Projects/Nepomuk/OntologyBasics: Difference between revisions

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    Nepomuk unlike other conventional projects, does not use a relational database to store its data. Instead it uses a RDF store. RDF stands for Resource Description Framework. It's the big framework that was written for describing abstract data on the web, and is the basis of the semantic web. While RDF, has many concepts, Nepomuk mainly utilizes a couple of them.
    Nepomuk unlike other conventional projects, does not use a relational database to store its data. Instead it uses a RDF store. RDF stands for Resource Description Framework. It's the big framework that was written for describing abstract data on the web, and is the basis of the semantic web. While RDF, has many concepts, Nepomuk mainly utilizes a couple of them.
    = Resources =


    =Statements=
    =Statements=


    The simplest concept is that of a sentence. All the data in Nepomuk is stored in the form
    The simplest concept is that of a sentence. All the data in Nepomuk is stored in the form <code><subject> <predicate> <object> <graph></code>. These 4 values combined are called a statement.


    <syntaxhighlight lang="text"><subject> <predicate> <object> <graph></syntaxhighlight>
    If we look at this in terms of an object oriented programming language, the <code><subject></code> is an OOP Object, and the (predicate, object) pair can be looked as (key, value) pairs which define properties on that OOP Object.  
     
     
    These 4 values combined are called a statement.
     
    If we look at this in terms of an object oriented programming language, the `<subject>` is an OOP Object, and the (predicate, object) pair can be looked as (key, value) pairs which define properties on that OOP Object.  


    Here are some examples of common statements -
    Here are some examples of common statements -
    Line 32: Line 25:


    Each predicate is generally assigned a domain and range which governs the kind of <subject>s and <object>s it can be connected to.
    Each predicate is generally assigned a domain and range which governs the kind of <subject>s and <object>s it can be connected to.
    Some examples of commonly used predicates are <code>rdf:type</code> <code>nao:lastModified</code> and <code>nao:hasTag</code>
    Some examples of commonly used predicates are <code>rdf:type</code> [http://oscaf.sourceforge.net/nao.html#nao:lastModified <code>nao:lastModified</code>] and [http://oscaf.sourceforge.net/nao.html#nao:hasTag <code>nao:hasTag</code>]


    == Object ==
    == Object ==
     
    The object is the slightly tricky part of statement. Objects can either be uris or literals. Uris refers to other subjects, and literals can be any of the conventional data types - integers, strings, datetime, boolean.


    == Graph ==
    == Graph ==
    Line 50: Line 43:


    = What is an Ontology =
    = What is an Ontology =
    An ontology is a formal description of the all the properties and types that resources can posses. It can be viewed as the database schema or like the architectural plans.


    = RDF and RDFS =
    = RDF and RDFS =
    The [http://www.w3.org/RDF/ Resource Description Framework] and [http://www.w3.org/2001/sw/wiki/RDFS Resource Description Framework Schema] are the two main schemas that were defined by the Semantic web. They are important because they provide the building blocks for writing all of the other ontologies.
    == Type System ==
    == Type System ==
    The RDF and RDFS provide the foundation for an entire type system. It is slightly simpler than that of any programming language. It just provides single and multiple inheritance.
    Eg -
    <syntaxhighlight lang="text">
        nfo:RemoteDataObject
              a      rdfs:Class ;
              rdfs:comment "A file data object stored at a remote location. Don't confuse this class with a RemotePortAddress. This one applies to a particular resource, RemotePortAddress applies to an address, that can have various interpretations." ;
              rdfs:label "RemoteDataObject" ;
              rdfs:subClassOf nfo:FileDataObject .
    </syntaxhighlight>
    We use the <code>rdfs:subClassOf</code> property to indicate one class is a sub-type of another class.
    == Property System ==
    == Property System ==
    Unlike other conventional system, RDFS also defines a property inheritance system. Each property can be defined a domain, range, cardinality and super-properties.
    <syntaxhighlight lang="text">
        nfo:fileName
              a      rdf:Property ;
              rdfs:subPropertyOf nao:prefLabel ;
              rdfs:comment "Name of the file, together with the extension" ;
              rdfs:domain nfo:FileDataObject ;
              rdfs:label "fileName" ;
              nrl:maxCardinality "1" ;
              rdfs:range xsd:string .
    </syntaxhighlight>
    In the above example a property called <code>nfo:fileName</code> has been declared. This property is a sub-property of the <code>nao:prefLabel</code> ontology.  It additionally defines a <code>nrl:maxCardinality</code> as 1. This indicates that no resource should have more than 1 filename. And finally it defines a domain of <code>nfo:FileDataObject</code> and range of <code>xsd:string</code>.
    Nepomuk currently enforces these ontologies. So if you should always check the domain and range of properties before using them.


    = Common Ontologies =
    = Shared Desktop Ontologies =
    ==NFO==
    ==NFO==
    ==NIE==
    ==NIE==
    ==NMM==
    ==NMM==
    ==NMO==
    ==NMO==

    Revision as of 08:21, 23 August 2012

    Nepomuk unlike other conventional projects, does not use a relational database to store its data. Instead it uses a RDF store. RDF stands for Resource Description Framework. It's the big framework that was written for describing abstract data on the web, and is the basis of the semantic web. While RDF, has many concepts, Nepomuk mainly utilizes a couple of them.

    Statements

    The simplest concept is that of a sentence. All the data in Nepomuk is stored in the form <subject> <predicate> <object> <graph>. These 4 values combined are called a statement.

    If we look at this in terms of an object oriented programming language, the <subject> is an OOP Object, and the (predicate, object) pair can be looked as (key, value) pairs which define properties on that OOP Object.

    Here are some examples of common statements -

    <nepomuk:/res/some-uuid> rdf:type nao:Tag
    <nepomuk:/res/some-uuid> nao:lastModified 2012-03-16T18:41:09.006Z
    <nepomuk:/res/some-uuid> nao:prefLabel "Sample"
    
    <nepomuk:/res/some-other-uuid> rdf:type nfo:FileDataObject
    <nepomuk:/res/some-other-uuid> nao:hasTag <nepomuk:/res/some-uuid>
    

    Subject

    The subject, like in conventional english, is the main part of the sentence. It highlights what we are talking about. In the above example there are different subjects nepomuk:/res/some-uuid and nepomuk:/res/some-other-uuid. In each of these statements the subjects have certain properties and objects bound to them.

    Predicate

    Predicates are often also called properties, and they used to connect the <subject> with the <object>.

    Each predicate is generally assigned a domain and range which governs the kind of <subject>s and <object>s it can be connected to. Some examples of commonly used predicates are rdf:type nao:lastModified and nao:hasTag

    Object

    The object is the slightly tricky part of statement. Objects can either be uris or literals. Uris refers to other subjects, and literals can be any of the conventional data types - integers, strings, datetime, boolean.

    Graph

    The graph contains some information about the triple - (<subject> <predicate> <object>) such as when it was created, and last modified. Unless you're working on the absolute core parts of Nepomuk, you won't ever have to interact with graphs, and they'll just work seamlessly in the background.

    Resources

    What they are - all about

    Collection of Statements

    URIs

    What is an Ontology

    An ontology is a formal description of the all the properties and types that resources can posses. It can be viewed as the database schema or like the architectural plans.

    RDF and RDFS

    The Resource Description Framework and Resource Description Framework Schema are the two main schemas that were defined by the Semantic web. They are important because they provide the building blocks for writing all of the other ontologies.

    Type System

    The RDF and RDFS provide the foundation for an entire type system. It is slightly simpler than that of any programming language. It just provides single and multiple inheritance.

    Eg -

        nfo:RemoteDataObject
              a       rdfs:Class ;
              rdfs:comment "A file data object stored at a remote location. Don't confuse this class with a RemotePortAddress. This one applies to a particular resource, RemotePortAddress applies to an address, that can have various interpretations." ;
              rdfs:label "RemoteDataObject" ;
              rdfs:subClassOf nfo:FileDataObject .
    


    We use the rdfs:subClassOf property to indicate one class is a sub-type of another class.

    Property System

    Unlike other conventional system, RDFS also defines a property inheritance system. Each property can be defined a domain, range, cardinality and super-properties.

        nfo:fileName
              a       rdf:Property ;
              rdfs:subPropertyOf nao:prefLabel ;
              rdfs:comment "Name of the file, together with the extension" ;
              rdfs:domain nfo:FileDataObject ;
              rdfs:label "fileName" ;
              nrl:maxCardinality "1" ;
              rdfs:range xsd:string .
    


    In the above example a property called nfo:fileName has been declared. This property is a sub-property of the nao:prefLabel ontology. It additionally defines a nrl:maxCardinality as 1. This indicates that no resource should have more than 1 filename. And finally it defines a domain of nfo:FileDataObject and range of xsd:string.

    Nepomuk currently enforces these ontologies. So if you should always check the domain and range of properties before using them.

    Shared Desktop Ontologies

    NFO

    NIE

    NMM

    NMO