Introduction to the Semantic Web

Lalana Kagal

6.898: Transparency and Accountability Architectures

World Wide Web

What is the Web? Information represented in natural language Documents contain graphics, multimedia, layouts Suitable for humans Properties of the Web Is a web of documents connected by anchors URIs are used for documents and anchors Standards include HTTP, xHTML, SVG, DOM, XSLT...

Semantic Web

What is the Semantic Web Web resources include meta data in a machine understandable format such as XML Helps in better searching, making inferences, etc. Extends existing web and does not replace it Properties of the Semantic Web Is a web of data URIs are used for documents, concepts, properties that connect concepts Standards include RDF, OWL, SPARQL, RIF...

Uniform Resource Identifiers (URIs)

On the Web A URI identifies a document HTTP allows a client to get a representation of the document On the Semantic Web A URI identifies any thing: Instead of using the word "colour" the concept <http://example.com/2002/std6#col> is used Concepts such as car, parking garage, university Properties of these concepts such as color, number of spaces, location of the university

Representing RDF

• Turtle (http://www.dajobe.org/2004/01/turtle/)

  <http://example.org/mit#pat> <http://xmlns.com/foaf/0.1/knows> <http://example.org/cmu#jo>. 

• RDF/XML (http://www.w3.org/TR/rdf-syntax-grammar/)

  <rdf:Description rdf:about="http://example.org/mit#pat">
       <foaf:knows rdf:resource="http://example.org/cmu#jo"/>
  </rdf:Description>
  	   

• http://example.org/mit, http://example.org/cmu, and foaf are called namespaces and are a collection of concepts
• As a shortcut it is possible to declare them prefixes and get the following

  mit:pat foaf:knows cmu:jo 

  <rdf:Description rdf:about="mit#pat">
       <foaf:knows rdf:resource="cmu#jo"/>
  </rdf:Description>
  	   

Modeling RDF

Similar to Object Oriented Programming

	 public class Course {
     public Person instructor;
     public string location;
	 public Person student;
    
	 // constructor
	 public Course(Person i, string l) {
	   instructor = i;
	   location = l;
         }
    
	 public void AddStudent(Person s) {
	   ...}

	 public List GetStudents() {
	   ...}
     }
   

Modeling RDF data

In RDF you can use rdf and rdfs namespaces to define concepts and their properties

   @prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .  
   @prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .

	 Course rdf:type rdfs:Class.
	 
	 instructor rdf:type rdf:Property.
	 instructor rdfs:domain Course.
	 instructor rdfs:range Person.
	 
	 student rdf:type rdf:Property.
	 student rdfs:domain Course.
	 student rdfs:range Person.	   
	 
	 location rdf:type rdf:Property.
	 location rdfs:domain Course.
   

Representing RDF

More shortcuts, rdf:type = a, property-values of same concept can be separated by ';'

	  6.898 a Course;
	     instructor <http://csail.mit.edu/~lkagal/foaf#me>;
 	     instructor <http://swiss.csail.mit.edu/users/cph/foaf#cph>;
	     student <http://mit.edu/~yosi/foaf#yes>;
	     location <http://mit.edu/36-153>.
	

Friend Of A Friend (FOAF)

• Is a collection of concepts that lets you define yourself, your school, the projects you're working on, and the people you know
• by following links to people you know, and the people they know, it is possible to calculate the degree of separation between people on the web
• Example

      <>  a foaf:PersonalProfileDocument;
          foaf:maker :me.
  
      :me  a foaf:Person;
           foaf:family_name "Kagal";
           foaf:givenname "Lalana";
           foaf:homepage <http://csail.mit.edu/~lkagal/>;
           foaf:knows <http://perich.net/foaf.rdf#FP>,
                  <http://swiss.csail.mit.edu/users/cph/foaf.rdf#cph>,
                  <http://www.w3.org/People/Berners-Lee/card#i>;
           foaf:name "Lalana Kagal";
           foaf:schoolHomepage <http://www.cs.umbc.edu/>;
           foaf:workplaceHomepage <http://www.csail.mit.edu/index.php> .
  	 

Reading/Browsing RDF data

• Some rdf data associated with our project: log.n3
• Install Tabulator Firefox extension (Download)
• Browse Chris's foaf file

Summary

• Semantic Web is a web of data
• Consists of machine understandable knowledge made up of triples (subject predicate object)
• This presentation borrows from http://dig.csail.mit.edu/2007/Talks/0108-sw-tbl/

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