Introduction to Semantic Web (Part 2)

In the first part i talked about the characteristics on which the Semantic Web is built. Now I'll introduce a couple of standards tools that support those characteristics.

The Standards and Tools i’ll cover are:

RDF
Sparql
RDFa


RDF (Resource Description Framewor): Is the De facto standard way of representing graph data for Semantic Werb, in a way that is both understandable by humans and machines. Is a language in which evrything is represented as a Resource (or a literal value), the subject, the predicate and the object. RDF can be represented in many ways. i’ll use the XML representation. An example RDF would be.

<rdf:rdf owl="http://www.w3.org/2002/07/owl#"

cc="http://creativecommons.org/ns#"
dc="http://purl.org/dc/terms/" fb="http://rdf.freebase.com/ns/"
rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xhtml="http://www.w3.org/1999/xhtml/vocab#" about="http://rdf.freebase.com/ns/en.the_other_side_of_midnight">

<fb:m.04l1354>
<dc:description>The Other Side of Midnight is a novel .....</dc:description>
<xhtml:license resource="http://creativecommons.org/licenses/by/3.0/">
<fb:media_common.adapted_work.adaptations resource="http://rdf.freebase.com/ns/m.04jn_x5">

<dc:creator resource="http://rdf.freebase.com/ns/en.sidney_sheldon">
<fb:book.book.genre resource="http://rdf.freebase.com/ns/en.thriller">
......

In this RDF we can see the three parts of every triple. The subject being in our case the rdf:about after the namespace declarations. Objects can be literals or resources, and predicates are resources. So in the previous example one triple would be:

rdf:about="http://rdf.freebase.com/ns/en.the_other_side_of_midnight"
dc:creator
rdf:resource="http://rdf.freebase.com/ns/en.sidney_sheldon"

In this case subject, predicate and object are all resources. As we can see this forms a directed graph.

Namespaces can be mixed as we see, so for example we can create our own custom namespace and mix it with the previous graph definition, extending the knowledge modeled.


SPARQL: Is the language that allow us to query RDF modeled data. From my point of view, it’s a lot like SQL but specifically for querying data in the Triple form. To understand it let’s see an example based on the previous RDF.

PREFIX fb:<”http://rdf.freebase.com/ns/">
PREFIX dc:<”http://purl.org/dc/terms/”>
PREFIX: rdf:<”http://www.w3.org/1999/02/22-rdf-syntax-ns#”>
select ?book
where ?book dc:creator <http://rdf.freebase.com/ns/en.sidney_sheldon>

The previous sparql would return us all the books created by Sidney Sheldon. Notice that all the where clauses would be in the form of Triples. Also binding variables (?book in our case) can be used in more than one triple clause to narrow the results further.


RDFa: Is a set of added constructs that allow us to embbed RDF in XHTML in the form of attributes to normal XHTML tags.For example if we want to expose our knowledge about “The other side of midnight” to the web, allowing potential crawler to see this information we can do something like:

<div xmlns:dc="http://purl.org/dc/elements/1.1/"
about="http://rdf.freebase.com/ns/en.the_other_side_of_midnight">
<span property="dc:description">The Other Side of Midnight is a novel... </span>
<span rel="dc:creator" resource=”http://rdf.freebase.com/ns/en.sidney_sheldon”> Sidney Sheldon</span>
</div>

These are maybe the main standards used in Semantic Web. If you are interested in the subject, you should also take a look at the OWL language and ontologies, and tools like Jena and Sesame for graph data repositories and servers.