct the survey questions.
Most hypermedia systems that provide semantic types, such as SEPIA (Streitz et al., 1992;
Thüring et al., 1995), LIRMM's
MacWeb [4] (Nanard & Nanard, 1995) and gIBIS, display the
types as labels within an overview diagram. Link labels appear next to
the arrows that represent links, while node labels may appear either as
the text of the node or next to the node. In NoteCards, authors
optionally can represent links as boxed labels positioned anywhere
within the node's white space (Halasz, 1988). Few systems embed labels in the
text itself next to the link marker. Although this should be
possible--distinguished in some manner, such as delimited by <angle
brackets>--it could clutter up the node's existing contents.
Alternatively, link labels could pop up when the user moves the mouse
cursor over them (Vitali, Chiu & Bieber, 1997). A hypermedia version of papers
from the August 1995
issue of the Communications of the ACM (Bieber & Isakowitz, 1995) designed for the
current Netscape viewer employs the latter approach (
Some systems display labels to help the reader distinguish among
multiple links from the same link anchor. While Intermedia (Yankelovich, Haan, Meyrowitz &
Drucker, 1988), ACM's Hypertext on Hypertext (Yankelovich, 1988) and
Microcosm (Davis, Hall, Heath, Hill
& Wilkins, 1992) only display labels in pop-up menus (preview
lists) when readers select anchors with multiple links, Max (Bieber & Kimbrough, 1992) always
displays the link type, even for single-destination links, so the reader can choose
whether
to follow it. Microcosm also has this option.
Vitali et al. (1997) propose a simple method called displets for authors to
include pop-up boxes with link labels for multiple links (as well as many other features
that could be specified from within HTML).
A third-generation type of authoring tool would provide authors with a
data structure containing a specific parameter for semantic types, and
the tool would display node and link labels automatically in overview
diagrams and in other content displays. A fourth-generation authoring
environment would add a magnitude of functionality. Authors would be
able to choose from a set of established node and link types, and would
have a high-level interface for creating new ones (or new sets for a
specific group of other authors). The system would associate semantic
types with appropriate browsing semantics (Garzotto, Mainetti & Paolini, 1996;
Schwabe, Rossi & Barbosa, 1996),
i.e., the behavior associated with objects of that semantic type. For
example, gIBIS would enforce its semantic structure. Following Table 2,
authors only would be allowed to attach certain types of nodes to a
particular link, and specific types of links between two particular
nodes. Indeed, authors could just choose from a menu item "I wish to
challenge this position" and the system would automatically create the
correct link and open up the appropriate node type for the author's
comment. In the insurance example, selecting a generic "client-view,"
"public view," or "agent view" link (each automatically attached to all
documents) would generate the appropriate view, assuming the user has
the correct access permissions (see Section
3.4). A client requesting to file a claim would automatically
receive the correct "claim form" node template. Once complete, the form
could follow an established workflow path, mailing itself to the
appropriate claims officer for approval. If the form then must go to
others, it would automatically route itself accordingly. The
fourth-generation environment would allow authors to specify these
behaviors based on node and link types at a very high level.
Textnet (Trigg et al., 1986), SEPIA
(Streitz et al., 1992),
MUCH (Wang & Rada, 1995) and
Debate Browser (Oinas-Kukkonen, 1996) enforce different
sets of link types. Other systems, such as LIRMM's MacWeb, NoteCards and
Linking Ability (Oinas-Kukkonen, 1997b) permit arbitrary
link labeling. MacWeb, as a knowledge-based hypertext development
environment, allows authors to type links, nodes, groups of nodes,
multimedia items within nodes and link anchors. Not only do these give
authors a high degree of semantic specification, but authors can create
scripts based on any of these types. Arbitrary labeling gives authors
more flexibility to express exact meaning, but carries the danger of
inconsistent type names (Rada,
1990). Wang and Rada also point out that authors often do not take
the time to incorporate link types, even when appropriate (Wang et al., 1995), so system designers
providing semantic typing should make them as easy to specify as
possible.
Even when the reader's hypermedia environment does not display node and
link types, Nanard et al. encourage system designers to facilitate
typing as far along the design process as possible, to assist authors in
structuring and organizing their work (Nanard et al., 1995).
Bieber similarly encourages application developers to determine the semantic
relationships within an application as part of the systems analysis and design process,
even if users will not gain direct access to these relationships through links in the final
application (Bieber, 1996).
The hypermedia research literature has produced other link taxonomies
designed purely for the benefit of researchers and system developers,
which systems will never display to readers (Bieber et al., 1997;
DeRose, 1989;
Parunak, 1991a;
Rao & Turoff, 1990). These include categories
such as extensional, inclusive, intensional, implicit and isomorphic (DeRose, 1989). System developers
can use these taxonomies in determining appropriate navigation
strategies and ways of conveying context for different categories of
links underlying fourth-generation environments. Rao et al. also include
a theoretical node taxonomy for the same purpose. In fact, Rao et al.
have suggested a general semantic framework for hypermedia functionality
based on Guilford's Structure of Intellect model. This framework
classifies nodes into six different semantic types and links into twelve
different types. Nodes represent cognitions or ideas. Links represent
the relationships between ideas. Links are further classified into
convergent links and divergent links. Convergent links help in focusing
or narrowing the pattern of relationships between ideas whereas
divergent links expand or broaden them.
Actually, the Web has always had a mechanism for specifying link types,
and has even a set of types, which slowly is becoming an accepted
standard (Maloney,1995).
Few browsers, however, take these into account. The CLASS, REL and REV
attributes of HTML's A and LINK tags allow authors to specify a complex
typology of links, while the TITLE attribute helps attach a text label
to the anchor. The REL attribute specifies the relation with the link's
destination. REL values include support for sequences (NEXT, PREVIOUS),
hierarchy (CHILD, PARENT, TOP), navigation (TOC, INDEX), home-defined
links (HOME, BACK), and common types of relations (CITATION, DEFINITION,
BIBLIOENTRY, FOOTNOTE, AUTHOR, COPYRIGHT, DISCLAIMER, etc.) The REV
attribute contains, where appropriate, the reverse relation (e.g., an
anchor with REL=NEXT will have REV=PREVIOUS). The CLASS attribute
contains free text further describing the link's type (so that it
becomes possible to define several links with REL=NEXT, e.g., each with
a different CLASS--NOVICE, INTERMEDIATE, EXPERT).
Unfortunately, discussions within the HTML standards working group cover
the acceptable syntax and values of these keywords, but make no mention
of implementation or use, leaving these to Web editor and browser
developers to figure out. Web browsers, for example, could display
semantic link types in several different ways, including in pop-up
windows or in separate frames as with the aforementioned
Communications of the ACM issue. Web-based editors could make it
easy for authors to assign a semantic type to their links, both through
pop-up lists of available links and by devoting a special input field in
the link dialog allowing for new type specifications. While authors must
include semantic types as a parameter, systems could infer most
structural information from the HTML/SGML (International Standards Organization, 1986)
layout of the link's source and destination documents.
Vitali et al.'s proposed displets provide support for exact rendering of new HTML
elements (Vitali et al., 1997)). The displets (special Java classes) are called to perform
the display of newly defined HTML tags, thus allowing fine control over appearance of the
document elements, while maintaining the descriptive markup of HTML.
A fourth-generation authoring environment would provide link
customization features. The Linking Ability system enables team managers
to define link subtypes and keywords for each team. Authors then choose
link subtypes from these (Oinas-Kukkonen, 1997b). In our
insurance scenario, an author might want to create an annotation link of
subtype "legal opinion" with keyword "highly important." Admittedly,
defining attributes takes effort, but the payback comes when readers
receive structural information in vast information collections,
distributed environments or application domains, in which they can see
and exploit dependencies among pieces of information.
While many hypermedia systems provide a text string search facility for
node content, they lack a structure-based query based on node and
link attributes (Halasz,
1988; Lucarella,
1990;
Lee, Yoo, Yoon & Berra,
1996;
Maurer, 1996).
Using the insurance company example, a team manager may want to find:
HyTime (Newcomb, Kipp & Newcomb,
1991; DeRose & Durand,
1994)--the hypermedia extension to SGML--contains provisions for
using structure-based queries to determine link endpoints. The first
version of the HyTime standard contains a query language, called HyQ,
which can be used to identify document elements satisfying given
structural properties (such as "the third paragraph of all annotations
created after 3/31/96"). Although a few partial implementations of
HyTime engines exist, the potential of queries as part of location
specification has barely begun to be realized.
Attributed links and structure-based query becomes especially useful in
complex, knowledge-intensive domains that emphasize teamwork, such as
collaborative software design and inspection (Tervonen & Oinas-Kukkonen, 1996). As Tervonen, Kerola & Oinas-Kukkonen
(1997) describe, collaborative hypermedia functionalities can
provide a solid platform for the growth and intuitive use of an
organizational memory (Stein & Zwass,
1995).
Transclusions (or inclusions) was one of the first hypermedia
features to be proposed by hypertext visionaries, but thereafter ignored
by hypermedia implementors. Transclusions constitute the core concept
within Ted Nelson's proposed system Xanadu (Nelson, 1987<;
Nelson, 1995). Nelson proposed transclusions as a
mechanism for having the exact same object (document content) exist in
multiple places. Whereas copying and pasting creates an identical copy,
transclusions act similar to pointers that connect the original copy to
all places that use it. Transcluded data is alive, still
connected to the original and automatically updated. Through
transclusion, readers always have access to the original and therefore to
its original context (through a context link). Xanadu's virtual
document structure is built around transclusions: each document is a
list of pointers to pieces of data, which originate in that document or
are "included" from others.
In the context of our insurance scenario, transclusions create a
reliable and maintainable set of similar documents (for instance, a
customized document containing policy information for a customer,
including the policy's payment and payout schedule). The complexity and
number of contracts inevitably results in a huge amount of documents
containing the same basic items, but each different in a specific way.
Storing each document as a separate, complete entity would mean vast
redundancy and complex update whenever a fairly common clause in many
needs to be changed. Using massive transclusions could reduce or
eliminate redundant storage and greatly simplify the update process.
Authors would specify documents in terms of transcluded elements. For
example, a document may comprise standard clauses 1 to 12, clause 13
from document Y, and a special clause 14 belonging to only this
kind of document. The same thing can be accomplished by component
documents and republishing all document that use a component when that
component changes. (Xanadu's implementation of transclusion inherently
incorporates versioning (Vitali &
Durand, 1995), which would satisfy the insurance company's need to
maintain old document versions.)
Transclusions are related to but differ from composites. Whereas
composites intrinsically refer to whole nodes (Halasz, 1988), transclusions refer to subparts of
a node. Thus the problem with composites is not with composition but
with decomposition: the granularity of the atoms forces the minimal
granularity of a composite's elements. It is thus impossible to include
in a composite a chunk of text smaller than the full granularity of the
text's unit. This affects the types of composites possible. For
instance, authors could build the policy documents using composites;
storing each clause as an atomic object and creating standard "sets" or
collections of clauses would allow authors to compose modular documents
very easily. But in the case of two documents differing only in a part
smaller than a clause (for instance, the policy term), authors would be
forced to create two distinct clauses identical in everything except the
term, thereby losing the notion (and operational benefit) that these
clauses essentially are identical.
Warm and hot links are relationships that create a channel between the
two end-points, through which data flows from one document to the other
(Meyrowitz, 1989).
Warm
and hot links are not pointers, but actual copies of the data, which can
update automatically. With warm links, users explicitly ask to
update the node content. With hot links, content is verified and
updated automatically as soon as the data is displayed. Transclusions
and warm linking differ basically only on implementation details.
Transclusions are pointers to data. With warm and hot links, the host
document contains both a copy of the data and a reference to the
original source. Simple references avoid duplications and redundancies,
while copies allow improve reliability and availability--the system
could still display a (possibly outdated) copy even if the original
document were deleted or inaccessible).
Curiously, while few existing hypermedia systems have ever implemented
hot and warm links (Catlin, Bush &
Yankelovich, 1989), implementations have come from an important
class of commercial systems: operating systems. Publish/subscribe in the
Macintosh operating system and dynamic data exchange (DDE) in Microsoft
Windows allow applications to create live channels for displaying
portions of data from one application to another. The difference between
transclusions (or warm/hot links) and composites can be shown clearly by
noting composites' parallel to UNIX symbolic links, MacOS
aliases, and Windows'95 short cuts. Just like composites,
symbolic links allow containers (i.e., folders) to share the same
components without duplication; and just like composites, only whole
files can be shared among containers. DDE, on the other hand, allows
arbitrarily-sized chunks of data to be shared among documents, as
transclusion would dictate.
A form of transclusion exists in the WWW environment. Through server
includes authors can create documents, the content of which is
decided dynamically, just before the document is sent over to the
client. Within the HTML code, authors can include special commands that
the server parses, substituting the resulting values every time the
client requests a document. Commands include invoking CGI applications,
inclusion of whole files and substituting variables. Although
potentially quite powerful, currently authors use this mechanism
primarily for simple variable substitutions, such as the number of
visitors to a WWW site, or automatically placing standard headers and
footers automatically on all documents belonging to a site.
Mosaic 2.4 (National Center for
Supercomputing Applications, 1995) provides an annotation facility.
Selecting the "annotation" menu item brings up an edit window into which
the reader enters the annotation text. Committing the annotation saves
its content to a file in the reader's personal workspace, attached by
default to the currently displayed document. Henceforth, every time the
reader displays this document, a link to the annotation will appear at
the bottom of the document. But unlike most hypermedia systems, readers
cannot associate an annotation with the specific spot in the document
upon which the reader wishes to comment. This potentially increases both
disorientation and overhead. However it would be a simple matter to
alter this to operate as open hypermedia systems do, associating the
annotation with a specific position without embedding it in the document
(see Section 3.6).
An additional issue is whether to allow users (and, especially, which
users) to customize the data, the way it is displayed, or the
available links. Creating views and only permitting certain readers to
access certain information requires customization based both on user
role and access permissions. While restricting access is an anathema to
hypermedia purists, real-world organizations demand such security. Our
insurance company will not want to run the risk of disgruntled employees
or the general public adding negative annotations on the annual report.
Similarly, only employees of the legal department should be able to add
links to certain documents where available information must be within
exact legal guidelines. Bieber and Kacmar
(1995) give additional examples for the domain of geographic
information systems.
Besides standard read and write access permissions on documents, system
administrators may consider analogous read and write permissions for
links and annotations. For certain documents one could allow link
authoring and update while prohibiting write access to the underlying
content. One also could restrict author access to certain node and link
types. For example, only employees of the legal department could author
a "legal opinion" link. Furthermore, one could regulate link and
annotations to specific views.
Within the ABC collaborative writing environment, access permissions
regulate group collaboration instead of acting philosophically to
prohibit free communication (Shackelford et al., 1993). Turoff has developed
twenty-five kinds of access permissions (as well as "tickets" to
override them) for computer-mediated communications, many of which would
apply to collaborative hypermedia applications (Turoff, 1991). In fact, access permissions may
prove most useful for allowing individuals to maintain a personal set of
nodes, links and annotations, while collaborating work groups would
maintain a shared set of these. HyperWave uses access rights assigned
to links to implement private and group networks (Maurer, 1996).
Mosaic's annotation facility supports three classifications: personal,
workgroup and public annotations. Later versions support workgroup
annotations through a server providing annotations to any person who has
the appropriate access permissions. The Distributed Link Service
implemented a similar approach (Carr,
De Roure, Hall & Hill, 1995). In each case, the document server
and link server can be completely different entities. This requires the
link server to employ externalized link databases (see
Section 3.7).
With personal and workgroup annotations, annotation authors implicitly
are trusted once they log onto the supporting network, hence there is
little need for annotations to be vetted for suitability. A public
annotation facility, however, may or may not permit annotations to be
added by an unvetted public. Thus these systems distinguish read-only
public annotations from unrestricted public annotations. For example, a
site may publish a database of annotations or links which cannot be
augmented by the public (as the Distributed Link Service does).
Alternatively, an annotation/link server may permit readers from
anywhere to add publicly-readable links and annotations to any document.
These two classifications respectively are directly analogous to the
concepts of moderated and unmoderated newsgroups. For this very reason,
implementors of public link and annotation databases may find it
profitable to consider the techniques of network bulletin board
management (such as the use of "kill" files to filter out irrelevant or
obnoxious annotations). For instance, our insurance company may publish
a page calling for clients to make comments about the service provided
by the company, yet they may be understandably reluctant to make public
any negative comments. Thus they could implement a form of moderated
public annotations for this purpose.
Schloss (1996) presents an
annotation mechanism on the World Wide Web for augmenting electronic
commerce applications with advisory systems. His advisory service
enables third parties to provide supplementary information to support,
augment or critique the contents of Web pages. Organizations will be
able to offer rating services (e.g., by special interest groups or along
the lines of Consumer Reports), active advice (e.g., discounts
for preferred customers) and related information (e.g., relevant
magazine articles). We anticipate this functionality to have significant
social impact. Schloss' advisory service, as well as external link
databases (see Section 3.6), render many
security issues irrelevant, but in the meantime but organizations still will need
content and annotation control on intranets behind company
firewalls.
For example, a sales agent in the insurance company may wish to create a
personalized link computation specification which will take the name of
any customer and link it to every insurance policy ever held by that
customer. Rather than have to create this set of links by hand every
time a policy is sold, the sales agent sets up a sort of "standing
order" with the hypermedia system, so that it will automatically create
these links whenever a new policy is taken out.
Tailoring of link computation specifications is not well-supported in
general. There are few hypermedia systems which permit users to create
their own link computation specifications, and not many more which
permit administrators to create link computation specifications for
corporate use. Where this has been enabled, the specifications for the
computation of links from one link type are expressed in some formal
language. One implementation of tailorable link types uses a relational
database to store these specifications (Verbyla et al., 1994). The Web's link computation
specifications (i.e., CGI scripts and mobile code applets) are usually
stored in directories on the host machine operating system and hence are
not immediately tailorable. However, the advent of client-supplied code
in the form of servlets (Sun, 1996)
will enable unlimited link type tailorability. Alternatively,
computation specifications can be attached to classes of objects, and
while the reader may not necessarily be able to alter the class (i.e.,
the domain to which the link type applies), it is possible to edit
specifications for computing the destinations of links (Monnard & Pasquier-Boltuck, 1992; Rizk & Sauter, 1992).
The key issue in computing private links is that the computation
specifications do not form part of the software of a hypermedia system,
hence they can be manipulated without the need to recompile (and hence
alter) the software. This allows both readers and system administrators
to create and maintain link computation specifications most relevant to
their needs. This is a form of externalized link database, except that
the stored objects are not actual links but are specifications for
computing links.
A typical situation would be where users want to apply their own link
specifications to data from a remote site or from read-only media, or
where documents being linked must remain inviolate. For example, our
insurance company may wish to link client correspondence to the client
file. For legal reasons, the correspondence must not be altered in any
way, hence links coming from the correspondences must be maintained
externally. Another case where documents must remain inviolate is for
accountability purposes. For example, links to previous versions of a
product brochure must be maintained for historical, legal or temporal
context. The advertising department may wish to link the advertising
pages of other insurance companies to its own advertising pages when it
believes the comparison in its favor. Since our insurance company is
most unlikely to get permission to create such links from their
competitors' pages, they must use an externalized link database.
Microcosm (Davis et al., 1992)
implements
every linkbase to be external and independent of all the documents it
connects. In addition to any communal linkbases, each user has their own
linkbase which they can carry with them on a floppy disk. Readers have
access to all links in all available linkbases. This shows the
feasibility of private linkbases containing links connecting public
documents. External link databases are being tested on a larger scale in
the Distributed Link Service (Carr et
al., 1995), in which databases of precomputed links are published
along with documents. A reader can connect to a server to retrieve
documents, but (with the appropriate browser or preprocessing engine)
can also connect to a server to retrieve links appropriate to a
document. The HyperWave distributed hypertext system (Maurer, 1996) employs external
link databases in order to provide bidirectional point-to-point links
within multiple media with rich typing, individual access permissions
and guaranteed consistency. Whenever documents are removed, every link
pointing to it is removed from view (i.e., it is not deleted, but it
becomes hidden to readers).
The Gentler system (Thimbleby, 1997 [this
issue]) uses an external link database for consistency maintainance, embedding
links in the HTML code only when creating the final Web page for display.
To support links, the endpoints of which occur at specific named points within
documents, links must include not only document names and network nodes,
but also internal locations. The addressing schemes of Xanadu and
Microcosm both refer to internal locations in documents using a document
name/byte offset form (much like Xanadu's tumbler addressing scheme (Nelson, 1988)). Other systems
dynamically compute link locations with, e.g., string-matching
algorithms. These links are either reconciled to the document by a
special-purpose browser, or the HTML is amended by some preprocessing
agent before it is tendered to a plain browser. While "published" link
databases are intended for widespread consumption, there is no reason
why a user's private link database should not be used in the same
way.
Not embedding links faces the problem that information update can
desynchronize the link's reference in a linkbase and its new location in
its document. Before using it, one needs to check whether the link is
still consistent. This means checking link validity, i.e.,
whether the nodes still exist, whether they are still reachable, and
whether the stored references still point to the exact location within
the nodes. It also means checking link relevance, i.e., whether
the link still has a reason to exist, or, more exactly, whether the
changes in the documents have made the link useless, inappropriate or
wrong. These problems arise when there is loose control between the link
and the connected nodes, i.e., when a change in one of them does not
necessarily imply a modification in the others.
Our insurance company can end up in difficulties when URLs become
invalid. They may have taken out a large paper advertising campaign,
distributing leaflets which all have the company's home page URL. But if
that URL becomes invalid for whatever reason (such as a change of
Internet service provider), the company runs the risk of alienating a
number of existing and potential customers.
Our insurance company can also run into problems when link relevance is
compromised. For example, a competitor may decide to undercut our
insurance company in price and suddenly changes the contents of their
advertising pages without warning or notification. The link which our
company uses to show that its price is better will henceforth
demonstrate exactly the opposite! Validity of links, on the other hand,
is at stake when the competitor rearranges the content of the document
so that the link points to an irrelevant part of the document, leaving
readers wondering about the link's purpose.
The World Wide Web is not immune from the problem of updating links.
HTML uses internal links, i.e., links that have end-points explicitly
stored along with the text, with <A HREF> tags as sources and
<A NAME> tags as destinations, with reference information about
the destination stored in the first tag. This reference is composed of a
URL (a locator of the destination) and the optional name of the
end-anchor. The URL, anchor names and document contents all are
controlled by the destination document's author, whereas the <A
HREF> tag is controlled by the source document's author. Since there
is no tight connection between the two documents, users find links
failing after any change in name of the anchor, the destination file or
of the network node, since embedded links (in this case URLs) are not
updated automatically.
Alternatively, suppose the document's location remains the same, but the
contents change. While byte-offsets make it possible for users to create
links on documents they do not own, byte offsets fail after even minor
modifications to the content (for instance, offsets are incorrect after
any insertion or deletion preceding them). While missing the real
endpoint by a few characters may seem a minor problem for navigation
links, it may become a vital problem if transclusions are realized in
the scale of the WWW. Because transclusions affect the actual content of
the node containing the reference, if the reference has not been
corrected after the referred to document has changed, then the content
of the node, and not just the link, becomes unreliable.
Universal resource names or URNs (URN
Implementors, 1996) should resolve these problems partially. URL
(Universal Resource Locators) and URN are two types of URIs (Universal
Resource Identifiers), which are addresses used by WWW software to
retrieve and access to network resources (World Wide Web Consortium, 1996). A URN provides a
reliable and persistent address to a network resource. Whenever a URN
address is requested, a network resolver determines the current and most
appropriate actual address (most often, a URL) for the client to
request. URNs most commonly comprise a directory system allowing users
and servers to register documents and other resources. The directory
service would then provide a persistent URN that could be reliably used
in anchor tags. Whenever a modification occurs, the person responsible
for the document would update its information in the directory service,
so that any subsequent access to the persistent URN name would lead the
user to the updated location. For the system to work, document owners
need to register them with a nominated document registry and update the
registration whenever the location changes. Link creator must remember
to use URNs instead of URLs. Of course our insurance company's
competition may decide not to register a URN specifically to avoid
persistent tracking, either by our company or by a negative advisory
service (see Section 3.4).
The URL addressing scheme could be seen as a 2nd-generation addressing
approach, since it requires an accurate knowledge of the storage details
of the document. URNs can be considered a 3rd-generation addressing
approach since the storage details are concealed from the author,
although the author still needs to know the unique identifier assigned
to a document.
Different philosophies (implemented in different systems) address the link update
problem:
Vanzyl, Cesnik, Heath and Davis
(1994) proposed heuristic techniques to retrieve the current
position through pattern matching, based on the fact that insertions and
deletions in parts of a document far away from the link end-point
usually do not affect the context around the end-point itself. Thus the
endpoint's unchanged content can be found again when needed. While
simple and powerful, the heuristic is not perfect, and fails in certain
situations such as multiple occurrences of the context or link text when
it cannot distinguish which instance we want;
Many versioning methods use a dynamic linking approach, e.g.,
(Maioli, Sola & Vitali, 1993;
Nelson, 1987). Being able to
check what changes have been performed on a document allows versioning
systems to compute current offsets based on old ones. In fact, the
Xanadu addressing schema, heavily based on versioned transclusions,
purportedly (Nelson, 1987)
never runs into the link update problem because original offsets are
always valid regardless of how a document is modified.
Intermedia provides overview diagrams, called Web Views, which provides
both spatial context ("Where can I go from here?") and temporal context
("When and by which path did I get here?") (Utting and
Yankelovich, 1989). Intermedia web views are dynamically updated,
local tracking maps that display all documents linked to the current
one. This provides local context as well as link previewing. Also,
selecting a link marker in the full content of a node highlights the
corresponding link line in the adjacent Web View, giving the user an
idea of the target without actually following the link. In Thoth-II, the
"Spiders" directed graph browser provides a contrasting type of overview
map (Collier, 1987). As the
reader interacts with the structure being viewed, new graphic objects
(nodes and links) are created. Activating a node expands it to show
links to other nodes which subsequently fan out to further nodes. The
network expands or fans out in two-dimensional space creating "spiders"
on the display. The window on which this structure was displayed could
be moved around to view the parts falling outside the viewing area.
Overview diagrams can be generated either by reading the links between
documents from a link database (see Section
3.7) (Andrews, 1996),
or by examining the documents themselves for embedded links. However, as
soon as overview diagrams for large systems become complex they might
introduce navigational problems of their own (Nielsen, 1990). Nielsen
suggests features which we believe an intelligent fourth-generation
overview map would provide. For example, in order to reduce the
propagation of links from the local diagrams to global diagrams, weights
can be assigned to the links based on their relevance to the user; this
will trim the edges of the graph at the global level. Similarly, readers
might want to know what proportion of the material in the current subweb
applies to their goal. (The scope line in Intermedia, e.g., informs the
reader about the number of documents and links in the web (Utting et al., 1989).) The overview
browser should apply user profiles to the node and link attributes,
perhaps querying the user for his or her interests, and then tailor the
overview appropriately and automatically.
In our life insurance company scenario, the amount of material to be
presented highlights the need for overview diagrams. An agent may be
interested in finding out the local and global context of a particular
product. Is a product part of a category of products with similar
characteristics? Is that category part of another higher-level group of
products? Is the product itself related to other products from other
groups? This kind of information is hard to assimilate when readers only
can function down at the actual content level, but much easier when the
relationship (meta)structure is shown in a graphical manner.
The World Wide Web does not have the necessary constructs to provide
local and global overviews or maps for two unrelated reasons. First,
while global overview maps can be provided in terms of imagemaps, their
automatic generation would be extremely complex and troublesome, having
to deal with CGI or Java applications parsing the documents for link
information, and creating the image (possibly a GIF) and the imagemap
table for relevant users' feedback. Second, WWW links cannot be
categorized by their roles; there is no easy way to differentiate
between structural links among parts of single concept or information
unit (e.g., the various details of an auto insurance policy) and
associative links among related concepts, (e.g., relationships between
two products such as auto insurance and homeowners' insurance).
Furthermore, many HTML authoring tools do not allow authors to visualize
the nodes and relationships which they are creating, although tools such
as Microsoft FrontPage and NaviPress allow authors to look at local
mini-webs as they are being created.
We can imagine endless opportunities to enhance our insurance company
scenario with trails and guided tours. Agents can see all correspondence
from a particular client regarding a particular claim in chronological
order. The company could establish a guided tour for the general public
about each of its major products and then nested guided tours within
this about particular product details. An agent might prepare a
customized tour for a client or potential client. The training
department could set up tutorials for new agents and those seeking
advanced training in various topics in the form of trails. Trainees
could follow the recommended path while having access to details and
tangential information.
Trail implementations vary in complexity. In simpler implementations,
such as with Textnet, the trail is an ordered list of nodes that the
user follows. Textnet trails can be viewed in a special window as a list
of nodes. Both the author and the reader can change this list at will to
reflect new discoveries or themes (Trigg
et al., 1986). Perseus represents trails as an ordered list of icons
(Mylonas & Heath, 1990). The
editor allows authors to build path entries to entire nodes or to a span
of text within a node, to rearrange path entries, and to annotate
entries with brief comments. In Intermedia, a path was a list of
documents readers visited earlier in a browsing session. The display of
a path consisted of the name of the document, an icon indicating the
type of event (opening or activating documents), and a timestamp
indicating when the event occurred. A user's path could be saved when
closing the web and restored when subsequently opening the web. Thus, a
path can be used to collect all interesting documents to form a linear
document that can be preserved in printed form (Utting et al., 1989).
More advanced implementations permit the author to annotate the trail
with fuller information about its overall purpose and execution, as
well as about each of its nodes. For example, NoteCards' (Halasz, 1988) guided tours
feature a graphical overview and customized tabletop screens at each
stop along the tour (Trigg,
1988; Marshall & Irish,
1989). The tour's author can add the following types of
metainformation in a separate area on the tabletop for the tour and for
each of the notecards or nodes included on the tour:
Cuff (1995) proposes a flexible
extension of trails, in which the system warns the reader that a link
will digress from the trail's theme. The system would also incorporate
guided tours, preventing the user from digressing at all. The author
could set the width of the trail, i.e., how many traversals away
from the main trail the reader may digress at any time.
Zellweger (1989) has
extended the notion of trails to procedural programmable paths with
active entries called scripts. This is analogous to a very
sophisticated macro facility where the author builds path macros
(Parunak, 1989) in which
entries can perform arbitrary actions ranging from issuing system
commands to manipulating variables via an interpreter. Sample actions
include playing back a voice annotation, animating a picture, or
querying a database.
This leads to the school of thought that considers hypermedia as a way
of representing and implementing processes. One can represent a process
as an associative network which users invoke through navigating.
Representing the process steps as nodes and transitions as links enables
developers and users to augment the process with annotations and with
links to related information (Scacchi, 1989). Similarly, some developers
structure programs as associated networks (e.g., petri nets) controlling
which links to activate based on the user's previous interactions (Stotts & Furuta, 1989).
In a third-generation style authoring environment, authors manually
place each entry in the tour list and manually tailor the contents of
each tour node. The system could generate previous step, next step,
first entry and exit navigation buttons automatically, as well as
generating an overview map. A fourth-generation trail and tour authoring
environment would use semantic types, keywords and other attributes,
access permissions, views and user information to organize and tailor
the tour contents dynamically.
On the World Wide Web, the Java tutorial is a guided tour with great
depth and comprehensiveness (Sun,
1995). New York University's Information Systems Department
Information System (New York University,
1996), designed using the RMM formal hypermedia
design methodology (Isakowitz, Stohr
& Balasubramanian, 1995), generates guided tours automatically
from a database of course, professor and publication information.
Trails (and tours) on the Web tend to contain only documents owned by
the trail author. Authors need write access to documents to embed
"previous" and "next" links, and to otherwise annotate and tailor the
documents. System developers could provide environments for authors to
build trails containing any document by pre-processing each document to
append tour commands, supplemental links, annotations, etc., and remove
any links the author wishes to filter out for tours. The environment
also might build a trail map which the author could modify. System
developers could take advantage of the data model and comprehensive set
of navigation commands for implementing traversal within single and
multi-level nested guided tours (Garzotto et al., 1996). These include starting,
suspending, resuming, going to previous and next steps, etc.
Bush foresaw "a new profession of trail blazers, those who find delight
in the task of establishing useful trails through the enormous mass of
the common record" (Bush, 1945).
Given the right tools, the Web can provide this massive common record
for trail blazers.
Rosenberg notes three other reasons for backtracking: to review the
content of a previously visited node, to recover from a link chosen in
error and as part of undoing (Rosenberg, 1996). Backtracking reduces
cognitive overhead in that readers know they can always return from a
detour or an incorrect traversal. Conceptually, however, backtracking
differs from undoing in that backtracking returns the reader to a
previously visited node in its current state. This could disorient
readers if the node contents have changed and appear different than
expected. Many WWW browsers cache node contents, so readers unknowingly
may see outdated contents upon backtracking.
Different multi-window hypermedia systems implement backtracking
differently (Bieber and Wan, 1994).
Assume the reader uses one of the many hypermedia systems which place
each node in a separate window (thus maintaining a level of orientation
as the reader can see both the link source and destination). The reader
opens node A, traverses a link from node A to node B, traverses a link
from node B to node C, clicks on node B and then traverses a different
link from node B to node D. Then the user wants to backtrack from node D
back to node A. Figure 1 represents this navigation.
Different hypermedia systems implement backtracking differently, as
Figure 2 shows.
Hypermedia researchers envision full 4th generation environments in
which users will work in multiple hypermedia applications simultaneously
(hypermedia email, electronic commerce application, a spreadsheet, a
word processor, etc.). Similarly, on the WWW we envision that users will
have multiple windows open in a single editor/browser client and work on
several tasks at once. This inspired Bieber and Wan to distinguish
chronological backtracking from task-based backtracking
(Bieber et al., 1994). Chronological
backtracking returns the reader to nodes in the reverse order visited.
Task-based backtracking restricts backtracking among nodes within the
current logical "task". They propose a simple technique, inferring that
the task potentially changes whenever the user opens or clicks on a
different window. More sophisticated analysis would infer which groups
of windows belong to the same task. Their paper presents a data model
and algorithm for each type of backtracking.
Garzotto et al. model qualified backtracking (Garzotto et al., 1996). Parametric
backtracking, written as go-back(X), allows the user to
specify a node characteristic in parameter X, which causes the
system to backtrack to the most recently departed node with that
characteristic. Conditional backtracking, written as
go-back(query-expression) evaluates query-expression and returns
the reader to the most recently departed node satisfying it.
While backtracking one node at a time eventually brings the reader back
to a particular previous node, readers often want to backjump
directly to a particular departure point. The system should display a
session log or history list in a clear enough format for the
reader to distinguish the node to which he or she wants to return. Often
the reader must pick this from a textual list of node title's. Nielsen
augments the node name with the amount of time since the user last
visited the node (Nielsen,
1990). The Interactive Graphical Documents system represents the
history list as a graphical timeline displaying a miniature picture of
each node each time it was visited, along with the time it was visited
(Feiner, 1985;
Utting et al., 1989). The LINKBase
system displays a copy of the session log as as standard part of each
page, so the user always can see how he or she arrived at the current
page (Balasubramanian, Ma &
Yoo, 1995; Balasubramanian, Bieber
&
Isakowitz, 1997; SIGLINK
1994).
Many WWW browsers implement the history list as a stack augmented with
all nodes on the current history path. When the reader backtracks or
backjumps to a certain point and then traverses a new link, the browsers
remove all nodes on any history paths emanating from that point. For
example, in the following diagram, suppose the reader traverses from A
to E, and backtracks back to B. At this point, nodes C, D and E will
still appear on the history list. When the reader traverses to F, the
system removes the history path C through E from the history list.
Nielsen's system, in contrast, treats the history list as a session log.
Backtracking adds the nodes onto the list again. When the reader
backjumps from the list, the system both adds the nodes onto the list
again and places a checkmark on the backjumped destination to remind the
reader of this action when viewing the history list in the future.
Still, despite timestamping each entry and these other orientation cues,
readers may find a linear session log cannot adequately capture
repetitive traversal within a non-linear network. Implementing the
session log as a graphical overview (see Section 3.8) may orient the user better. Authors
building guided tours and trails (see Section
3.9) will find a session log especially useful. In systems such as
Microcosm (Davis et al., 1992),
authors can browse normally to find all nodes and the proper traversal
among them, and then package this up as a trail by modifying a copy of
the session log.
NoteCards takes an even broader view of the history list, maintaining a
full log of all author and reader actions. Users can specify which
events to record (node and link creation, traversal, deletion, etc.),
making this a tailorable log. While all this information is not
necessary for backtracking or jumping, it does provide additional
orientation for the user in recalling all steps taken to arrive at the
current application state.
Backtracking within a fourth-generation hypermedia environment would
allow not only the reader to specify parameter and conditions in an
intuitive manner, but would allow an author to set up standard sets of
parameters and conditions for a particular application domain. The
fourth-generation environment also would support sets of predefined
task-based backtracking options tailorable to particular user
domains.
Conditional or parametric backtracking would require WWW browsers to
maintain whichever additional parameters that users may wish to query in
the history list. These could include those discussed in other sections
of this paper, such as semantic link type or document type, title and
location, or perhaps the parameters passed during traversal. For
conditional or parametric backtracking based on document content, the
system could search the contents saved in the browser's cache. Creating
trails and guided tours will require the system to maintain a
supplementary session log. Task-based backtracking will require the
system to capture how each document is activated: by opening, clicking,
closing the window above it, through backtracking, etc.
Associative links connect information in some way related, while
annotative links represent the subset of nodes that comment upon
others. While most second generation links are hand-crafted, the
larger the information base, the less feasible authors find it to
compose all links manually. For domains where the hypermedia network
represents applications, which generate display contents at run time
(e.g., database management systems, decision support systems, expert
systems and geographic information systems), hypermedia support must be
computed or mapped automatically (Bieber, 1992;
Bieber et al., 1992;
Bieber & Kacmar, 1995;
Wan, 1996). Also, automated tools such as link
apprentices (Bernstein,
1990) can propose lexical links between documents to authors,
forming a preliminary tool for fourth generation environments. When
dynamically computed links can be created periodically instead of
the last minute, then they could be precomputed to save time when
the user wants to traverse them. Without an external linkbase, links are
inherently unidirectional. Most hypermedia systems support
bidirectional links, which users can access from either
endpoint.
Landmarks are one-way links from everywhere to a specific place,
such as a home page. Normally landmarks appear at the top or bottom of
every document, in frames (in browsers such as Netscape), or in menus.
Authors create landmarks, whereas readers create bookmarks, which
essentially constitute personal landmarks. Web browser hot lists are
hypermedia bookmarks. While the concept of hypermedia concerns browsing,
many hypermedia systems also incorporate content-based information
retrieval mechanisms. Readers can use content-based search to locate
a relevant starting point for browsing.
Hypermedia structuring and access applies to anything that can be
represented as an associative network. As long as the hypermedia engine
has some intermediate access or mapping function
(Bieber, 1995),
we should be able to
annotate and access metainformation about objects anywhere, including
those in heterogeneous data, process and knowledge repositories
(Noll & Scacchi, 1996).
Automatic link propagation concerns link update. When information
such as the price of an insurance policy changes, all direct occurrences
of that item should be updated, as well as any calculations depending on
this price. As part of the Chimera environment, Anderson, Taylor and Whitehead (1994) currently are
developing a set of hypermedia-aware widgets containing a degree
of hypermedia functionality for X-Windows. Any application incorporating
these will get this hypermedia functionality for free (Anderson, 1996). We should
not only have links to and from HTML documents, but links to data in
any application. HTML is but one standard for content markup and
linking. HyTime provides hypermedia support to SGML documents, but
also can express locations within documents which are not in HyTime,
SGML or HTML (DeRose et al., 1994).
Returning to the list of challenges posed in the insurance scenario that
opened Section 3, we conclude that applications
can leverage the WWW's hypermedia infrastructure to improve
effectiveness in many ways. The high-level structuring, navigation and
annotation features we discuss help authors organize information so that
users can access it directly while maintaining context and orientation.
Applications could employ annotations and semantically typed templates
to create fourth-generation environments for collaborative authoring.
These authoring environments and the information they produce both can
be customized for specific uses and users. Transclusions, warm and hot
linking, and versioning can support this customization as well as
information update and access to historical information. Access
privileges extend security to hypermedia-structured information.
In
summary, application developers should strongly consider hypermedia
structuring and navigation for organizing, merging and giving users
flexible access within large bodies of information. But they only will
do so en masse when the appropriate tools exist. We call on WWW
developers to provide high-level hypermedia features and their
respective sophisticated authoring environments so that every
application developer, author and reader can take advantage of them.
Acknowledgments
We wish to thank the participants of the Hypertext Functionality
Workshops I and II, held in conjunction with the European Conference on
Hypermedia Technologies (ECHT) 1994 and the Hypertext'96 Conference.
Special thanks to Keith Andrews, Alejandra Garrido, Paul Kahn and
Hermann Maurer for reviewing drafts of this paper. This research
generously has been supported by the NASA JOVE faculty fellowship
program, by the New Jersey Center for Multimedia Research,
by the New Jersey Commission on Science and Technology, by the New
Jersey Institute of Technology (NJIT) under grant #991967, by the
National Center for Transportation and Industrial Productivity at NJIT,
and by a grant from the AT&T Foundation.
Footnotes
* This scenario is discussed in more detail in
Balasubramanian, Bashian and Porcher (1997).
[1] The authors of this survey note that
the respondents did not necessarily represent a true cross-section of
Web users, due to the targeted circulation of the survey questionnaire
and its voluntary nature.
[2]
Tauscher and Greenberg (1997) note that this feature has now
been added to some Web browsers.
[3] gIBIS has a ninth link type,
"unspecified," which its authors do not discuss in any of their
papers.
[4] MacWeb is a knowledge-based hypertext
system under development at LIRMM since 1989. It has no relationship to
the World Wide Web client developed later with the same name.
[5] Vannevar Bush was Director of the
Office of Scientific Research and Development during World War II for
President Franklin D. Roosevelt. Brown University recently sponsored a
50-year symposium to honor his work
(Simpson, Renear, Mylonas & Van
Dam, 1996;
Brown Computer
Graphics Group, 1995).
[6] We especially wish to thank Carolyn
Watters from Acadia University in Canada, a participant in the Hypertext
Functionality Workshop II at the Hypertext '96 conference (Ashman et al. 1996), for coming up
with the structure of Table 3's feature list.
[7] Smith, Newman and Parks (1997), for example, conclude that
hypertext usability research can not be utilized as the basis for Web usability research
because of the inherent differences caused by scale between primarily small hypertext
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3.2 Link attributes and structure-based query
In addition to types, other semantics can be attached to links, such as
labels, names, keywords or timestamps. A single link type defining a
conceptual relationship between nodes can have multiple labels
representing the type tailored for different contexts (Rao et al., 1990). The keywords attached to a link
can include its type and labels. All users--including readers--may be
allowed to define keywords on links. Any number of keywords can be
attached to a link.
Executing the query results in a hit list consisting of the links that
satisfy the query or an overview diagram of the system with just these
links highlighted. Readers can inspect the dependencies between
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Implementing structure-based query will be facilitated greatly by
maintaining external link databases (Section
3.6). 3.3 Connecting occurrences: transclusions, warm links and
hot links
Transclusion, warm links and hot links all connect two occurrences of
the same information. They differ in philosophy, functionality and
implementation. 3.4 Annotations and public or private links
Hypermedia researchers have always considered private annotations
(comments) a basic right for hypermedia readers as well as a basic tool
for collaboration and exchange of ideas. Almost every major hypermedia
system provides support for annotations: KMS (Akscyn, McCracken & Yoder, 1988;
Yoder, Akscyn & McCracken, 1989),
Intermedia (Catlin et al., 1989),
Aquanet (Marshall, Halasz, Rogers
& Janssen, 1991), the Virtual Notebook System (Burger, Meyer, Jung & Long, 1991), StorySpace
(Joyce, 1991), etc. While WWW
committees have discussed annotations (La Liberte, 1994;
Gramlich, 1994), few browsers support
annotation as it is difficult to embed annotation links within HTML
documents belonging to others. 3.5 Computed, personalized links
An important and powerful feature of 4th generation hypermedia systems
is the automated creation of private links from computations. This
necessarily means that readers must be able to specify their own
computations in order to create the private links, that is, the reader
must be able to tailor their own link computation specifications. This
allows readers to create entire collections of links which are
specialized to their needs. 3.6 External link databases and link update
mechanisms
Embedding link endpoints, as in HTML, presupposes that the link creator
has write access to the information being linked. External links, which
are stored in external linkbases separate from the document
content are used where write access is not available, or when
user tailorability of links is supported. For example, the field of
open hypertext systems
(Wiil, U. & Demeyer, S., 1996<;
Osterbye & Wiil, 1996)
uses external links for linking to data
owned by legacy applications and other non-hypermedia applications
without disturbing the original format of that data (Davis, 1995a;
Davis, 1995b;
Kacmar, 1996). We also see in this paper that
maintaining external linkbases supports link and view customization,
guided tours and trails.
Fully implementing many fourth-generation hypermedia features, such as
views, private link collections and transclusions will rely on external
linkbases. Developers ought not be discouraged by the link maintenance
problem. Versioning is absolutely reliable as it employs a form of the
'no change' approach where document versions are never deleted or
altered--new versions are constantly added. Authors, however, will be
able to use only versioning-aware tools when creating and modifying
links. A versioning mark up language for the WWW has been proposed (Vitali et al., 1995), and its
implementation in HTML editors and browsers would contribute to a
natural and widespread use of reliable update techniques.
3.7 Local and global overviews
Overview diagrams (or maps) provide a view of the hypermedia node and
link network from above. They show its structure in a graphical manner
(e.g., with icons and arrows). Global overview diagrams provide an
overall picture and can also provide anchors (entry points) to local
overview diagrams. Local overview diagrams provide a fine-grained
picture of the local neighborhood of a node. Both can serve as excellent
navigational aids. Overview diagrams or maps, both at the local and
global levels, improve spatial context and reduce disorientation in a
hypermedia network. Local overview diagrams also help minimize cognitive
overhead by showing a small, relevant part of the network. 3.8 Trails and guided tours
In what people now recognize as the first article on hypermedia, Bush [5] introduced the idea of associative
trails (Bush, 1945) or
paths. Trails connect a chain of links through information
spaces. They provide a context for viewing and understanding a series of
documents. Trails can record a path of information that the reader may
wish to remember and share with others. Authors (or interested third
parties) can prepare multiple "recommended" trails through an
associative network focusing on different aspects or tailored to
different readers (a novice, an expert, a teacher, a student, etc.)
Bieber notes that analysts might use trails to document a decision
analysis for management or colleagues (Bieber, 1992). Continuity and guidance
distinguish trails from random links in documents. The trail should be
clearly marked, so users will know which links keep to the trail and
which constitute detoursfrom the trail. The trail designer could filter
out links, making only the ones most relevant to the current item available
as detours. Guided tours restrict users to the trail,
prohibiting detours. Nodes viewed during the guided tour will have links off the tour
dimmed or hidden. Users have to suspend or exit the tour to access these
(Garzotto et al., 1996). While
trails lower cognitive overhead by recommending the next logical link to
take, guided tours reduce overhead further by removing all other
choices. This notwithstanding, trails and tours could contain branches
allowing the reader to pick one subpath over another (Thüring et al., 1995). In addition, the system
could provide an overview or map, so readers can maintain their
orientation along the trail or tour.
Hypermedia environment developers could allow both readers and authors
to annotate nodes on tours and trails, and facilitate these appearing
only in conjunction with that trail. When a trail or tour includes
regular documents and nodes, special annotations and labeling could
alleviate the following danger. Garzotto et al. note that understanding
a node's content sometimes depends on reading other nodes first, or on
titles and supplemental links that authors sometimes remove to fit that
node into the tour. They give several examples of how including both
textual and multimedia nodes out of their normal context could make the
contents confusing (Garzotto, Mainetti
& Paolini, 1995).3.9 Backtracking and History-based Navigation
Hypermedia-style interfaces help readers to explore information networks
safely and confidently. A reader may be working on a particular task
when a link anchor catches the eye and decides to take a detour.
The reader should be able to select this link knowing that if it is not
interesting, or when done exploring that tangent, he or she can return
readily to the point of departure and continue the original task. In the
insurance scenario, for example, a potential client may be reading
information about homeowners insurance and decide to follow an optional
"detour," e.g., a link describing the difference between tenant and
homeowners insurance or to a customer testimonial. When done, the client
can then backtrack to the main materials. Indeed, the insurance company
may wish to take advantage of easy returning to provide several detours
that promote their other products, knowing that the customer will not
get "lost" by examining these. Bieber and
Kacmar (1995) provide examples of backtracking within a geographic
information system.3.10 Additional hypermedia functionality
The papers and presentations at the Second International Workshop on
Incorporating Hypertext Functionality into Software Systems (Ashman et al., 1996) held in
conjunction with the ACM Hypertext `96 conference addressed a
wide variety of hypermedia features in software systems. Table 3
classifies these into node, link, navigation, and miscellaneous issues
[6]. The present list is by no means
complete, but rather gives one an idea of the richness and applications
of hypermedia functionality research. Unfortunately, in this paper we
could describe only a small, but important selection of these. In what
follows we briefly describe the ones we left out.
Hypermedia features addressed in the HTF II
workshop
A. Node issues
Annotations
Typed nodes
B. Link issues
Associative and annotative links
Computed and hand-crafted links
Dynamically computed and precomputed links
Link attributes
Transclusions
Unidirectional and bidirectional links
C: Navigation issues
Author-created landmarks
Backtracking and interaction history
Browsers and overviews
Content-based information retrieval mechanisms
Different views of a single hypermedia network
Link traversal mechanisms
Node participation in different views
Reader-created bookmarks
Structure-based information retrieval mechanisms
Trails and guided tours
D. Miscellaneous issues
Access to heterogeneous repositories
Automatic link propagation
Hypermedia-aware user interface controls or widgets
Links to data in any application
Mapping of application objects to hypermedia objects
Standards for content markup and linking
4. Looking back and looking ahead
The World Wide Web emerged among a group of globally dispersed physics
researchers endeavoring to share information. Hypermedia has grown
primarily from computer science and English literature researchers.
While both fields build upon interrelationships, hypermedia researchers
got a head start on designing the high-level features we discuss. Yet
just as WWW environments can benefit greatly from hypermedia
functionality, hypermedia researchers are learning tremendously about
distribution and scale (Engelbart, 1990; Malcolm et al., 1991;Parunak, 1991b) from the Web community
[7]. The
World Wide Web is a wonderful delivery vehicle for hypermedia support.
We urge each field to embrace and prosper from the successes of the
other.References