Explorer's Guide to the Semantic Web

The Web is incomplete, meaning there can be no guarantee that every link will work or that all possible information will be available. It can be inconsistent: Anyone can say anything on a web page, so different web pages can easily contradict each other. Information on the Web will never be fully consistent, and it also changes constantly. Just think of all the web pages you've returned to that changed since you last visited them, or that don't even exist anymore. Software that wishes to draw logical conclusions from data on the Web must work with reasonable reliability in the face of all this change, potential inconsistency, and incompleteness.

1.3.6 The Web and the Semantic Web

For the Semantic Web to follow the current web model, then, it should use key aspects of the current World Wide Web:

8 The domain name is the general part of the server’s name—usually, many servers share a domain name. For example, in the URL www.cnn.com, the domain name is cnn.com.

1.4 The Semantic Web layer cake

The W3C web pages on the Semantic Web include a diagram labeled Architecture. This diagram, sometimes called the “Semantic Web layer cake,” has been reproduced often, and our own version of it is depicted in figure 1.1. Descriptions of the layers are as follows:

Figure 1.1 The layered technologies of the Semantic Web, according to Tim Berners-Lee and the W3C. Each layer is seen as building on—and requiring—the ones below it. The W3C has developed, or is in the process of developing, standards and recommendations for all but the top two layers, and the W3C recommendations for digital signatures and managing encryption keys will also play roles in the Trust layer.

Each layer is seen as building on the layer below. At the base, most data is expected to be created in XML formats. Each layer is progressively more specialized and also tends to be more complex than the layers below it. A lower layer doesn’t depend on any higher layers. Thus the layers can be developed and made operational relatively independently. XML is in place now, and XML Schema has recently become standardized. RDF has been released as a W3C Recommendation, 9 (and has just been re-released with changes). The other layer sare under development, and their form and direction are progressively uncertain
according to their altitude in the layer cake.

You should realize that this diagram represents the W3C view, and most of the technologies depicted in the diagram are W3C developed or endorsed. There are potential alternatives for some of the layers. Among others, alternative schemas exist for XML documents, and there are quite a few alternative efforts to develop ontology systems.

If you noticed that there is no layer labeled Web Services, it’s true that services don’t fit neatly into this layer cake. Such technologies make use of several layers, such as XML and XML Schemas—and perhaps, in the future, RDF and Ontology. This book also discusses other technologies and subjects that don’t appear in the layer-cake diagram.

9 The W3C publishes technology standards like HTML, the common Hypertext Markup Language. It calls them Recommendations, even though many people informally call them standards or specifications. In the W3C process, a document proceeds through a series of draft stages, moving from Working Draft through Candidate Recommendation before it gets released as an approved Recommendation.