Modelling Context using Named Graphs

Bizer & Carroll

Trust + TriX

Semantic Web Interest Group

1-2 March 2004

Context

What is a context?

• Lots of answers, confusion

Minimalist answer:

The context of a statement is its surrounding graph

Hypotheses

• The context of a statement is its surrounding graph addresses enough use cases
• Named graphs can articulate that context
• A pragmatic trust layer gives assertedness
• de re, de dicto? All URIs have a single interpretation. (Thumbs down to log:implies)

What is a Named Graph

A uriref or a blank node is interpreted as a graph.

Syntax

 G1 (Monica ex:name  Monica Murphy . 
      Monica rdf:type ex:Person . 
      Monica ex:homepage  )
 _:G3 (G1 dc:author Chris. 
      _:G3 dc:author Chris )
 

Also TriX (Carroll-Stickler), N3

TriQL (extending RDQL)

SELECT ?x, ?y 
WHERE 
    ?a (?b rdf:type ex:Person . 
        ?b rdf:hasName ?x . 
        ?b ex:homepage ?y) 
    (?a dc:date ?c) 
AND ?c > "1/1/2003"

Data Syndication

• Provenance tracking
• Provenance chains

Provenance Tracking

G1 (Monica ex:name  Monica Murphy . 
    Monica rdf:type ex:Person) 
G2 (G1 dc:author Chris. 
    G1 dc:date  2/10/2004 ) 
G3 (Monica ex:hasSkill ex:Programming) 
G4 (G3 dc:author Peter. 
    G3 dc:date  2/3/2004 )

SELECT ?x ?y ?z 
WHERE ?a (?x ?y ?z) 
     (?a dc:author Chris)

Signing RDF Graphs

G1 (Monica ex:hasStatus Admin. 
    Monica rdf:type ex:Person. 
    G1 ex:author Andy. 
    G1 dc:date  2/10/2004 ) 
G2 (G1 ex:hasSignature  xd2shfl22k4jdsre& . 
    G1 ex:Signer Andy) 
G3 (Andy ex:publicKeyURL http://bla.bla.bla)

SELECT ?a ?b ?c ?d 
WHERE ?d (Monica ex:hasStatus Admin) 
 (?d dc:signer ?a) 
 (?a ex:publicKeyURL ?b) 
 (?d ex:hasSignature ?c)

Scoping Assertions

Unconvincing?

 A consistsOf Metal . 
B consistsOf Water . 
{ Metal denserThan Water } log:implies { A sinksIn B } .

{ Mass a FixedValue } in NewtonianMechanics . 
{ Mass a Variable } in RelativityTheory . 
{ RelativityTheory approximates NewtonianMechanics } 
     when { RelativeVelocities lessThan halfC } .

Liar Paradox

{ <http://example.org/liar> 
     log:implies { 
       <http://example.org/noone> a owl:Nothing .
     } . 
} owl:sameAs <http://example.org/liar> .
<http://example.org/liar> a log:Truth ?

De Re or De Dicto

Against De Dicto

1. SemWeb needs shared conceptualization
2. Appropriate treatment of graph implicit in context of use.
3. integrating info from multiple graphs requires de re; transformation from de dicto needs trust mechanisms concerning correct URI usage.

Workaround for de dicto "ex:thisURI"^^xsd:anyURI

Some Other Use Cases

Access Rights

G1 (Monica ex:hasStatus Admin. 
    Monica rdf:type ex:Person. 
    G1 requiresAccessRight Admin)
  

• Expressing Privacy Preferences

Beliefs

G1 (Monica ex:name  Monica Murphy . 
    Monica rdf:type ex:Person. 
    Monica ex:hasSkill ex:Programming. 
    G1 dc:author Chris) 
G2 (Monica ex:hasSkill ex:Programming) 
G3 (G2 ex:truthValue ex:false) 
G4 (G2 dc:author Peter. 
    G3 dc:author Peter)
  

Trust Evaluations

SW statements are claims not facts.

We evaluate claims pragmatically, based on who said them, in what context, etc.

Reputational info, and digital sigs helpful but neither necessary nor sufficient.

Example Trust Policies

"Distrust everything a vendor says about its competitor."

"Believe information which has been stated by at least 5 independent sources."

Issue List

• de re, de dicto
• liar and log:implies
• assertional status
• document URL uses
• top level graph
• literal as subject
• blank predicate
• blank node scope

Issues - Document, Graphset and Graph

Do not use document URL to identify either a graph or a graph set.

Use explicit names for graphs

Use RDF collections and containers for graphsets

More Issues

Need top level graph?

N3 - yes

TriX - no

Literals as subjects

Yes, but not blank predicates?

Blank Node Scope

File

Issue List

• de re, de dicto
• liar and log:implies
• assertional status
• document URL uses
• top level graph
• literal as subject
• blank predicate
• blank node scope