Semantic Web

• "The Semantic Web is not a separate Web but an extension of the current one, in which information is given well-defined meaning, better enabling computers and people to work in cooperation."
  • from "The Semantic Web" by Berners-Lee et al., Scientific American, May 2001.
• The idea is to add semantically well-defined metadata to web pages to support more sophisticated (AI) kinds of information retrieval.
• The metadata is often referred to as the ontology for a web page or as being defined within an ontology
• Ontology: (1) the branch of metaphysics that studies the nature of existence or being as such. (2) (loosely) metaphysics.
• There are two standard notations for specifying metadata: RDF and OWL

OWL

• OWL may be thought of as a better defined RDF
• Both are similar, but OWL has formally defined semantics and associated tools such as theorem provers
• OWL was developed by the theoretical end of computer science, not by engineers
• OWL Constructs: classes, subclasses, properties (binary relations) over classes and xsd datatype, and triples (instances)
• Example

OWL: Three Versions

1. OWL Lite supports those users primarily needing a classification hierarchy and simple constraints.
2. OWL DL supports those users who want the maximum expressiveness while retaining computational completeness (all conclusions are guaranteed to be computable) and decidability (all computations will finish in finite time). OWL DL is so named due to its correspondence with description logics.
3. OWL Full is meant for users who want maximum expressiveness and the syntactic freedom of RDF with no computational guarantees. It is unlikely that any reasoning software will be able to support complete reasoning for every feature of OWL Full.

Why the Semantic Web is Slow to Grow

• The formal specifications (both RDF and OWL) require a web page designer to essentially specify the page twice: once for display and once to capture semantics
• Capturing semantics is hard
• Notations are difficult to understand and apply
• Scalability
• Other criticisms
  • The 7 (f)laws of the Semantic Web
  • Introduction to Microformats

Microformats

• History is a little unclear
• Started as an attempt to standardize certain XHTML/XML structures
• "...microformats are a set of simple, open data formats built upon existing and widely adopted standards.
• Primary source of information: http://microformats.org
  (I really hate the PR associated with this stuff)

Microformat Specifications

• Abuse various XHMTL features to define various standarized formats
• <abbrev>, <div>, <span>, class=, rel=, ref=
• Examples:
• http://microformats.org/wiki/Main_Page
• http://microformats.org/wiki/selected-test-cases-from-the-web
• http://microformats.org/wiki/xfn

Microformat and the Semantic Web

• In effect, microformats are a sort of bottom up approach to web semantics
• They standardize various small semantic domains
• In theory, it ought to be possible to do a OWL => microformat conversion and use OWL as the formal specification for microformats
• But, you would lose information such as cardinality and subclassing
• But, this could be checked by a "lint" for microformats

hCard/vCard Examples

• References:
• An Ontology for vCards
• hCard Microformat

Observations WRT OWL for Microformats

• Using OWL/RDF for formal definition might be useful, but...
• OWL needs a smaller and cleaner syntax (NS3 is a start) to make it more accessible to microformat designers
• OWL community has not actually focused on the things that can be usefully standardized (who cares about a wine shop). The microformat community seems to be doing a better job of this.
• It would be instructive to compare the existing microformats against the Dublin Core (a defined set of common domains)

Summary

• Microformats provide a bottom-up approach to web semantics
• Low entry barrier
• Abuse existing XHTML tags and attributes
• Semantics not formally defined