Machine Understandable information: Semantic Web

The Semantic Web is a web of data, in some ways like a global database. The rationale for creating such an infrastructure is given elsewhere [Web future talks &c] here I only outline the architecture as I see it.

The basic assertion model

When looking at a possible formulation of a universal Web of semantic assertions, the principle of minimalist design requires that it be based on a common model of great generality. Only when the common model is general can any prospective application be mapped onto the model. The general model is the Resource Description Framework.

See the RDF Model and Syntax Specification

Being general, this is very simple. Being simple there is nothing much you can do with the model itself without layering many things on top. The basic model contains just the concept of an assertion, and the concept of quotation - making assertions about assertions. This is introduced because (a) it will be needed later anyway and (b) most of the initial RDF applications are for data about data (”metadata”) in which assertions about assertions are basic, even before logic. (Because for the target applications of RDF, assertions are part of a description of some resource, that resource is often an implicit parameter and the assertion is known as a property of a resource).

As far as mathematics goes, the language at this point has no negation or implication, and is therefore very limited. Given a set of facts, it is easy to say whether a proof exists or not for any given question, because neither the facts nor the questions can have enough power to make the problem intractable.

Applications at this level are very numerous. Most of the applications for the representation of metadata can be handled by RDF at this level. Examples include card index information (the Dublin Core), Privacy information (P3P), associations of style sheets with documents, intellectual property rights labeling and PICS labels. We are talking about the representation of data here, which is typically simple: not languages for expressing queries or inference rules.

RDF documents at this level do not have great power, and sometimes it is less than evident why one should bother to map an application in RDF. The answer is that we expect this data, while limited and simple within an application, to be combined, later, with data from other applications into a Web. Applications which run over the whole web must be able to use a common framework for combining information from all these applications. For example, access control logic may use a combination of privacy and group membership and data type information to actually allow or deny access. Queries may later allow powerful logical expressions referring to data from domains in which, individually, the data representation language is not very expressive. The purpose of this document is partly to show the plan by which this might happen.

Source : W3C