The Free-For-All in Montreal

Facing the conflict between Topic Maps and RDF head-on, the conference staged a debate between Eric "RDF" Miller of OCLC and Eric "Topic Maps" Freese of ISOGEN[10]. Freese and Miller provided this comparison between the two specs:

Both specifications

• are hard to read
• share a goal: to tie semantics to document structures
• provide a systematic way to declare a vocabulary and basic integrity constraints
• provide a typing system
• provide entity relationships
• both work well with established ontologies

The correspondences between the specs look something like this:

RDF	Topic Maps
Resource	Topics
RDF schema	TM templates (proposed)
Properties	Facets and association roles
URIs	Topic identity, scope
Reification	Association IDs

Differences between the two specifications

• Topic Maps are not XML-specific and have so far been standardized for SGML only. The XML Topic Map activity under the GCA's IDEAlliance is drafting a proposal for such an implementation.
• RDF is also not XML-specific, but to date has been implemented only in XML
• RDF now has a schema specification which provides a standard way to express and link an ontology; such a schema is proposed for Topic Maps
• RDF uses XML linking, Topic Maps use HyTime linking
• Topic Maps have explicit scoping
• Topic Maps start with the abstract layer and (optionally) link to resources; RDF starts at the resource layer and (optionally) creates an abstract layer

Modeling Topic Maps with RDF "loses the distinction between topics and resources," according to Freese.

In preparation for Montreal, he put out a call for suggestions on how to combine the two to end up with the best that each has to offer. Here are some of the suggestions:

• consider topics as collections of resources (anchors) or links such that one object can be a link by a link interpreter and a topic by a Topic Map interpreter
• add RDF's frame-based notation to Topic Maps to attach properties to resources
• model RDF as a Topic Map application, gaining the scoping, merging, and inheritance mechanisms

David Dodds provided one view of an RDF/Topic Map alliance in his paper, "Simultaneous Topic Maps and RDF Metadata Structures in SVG." [11] In this application, he embedded Topic Map constructs in RDF metadata within SVG resources. With this notation, a graphics application would then know that a bar chart is a bar chart, and that each bar represents a certain scale and quantity. Since the RDF is embedded in a map, an external Topic Map processor can also manipulate the image.

Freese's example of the best of both worlds would look like this:

<topic xlink:type="extended"...etc...>
  <resource xlink:type="locator" xlink:href="...etc..."
     rdf:type="dublinCore">
       <dc:author>Dr Livingstone</author>
       <dc:language>english</language>
       ...etc...
  </resource>
</topic>

This example attaches a set of properties to a locator, which is a link. The topic could also be an RDF frame and, therefore, could contain any kind of property.

The reaction of the user community in Montreal was strong and unequivocal: merge the two or at least make them compatible. Among the desirable outcomes that were mentioned was a new syntax for RDF that would retain the graph notation but be less difficult to use.

The Next Round

Before SGML, there were GML and GenCode, which had no doctypes, only generalized instance syntax. Then SGML and XML provided a means to declare shared syntax for a type of document. There are several schema languages for XML that provide even stronger typing. Now we are seeing the binding of instances to even more powerful and more abstract models of information. As others have said, you always need one level of indirection more than you have.

As a result of Montreal, representatives from the W3C RDF activity and the XML Topic Maps group have committed to a series of unofficial joint teleconferences to begin this month. One of the first topics of discussion will be an RDF schema definition of a topic map. According to Freese and Miller, it looks like this is a viable and sound basis on which to open discussion on the possible convergence of the two specifications.

But as C. M. Sperberg-McQueen reminded us in his closing keynote, meaning is always in the instance. It would be reassuring to think that the Topic Map and RDF folks will hold this in mind as they convene their joint meetings and deliberate on the future of angle brackets with metadata. Reducing Tosca to a Topic Map, or a set of directed graphs, and calling the libretto "mere information," while calling the metadata schema "knowledge," misses a very large and important boat. Again, as Sperberg-McQueen put it, we should all "resist the temptation to be more meta than thou," and not lose sight of the importance of the instance itself.

Resources

• TopicMaps.com: a site from STEP UK providing a good introduction to Topic Maps.
• Infoloom: a Topic Maps vendor.
• XML Topic Maps: mailing list for work on XML Topic Maps.
• W3C RDF Page: a good collection of RDF resources from the W3C.

Extreme Markup Papers

[1] The descriptive/procedural distinction is flawed
Allen Renear, Brown University

[2] Semantic interoperability on the Web
Jeff Heflin and James Hendler, both of University of Maryland

[3] Using UML to define XML document types
W. Eliot Kimber and John Heintz, both of DataChannel, Inc

[4] Markup's current imbalance
Paul Caton, Brown University

[5] Meaning and interpretation of markup
C. M. Sperberg-McQueen, W3C, MIT Laboratory for Computer Science, Claus Huitfeldt, University of Bergen, and Allen Renear, Brown University

[6] Management of XML documents in an integrated digital library
David Smith, Anne Mahoney, and Jeffrey A. Rydberg-Cox, Perseus Project

[7] Constructing a navigable Topic Map by inductive semantic acquisition methods
Helka Folch, Eléctricité de France, Benoit Habert

[8] Building dynamic Web sites with Topic Maps and XSLT
Nikita Ogievetsky, Cogitech Inc.

[9] Validating Topic Maps with constraints
Hans Holger Rath, STEP Electronic Publishing Solutions GmbH

[10] Topic Maps and RDF
Eric Freese, ISOGEN/DataChannel

RDF and Topic Maps
Eric Miller, Online Computer Library Center

[11] Simultaneous Topic Maps and RDF metadata structures in SVG
David Dodds, Open Text

Other presentations available online.