Semantic Web: A McCool Way of Explaining It

Yahoo's Rob McCool argues in Rethinking the Semantic Web, Part 1 that the Semantic Web will never happen. Why? Because the Semantic Web has three fundamental parts, and they just don't fit together based on current technologies. Here is what we have. The foundation is the set of data models and formats that provide semantics to applications that use them (RDF, RDF Schema, OWL). The second layer is composed of services - purely machine-accessible programs that answer Web requests and perform actions in response. At the top are the intelligent agents, or applications.

Reason? Knowledge representation is a technique with mathematical roots in the work of Edgar Codd, widely known as the one whose original paper using set theory and predicate calculus led to the relational database revolution in the 1980's. Knowledge representation uses the fundamental mathematics of Codd's theory to translate information, which humans represent with natural language, into sets of tables that use well-defined schema to defined schema to define what can be entered in the rows and columns.

The problem is that this creates a fundamental barrier, in terms of richness of representation as well as creation and maintenance, compared to the written language that people use. Logic, which forms the basis of OWL, suffers from an inability to represent exceptions to rules and the contexts in which they're valid.

Databases are deployed only by corporations whose information-management needs require them or by hobbyists who believe they can make some money from creating and sharing their databases. Because information theory removes nearly all context from information, both knowledge representation and relational databases represent only facts. Complex relationships, exceptions to rules, and ideas that resist simplistic classifications pose significant design challenges to information bases. Adding semantics only increases the burden exponentially.

Because it's a complex format and requires users to sacrifice expressively and pay enormous costs in translation and maintenance, McCool believes Semantic Web will not achieve widespread support. Never? Not until another Edgar Codd comes along our way. So we wait.