Submissions:Context Slices

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|Author=Chris Welty
|Author=Chris Welty
}}
}}
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{{Logical OP Description Template}}
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{{Logical OP Description Template
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|Motivation=Most information on the web is contextualized somehow, for example information may be believed by a person or organization, it may hold only for some time period, it may have been reported/observed by an individual, etc.  There are myriad proposals and logics for context, but none are standards and few have even prototype implementations. 
 +
 
 +
In RDF and other binary relation languages (like object oriented languages and description logics), one typical way to represent that a binary relation holds in some context is to "reify" the relation-holding (sometimes called obtainment) in the context as an object with a binary relation between the obtainment and each the two relation arguments and a third binary relation between the obtainment and an object representing the context itself.  The downside to this approach is the expressive ability of the language, especially in the case of description logics, is lost.
 +
 
 +
The motivation for context slices is to provide a logical pattern for encoding context information in standard RDF graphs that allows some of the expressiveness of OWL to be used in describing the relations that hold in contexts.
 +
 
 +
This is a generaliztion of the four dimensional ontology for fluents published in [Welty & Fikes, 2006].
 +
|Aim=To encode that a binary relation holds in a context.
 +
|Solution=As shown in the example, the idea of the context slices pattern is, rather than reifying the obtainment itself, to create a projection of the relation arguments in each context for which some binary relation holds between them.  Take for example the statement "Chris believes Sam is CEO of IBM".  Say we already have nodes in some graph representing Sam and IBM. 
 +
 
 +
We create, as shown in the diagram, the context c1 corresponding to Chris' belief, and two nodes representing Chris' belief about Sam and Chris' belief about IBM (shown as Sam@c1 and IBM@c1). 
 +
 
 +
This allows us to represent ceoOf as a binary relation, which seems more natural, and it allows us to use the expressivity of OWL in more ways. We can say of the ceoOf relation that it has an inverse, hasCeo.  In contrast, the reified obtainment approach has no "directionality" in relations, but rather represents the "roles" played by the arguments as binary relations to the obtainment.
 +
 
 +
We can express cardinality, e.g. a company may have only one CEO within a context. This axiom cannot be expressed in OWL for reified obtainments.
 +
 
 +
We can say that a relation is transitive or symmetric. Symmetry can be expressed in reified obtainments by using a single role for both arguments, but not transitivity.
 +
 
 +
We can express relation taxonomies in the usual way.  This is tricky to do using reified obtainments, though it is possible.
 +
|Elements=As shown in the diagram, the pattern uses two predefined classes, cs:ContextualProjection and cs:Context.  It uses two predefined properties, cs:projectionOf (ContextualProjection x TOP), and cs:hasContext (cs:ContextualProjection x cs:Context).  Properties that hold in a context can extend the property cs:contextualProperty.
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|Implementation=<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE rdf:RDF [
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  <!ENTITY owl "http://www.w3.org/2002/07/owl#">
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  <!ENTITY rdf "http://www.w3.org/1999/02/22-rdf-syntax-ns#">
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  <!ENTITY rdfs "http://www.w3.org/2000/01/rdf-schema#">
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  <!ENTITY xsd "http://www.w3.org/2001/XMLSchema#">
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]>
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<rdf:RDF xml:base=""
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        xmlns:owl="&owl;"
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        xmlns:rdf="&rdf;"
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        xmlns:rdfs="&rdfs;">
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<!-- Ontology Information -->
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  <owl:Ontology rdf:about="">
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    <rdfs:label xml:lang="en">Context slices ontology logical pattern</rdfs:label>
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    <owl:versionInfo xml:lang="en">1.0</owl:versionInfo>
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  </owl:Ontology>
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<!-- Classes -->
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  <owl:Class rdf:about="ContextualProjection">
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    <rdfs:comment rdf:datatype="&xsd;string">A contextual projection or slice of an entity or event. 
 +
The context defines the contextual extent of the slice.  If any relations hold in a context, they
 +
must be to other slices of the same context (there is no way
 +
to enforce this constraint in OWL).<rdfs:comment>
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    <rdfs:subClassOf>
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      <owl:Restriction>
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        <owl:cardinality rdf:datatype="&xsd;nonNegativeInteger">1</owl:cardinality>
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        <owl:onProperty rdf:resource="#hasContext"/>
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      </owl:Restriction>
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    </rdfs:subClassOf>
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    <rdfs:subClassOf>
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      <owl:Restriction>
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        <owl:cardinality rdf:datatype="&xsd;nonNegativeInteger">1</owl:cardinality>
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        <owl:onProperty rdf:resource="#projectionOf"/>
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      </owl:Restriction>
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    </rdfs:subClassOf>
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    <rdfs:subClassOf>
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      <owl:Restriction>
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        <owl:allValuesFrom rdf:resource="Context"/>
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        <owl:onProperty rdf:resource="#hasContext"/>
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      </owl:Restriction>
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    </rdfs:subClassOf>
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    <owl:disjointWith rdf:resource="Context"/>
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  </owl:Class>
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  <owl:Class rdf:about="Context">
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    <rdfs:comment rdf:datatype="&xsd;string">It is intended that this class should be equated
 +
with whatever class represents contexts, with possibly subclasses for different kinds such as
 +
BeliefContext, TemporalInterval, etc.</rdfs:comment>
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    <owl:disjointWith rdf:resource="#ContextualProjection"/>
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  </owl:Class>
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<!-- Object Properties -->
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  <owl:ObjectProperty rdf:about="#contextualProperty">
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    <rdfs:comment rdf:datatype="&xsd;string">A property that holds in a context.</rdfs:comment>
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    <rdfs:domain rdf:resource="#ContextualProjection"/>
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    <rdfs:range rdf:resource="#ContextualProjection"/>
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  </owl:ObjectProperty>
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  <owl:FunctionalProperty rdf:about="#hasContext">
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    <rdf:type rdf:resource="&owl;ObjectProperty"/>
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    <rdfs:comment rdf:datatype="&xsd;string">The relation from a ContextualProjection to the context in which some property holds</rdfs:comment>
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    <rdfs:domain rdf:resource="#ContextualProjection"/>
 +
  </owl:FunctionalProperty>
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  <owl:FunctionalProperty rdf:about="#projectionOf">
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    <rdf:type rdf:resource="&owl;ObjectProperty"/>
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    <rdfs:comment rdf:datatype="&xsd;string">The entity or event that a context slice is a slice of.</rdfs:comment>
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    <rdfs:domain rdf:resource="#ContextualProjection"/>
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  </owl:FunctionalProperty>
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</rdf:RDF>
 +
 
 +
}}
{{Logical OP Example Template}}
{{Logical OP Example Template}}
{{Logical OP Reference Template}}
{{Logical OP Reference Template}}

Revision as of 03:59, 18 August 2010


This pattern has been certified.

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Current revision ID: 9838

Graphical representation

Diagram

Image:Context.-slices-example.jpg

General information

Name Context Slices
Also known as
Author(s) Chris Welty
SubmittedBy Chris Welty


Description

Motivation Most information on the web is contextualized somehow, for example information may be believed by a person or organization, it may hold only for some time period, it may have been reported/observed by an individual, etc. There are myriad proposals and logics for context, but none are standards and few have even prototype implementations.

In RDF and other binary relation languages (like object oriented languages and description logics), one typical way to represent that a binary relation holds in some context is to "reify" the relation-holding (sometimes called obtainment) in the context as an object with a binary relation between the obtainment and each the two relation arguments and a third binary relation between the obtainment and an object representing the context itself. The downside to this approach is the expressive ability of the language, especially in the case of description logics, is lost.

The motivation for context slices is to provide a logical pattern for encoding context information in standard RDF graphs that allows some of the expressiveness of OWL to be used in describing the relations that hold in contexts.

This is a generaliztion of the four dimensional ontology for fluents published in [Welty & Fikes, 2006].

Aim To encode that a binary relation holds in a context.
Solution description As shown in the example, the idea of the context slices pattern is, rather than reifying the obtainment itself, to create a projection of the relation arguments in each context for which some binary relation holds between them. Take for example the statement "Chris believes Sam is CEO of IBM". Say we already have nodes in some graph representing Sam and IBM.

We create, as shown in the diagram, the context c1 corresponding to Chris' belief, and two nodes representing Chris' belief about Sam and Chris' belief about IBM (shown as Sam@c1 and IBM@c1).

This allows us to represent ceoOf as a binary relation, which seems more natural, and it allows us to use the expressivity of OWL in more ways. We can say of the ceoOf relation that it has an inverse, hasCeo. In contrast, the reified obtainment approach has no "directionality" in relations, but rather represents the "roles" played by the arguments as binary relations to the obtainment.

We can express cardinality, e.g. a company may have only one CEO within a context. This axiom cannot be expressed in OWL for reified obtainments.

We can say that a relation is transitive or symmetric. Symmetry can be expressed in reified obtainments by using a single role for both arguments, but not transitivity.

We can express relation taxonomies in the usual way. This is tricky to do using reified obtainments, though it is possible.

Elements As shown in the diagram, the pattern uses two predefined classes, cs:ContextualProjection and cs:Context. It uses two predefined properties, cs:projectionOf (ContextualProjection x TOP), and cs:hasContext (cs:ContextualProjection x cs:Context). Properties that hold in a context can extend the property cs:contextualProperty.
Implementation <?xml version="1.0" encoding="UTF-8"?>

<!DOCTYPE rdf:RDF [

 <!ENTITY owl "http://www.w3.org/2002/07/owl#">
 <!ENTITY rdf "http://www.w3.org/1999/02/22-rdf-syntax-ns#">
 <!ENTITY rdfs "http://www.w3.org/2000/01/rdf-schema#">
 <!ENTITY xsd "http://www.w3.org/2001/XMLSchema#">

]> <rdf:RDF xml:base=""

        xmlns:owl="&owl;"
        xmlns:rdf="&rdf;"
        xmlns:rdfs="&rdfs;">
 <owl:Ontology rdf:about="">
   <rdfs:label xml:lang="en">Context slices ontology logical pattern</rdfs:label>
   <owl:versionInfo xml:lang="en">1.0</owl:versionInfo>
 </owl:Ontology>
 <owl:Class rdf:about="ContextualProjection">
   <rdfs:comment rdf:datatype="&xsd;string">A contextual projection or slice of an entity or event.  

The context defines the contextual extent of the slice. If any relations hold in a context, they must be to other slices of the same context (there is no way to enforce this constraint in OWL).<rdfs:comment>

   <rdfs:subClassOf>
     <owl:Restriction>
       <owl:cardinality rdf:datatype="&xsd;nonNegativeInteger">1</owl:cardinality>
       <owl:onProperty rdf:resource="#hasContext"/>
     </owl:Restriction>
   </rdfs:subClassOf>
   <rdfs:subClassOf>
     <owl:Restriction>
       <owl:cardinality rdf:datatype="&xsd;nonNegativeInteger">1</owl:cardinality>
       <owl:onProperty rdf:resource="#projectionOf"/>
     </owl:Restriction>
   </rdfs:subClassOf>
   <rdfs:subClassOf>
     <owl:Restriction>
       <owl:allValuesFrom rdf:resource="Context"/>
       <owl:onProperty rdf:resource="#hasContext"/>
     </owl:Restriction>
   </rdfs:subClassOf>
   <owl:disjointWith rdf:resource="Context"/>
 </owl:Class>
 <owl:Class rdf:about="Context">
   <rdfs:comment rdf:datatype="&xsd;string">It is intended that this class should be equated

with whatever class represents contexts, with possibly subclasses for different kinds such as BeliefContext, TemporalInterval, etc.</rdfs:comment>

   <owl:disjointWith rdf:resource="#ContextualProjection"/>
 </owl:Class>
 <owl:ObjectProperty rdf:about="#contextualProperty">
   <rdfs:comment rdf:datatype="&xsd;string">A property that holds in a context.</rdfs:comment>
   <rdfs:domain rdf:resource="#ContextualProjection"/>
   <rdfs:range rdf:resource="#ContextualProjection"/>
 </owl:ObjectProperty>
 <owl:FunctionalProperty rdf:about="#hasContext">
   <rdf:type rdf:resource="&owl;ObjectProperty"/>
   <rdfs:comment rdf:datatype="&xsd;string">The relation from a ContextualProjection to the context in which some property holds</rdfs:comment>
   <rdfs:domain rdf:resource="#ContextualProjection"/>
 </owl:FunctionalProperty>
 <owl:FunctionalProperty rdf:about="#projectionOf">
   <rdf:type rdf:resource="&owl;ObjectProperty"/>
   <rdfs:comment rdf:datatype="&xsd;string">The entity or event that a context slice is a slice of.</rdfs:comment>
   <rdfs:domain rdf:resource="#ContextualProjection"/>
 </owl:FunctionalProperty>

</rdf:RDF>

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Reviews about Context Slices
Review article Posted on About revision (current is 9838)
VojtechSvatek about Context Slices 245545616 September 2010 1005810,058
EnricoMotta about Context Slices 245545919 September 2010 1011910,119
ValentinaPresutti about Context Slices 245546323 September 2010 1011910,119

This revision (revision ID 9838) takes in account the reviews: ValentinaPresutti about Context Slices

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