The School of Web Semantics Learning Semantic Web Techniques

In the previous article I presented the story about the birth and the development of the Semantic Web. There is no secret that for its full potential to unwind there is still something missing. You could talk a long time about what has happened, that after 20 years since the Web started, it still does not look like what Sir Tim Berners-Lee (pictured below) had in mind.

One way to present the vision for Web 3.0 development, actually the Semantic Web to be precise, is to analyze the so-called Semantic Web Layer Cake, which shows a stack of technologies and standards needed to create a fully functional Semantic Web. One of its first version (shown in 2002 by TBL) was as follows:

Semantic Web Layer Cake (2002) Source: http://www.w3.org/2002/Talks/04-sweb/slide12-0.html

The foundations of the Semantic Web are two technologies: URI (Uniform Resource Identifier) and Unicode; they enable us to identify and indicate any content or object (not just online) and describe it in any language. A URI is similar to a URL (Uniform Resource Locator) address, which we deal with every day on the Internet; URIs, however, enable (at least theoretically) to indicate the real-world objects, and thus to describe and to link them to documents on the Internet. Unicode enables one to write with characters from all currently known alphabets.

The second layer is a currently very popular standard called XML, and so-called namespaces associated with it. With XML you can save information so that a machine could process it (but not understand it!). In an upcoming article we will try to explain why introducing the XML layer to the "semantic cake" was not such a good idea.

A third layer defines RDF (Resource Description Framework), a standard that is inherently bound to the Semantic Web. Unlike XML, it allows you to create a (directed) graph in a form processable by machines; based on information stored in these graphs of meaningful relationships (i.e., semantics) a machine can attempt to understand the presented content.

And the next two layers? To assist the process of "understanding" of information stored in RDF, the next two layers define terms used in describing an RDF graph. Yes, you guessed it right: it is all about the infamous "ontologies" (coming up in following articles). RDF Schema can be used to define a simple ontology using a hierarchy of classes and properties; but these classes and properties have nothing to do with object-oriented programming (OOP) apart from sharing the same name. We will discuss this issue in a future article.

The sixth layer, the rules, enables to define (in the formal languages such as RIF) rules for processing of knowledge stored in RDF and ontologies. Together with logic frameworks rules will enable machines to understand the information, rather than merely process it.

The top two layers are necessary to ensure that machines have (some) autonomy in information processing and decision making on behalf of their owners. However, this requires a formal means of delivering evidence (called proof) that the reasoning process was correct; plus a way to determine a level of confidence in the reasoning process (called trust).

The other two elements, digital signatures and encryption, ensure the safety of operations in the Semantic Web.

Phew, a lot of layers, and worse still: all of them the same dry specifications, recommendations, and standards. Fortunately, over the years we managed to change this vision into reality, at least partly.

In recent years, the "Semantic Web Layer Cake" has evolved along with the implementations of the successive layers. Already two years after the initial diagram was published, TBL added an additional layer: queries and XML schemas. There were also the first tool (Annotea), dictionaries (P3P , CC / PP) and standards (Dublin Core, RSS) that implement the bottom layers of the cake:

Another version of the semantic cake (2004) http://www.w3.org/2004/Talks/0412-RDF-functions/slide4-0.html

The current version of the cake contains mainly the specifications that have become W3C standards that implement the different layers: OWL (Web Ontology Language), RIF (Rule Interchange Format), SPARQL (query language for RDF). It is important that ontologies, rules and logic have become a de facto single layer. Further, the role of XML has been limited, because in the Semantic Web we can handle stuff without the restrictions imposed by XML. There is also a new delivery layer: UI (User Interface) and applications. In addition, identification of objects is now possible using the IRI (Internationalized Resource Identifier) standard that extends URI with the ability to use non-standard ASCII characters to encode the object identifier.

Until recently, all the emphasis in the Semantic Web research domain was focused at understanding the meaning of the content by machines. Thanks to the success of Web 2.0, we understood the need and importance of human interaction with computer systems systems utilizing semantic techniques.

The current version of the semantic cake http://www.w3.org/2007/03/layerCake.png

Finally, the cherry on the semantic cake is the 3D version prepared by Benjamin Nowack. What it important here is the new element in the stack of semantic technologies: linked data. They are extremely important for the success of Web 3.0: without semantic, i.e., linked, data (often publicly available) no reasoning mechanisms can do anything.

Semantic cake in 3D, together with related data http://bnode.org/blog/2009/07/08/the-semantic-web-not-a-piece-of-cake