Semantic Web Tutorial

Part A1: Primer

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The Semantic Web

• Really simple
• Based on RDF standards
• N3 syntax makes tutorial easier

Subject, verb and object

All knowledge is just a set of statements

<#pat> <#knows> <#jo> .

• Everything is identified by URI

  Here a local URI but could point to ANY document

  Verb known as predicate in the statement

• The thing you use a predicate is a Property.
• Don't forget the period.

Object can be literal

<#pat> <#knows> <#jo> .
<#pat> <#age> "24" .

Note: noun form "age" preferred to the verb style "knows" for predicates.

Alternative forms

<#pat> has <#child> <#al> .

Just to make reading easier; no meaning. But...

<#al> is <#child> of <#pat> .

isand of reverse the direction

Saves having inverse relationships for everything (eg parent)

Saving space: the comma and semicolon

<#pat> <#child>  <#al>, <#chaz>, <#mo> ;
       <#age>    "24" ;
       <#eyecolor> "blue" .

• Comma: Here comes another object for same subject & predicate
• Semicolon: here comes another predicate for same subject.
• Aim? Easy scribbling of data.

Data .. eg a table

  <#pat>   <#age> 24;  <#eyecolor> "blue" .
  <#al>    <#age>  3;  <#eyecolor> "green" .
  <#jo>    <#age>  5;  <#eyecolor> "green" .

Unnamed things: Square brackets

<#pat> <#child> [ <#age> 4 ] , [ <#age> 3 ].

Note:

• Words used as IDs have no actual meaning.
• Unnamed nodes can't be used elsewhere.
• If things are names, make the name explicit

  [ <#name> "Pat"; <#age> 24;  <#eyecolor> "blue"  ].
  [ <#name> "Al" ; <#age>  3;  <#eyecolor> "green" ].
  [ <#name> "Jo" ; <#age>  5;  <#eyecolor> "green" ].

Congratulations.

You can now write RDF data in N3.

Sharing concepts

Semantic Web culture:

• Using the same URIs is effort but valuable
• For anything -- including predicates.
• URIs tend to be long so we use namespaces.

  Writing authoritative documents about shared concepts is useful.

Local concept

<> <#title>  "A simple example of N3".

Who or what knows what <#title> is?

Shared concept

<> <http://purl.org/dc/elements/1.1/title>
 "Primer - Getting into the Semantic Web and RDF using N3".

To save space:

@prefix dc:  <http://purl.org/dc/elements/1.1/> .
<> dc:title
  "Primer - Getting into the Semantic Web and RDF using N3".

Note

• No <> used when prefixed identifier
• Typically prefix stands for everything up to including a "#""
• Dublin core example alas does not give RDF schema directly: no "#"

Assumed prefixes

in this tutorial:

@prefix rdf:  <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
@prefix owl:  <http://www.w3.org/2002/07/owl#> .

and also

@prefix : <#> .

eg

:pat :child [ :age 4 ] , [ :age 3 ].

Making vocabularies

• A set of shared concepts
• Properties are things to use as predicates (verbs)

  Classes can be the type of an object

  rdf:type is just a property, abbreviated to "a" in N3

• You can invent them and describe Properties and Classes.

Equivalent:

:Person rdf:type  rdfs:Class

:Person a rdfs:Class.

which we could use with data;

:Pat a :Person.

Classes

• An object can be in many classes
• Classes can have many superclasses
• You can never know all the classes an object is in.
• These are classes of real things, not OO classes.

Examples: class and property

:Woman a rdfs:Class; rdfs:subClassOf :Person .

and a property:

:sister a rdf:Property.

Something about the Property :sister::

:sister rdfs:domain :Person; 
        rdfs:range :Woman.

Use:

:Pat  :sister  :Jo.

Note

• Convention Class ids start with capital, Property ids with lowercase
• More useful terms in the RDF Schema and OWL vocabularies

Best practice: Sharing

• Use other people's terms when you can
• Use your own when you need to
• Be prepared to declare them equivalent later

  = in N3 is short for daml:equivalentTo

  • daml: will become owl:

:Woman = foo:FemaleAdult .
:title a rdf:Property; = dc:title .

Best practice: Publishing

• Don't reuse a URI for something different
• Do publish a schema in RDF (XML or N3) if you can

  Do publish definition of local concepts in the file itself.

  Using OWL terms makes query systems smarter.

Congratulations

You can now define new vocabularies for new applications.

You can link together vocabularies which you and others have made.

You can start to weave the semantic web.

(end of section)