Topology.html
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<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta name="generator" content=
"HTML Tidy for Mac OS X (vers 31 October 2006 - Apple Inc. build 13), see www.w3.org" />
<title>
HyperText Design Issues: Topology
</title>
<nextid />
</head>
<body bgcolor="#FFC060" text="#302005">
<a href="OldDocs.html"><img src=
"../Icons/WWW/arch1990" /></a>TimBL
<hr />
<h1>
Topology
</h1>Here are a few questions about the underlying connectivity
of a hypertext web.
<h2>
Are links two- or multi-ended?
</h2>The term "link" normally indeicates with two ends.
Variations of this are liks with multiple sources and/or
multiple destinations, and constructs which relate more than
two anchors. The latter map onto logic description systems,
predicate calculus, etc. See the "Aquanet" system from Xerox
PARC - paper at HT91). This is a natural step from hypertext
whose the links are typed with semantic content. For example,
the relation "Document A is a basis for document B given
argument C". From now on however, let us restrict ourselves to
links in the conventional sense, that is, with two ends.
<h2>
<a name="14">Should the links be monodirectional or
bidirectional</a>?
</h2>If they are bidirectional, a link always exists in the
reverse direction. A disadvantage of this being enforced is
that it might constrain the author of a hypertext - he might
want to constrain the reader. However, an advantage is that
often, when a link is made between two nodes, it is made in one
direction in the mind of its author, but another reader may be
more interested in the reverse link. Put another way,
bidirectional linking allows the system to deduce the inverse
relationship, that if A includes B, for example, that B is part
of A. This effectively adds information for free. This is
important when a critical parameter of the system is how long
it takes someone to create a link.
<p>
KMS and hypercard have one-way links; Enquire has two-way
links.
</p>
<p>
There is a question of how one can make a two-way link to a
protected database. The automatic addition of the reverse
link is very useful for enhancing the information content of
the database. See also:<a name="2" href=
"Multiuser.html#3">Private overlaid web</a> , <a name="3"
href=
"../../History/19921103-hypertext/hypertext/Products/Microcosm/Microcosm.html#GenericLinking">
Generic Links</a> .
</p>
<p>
It may be useful to have bidirectional links from the point
of view of managing data. For example: if a document is
destroyed or moved, one is aware of what dangling links will
be created, and can possibly fix them.
</p>
<p>
A compromise that links be one-way in the data model, but
that a reverse link is created when any link is made, so long
as this can be done without infringing protection. An
alternative is for the reverse links to be gathered by a
background process operating on a basically monodirectionally
linked web. See <a name="13" href=
"BuildingBackLinks.html">Building Back-links</a>.
</p>
<h2>
<a name="12">Should anchors have more than one link?</a>
</h2>There is a design issue in whether one anchor may lead to
many links, and/or on link have many anchors. It seems
reasonable for many anchors to lead to the same reference. If
one source anchor leads to more than one destination anchor,
then there will be ambiguity if the anchor is clicked on with a
mouse. This could be resolved by providing a menu to the user,
but I feel this would complicate it too much. I therefore
suggest a many-to-one mapping. <a name="8" href=
"../People.html#groff">JFG</a> disagrees and would like to see
a small menu presented to the user if the link was
ambiguous.<a name="9" href=
"../../History/19921103-hypertext/hypertext/Products/Microcosm/Microcosm.html">Microcosm</a>
does this.
<h2>
<a name="4">Should links be typed?</a>
</h2>A typed link carries some semantic information, which
allows the system to manage data more efficiently on behalf of
the user. A default type ("untyped") normally exists in some
form when types are implemented. See also a <a name="1" href=
"LinkTypes.html">list of some types</a> . (Should a link be
allowed to have many types? (- <a name="6" href=
"../People.html#groff">JFG</a> ) I don't think so: that should
be represented by more than one link.(- <a name="7" href=
"../People.html#BernersLee">TBL</a> ))
<p>
Link typing helps with the generation of <a name="10" href=
"Navigation.html#6">graphical overviews</a> , and with
<a name="11" href="TracingLinks.html">automatic tracing</a> .
</p>
<h2>
Should links contain ancillary information?
</h2>Does the system allow dating, versioning, authorship,
comment text on a link? If so, how is it displayed and
accessed? This sort of information complicates the issue, in
that readable information is no longer carried within node
contents only. Pretty soon, following this path leads to a link
becoming a node in itself, annotatable and all. This perverts
the data model significantly, and I cannot see that that is a
good idea. Information about the link can always be put in the
source node, or in an intermediate node, for example an
annotation. However, this makes tracing more difficult. It is
certainly nice to be able to put a comment on a link. Perhaps
one should make a link annotatable. I think not.
<h2>
Should a link contain Preview information?
</h2>This is information stored at the source to allow the
reader to check whether he wants to follow a link before he
goes. I feel that the system may cache some data (such as the
target node title), or the writer of the node may include some
descriptive material in the highlighted spot, but it is not
necessary to include preview information just because access
may be slow. Caching should be done instead of corrupting the
user interface. If you have a fast <a name="5" href=
"Navigation.html#6">graphic overview</a> , this could remove
the necessity for a preview function.
</body>
</html>