6. Entwicklung computergest

1
Semantic Web and Policy Workshop _at_ ISWC 2005
Galway, Ireland, 7 November 2005
The TriQL.P BrowserFiltering Information using
Context-, Content- and Rating-Based Trust
Policies
Christian Bizer, Freie Universität Berlin,
Germany Richard Cyganiak, Freie Universität
Berlin, Germany Tobias Gauss, Freie Universität
Berlin, Germany Oliver Maresch, Technische
Universität Berlin, Germany
2
Agenda

• Trust policies in the context of the Semantic Web
• Demonstration of the TriQL.P browser
• Explanation of the technologies used by the
  browser

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Introduction

• The Semantic Web is envisioned as an open,
  dynamic network of autonomous information
  providers.
• They have different views of the world, different
  levels of knowledge, and different intentions.
• Statements published on the Semantic Web have to
  be seen as claims rather than as facts.
• Thus before Semantic Web information should be
  used its trustworthiness has to be evaluated
  according to task-specific criteria.

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How do we decide what to trust in the offline
world?

• In everyday life, we use a wide range of trust
  policies.
• These policies depend on the specific situation,
  our subjective preferences, our past experiences
  and the trust relevant information available.
• We might trust Andy on restaurants but not on
  computers,
• trust professors on their research field,
• believe foreign news only when it is reported by
  several independent sources and
• buy only from sellers on eBay who have more than
  100 positive ratings.

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Trust Assessment Methods

• Every trust policy employs one or more trust
  assessment methods.
• These methods can be classified into three
  categories
• Rating-based assessment methods
• Context-based assessment methods
• Content-based assessment methods

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Rating-Based Assessment Methods

• most proposals for the Semantic Web fall in this
  category.
• have a general problem
• they require explicit and topic-specific ratings.
• high effort for information consumers.
• Central question Are the users willing to rate?

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Context-Based Assessment Methods

• use background information about the information
  provider
• e.g. his role in the application domain or his
  membership in a specific group
• example policies Trust professors about their
  research field. or "Distrust everything a
  vendor says about his competitor.
• use information created in the information
  gathering process
• publishing and retrieval date and the retrieval
  URL
• information whether a signature is verifiable or
  not
• example policy Trust all information which has
  been signed and is not older than a month.

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Content-Based Assessment Methods

• use information content itself, together related
  information content published by other
  information providers.
• Example policies
• Believe information which has been stated by at
  least 2 independent sources.
• Distrust product prices that are more than 50
  below the average price.

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The TriQL.P Semantic Web Browser

• Browse
• General purpose RDF browser for exploring RDF
  datasets containing information from multiple
  sources.
• Collect information together with provenance
  meta-information from the Web.
• Filter
• Filter information using a wide range of
  user-definable reputation-, context- and
  content-based trust policies.
• Explain
• In order to facilitate the users understanding
  of the filtering decisions, the browser creates
  explanations why information fulfils a selected
  policy.
• The TriQL.P browser builds on Piggy Bank.

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Demonstration .
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How does it work?
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The Named Graphs Data Model

• A Named Graph is an entity which consists of
• A name, which is an URIref
• A graph, which is an RDF Graph
• A Named Graph is a resource, which can be
  described by RDF statements within the graph or
  in another graph.
• Pragmatic alternative to reification!
• Formal definition Jeremy Carroll, Christian
  Bizer, Patrick Hayes, Patrick Stickler Named
  Graphs. Journal of Web Semantics, Vol. 3, Issue
  4, 2005

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Named Graphs in W3C SPARQL
Set of Named Graphs
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NG4J Named Graphs API for Jena

• Extension to the Jena semantic web toolkit
• APIs for manipulating sets of Named Graphs
• Memory and database backed storage
• TriQL and SPARQL query languages
• Serialization using TriX, TriG and collections of
  RDF/XML files
• SWP API for signing sets of graphs
• Available under BSD license

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TriX
ltTriX xmlns"http//www.w3.org/2004/03/trix/trix-1
/"gt ltgraphgt lturigthttp//www.bizer.de/Informa
tionAboutRichardlt/urigt lttriplegt
lturigthttp//richard.cyganiak.de/foaf.rdfRichardCy
ganiaklt/urigt lturigthttp//xmlns.com/foaf/0.1/
mboxlt/urigt lturigtmailtorichard_at_cyganiak.delt/
urigt lt/triplegt lt/graphgt ltgraphgt
lturigthttp//www.bizer.de/ProvenanceInformationlt/ur
igt lttriplegt lturigthttp//www.bizer.de/Inf
ormationAboutRichardlt/urigt
lturigthttp//purl.org/dc/elements/1.1/authorlt/urigt
ltplainLiteralgtChris Bizerlt/plainLiteralgt
lt/triplegt lt/graphgtlt/TriXgt

• plays well with generic XML tools like XSLT or
  XQuery

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TriG

• Turtle subset of N3 extended with graph naming.

_at_prefix dc lthttp//purl.org/dc/elements/1.1/gt .
_at_prefix ex lthttp//www.example.org/vocabulary/gt
. _at_prefix lthttp//www.example.org/exampleDocume
nt/gt . G1 _Monica exname "Monica Murphy"
. _Monica exemail ltmailtomonica_at_murphy.or
ggt. G1 exdisallowedUsage exMarketing
G2 G1 exauthor Chris . G1 exdate
"2003-09-03"xsddate
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The Semantic Web Publishing Vocabulary

• Named Graphs provide the hooks on which we pin
• provenance information
• propositional attitudes
• digital signatures
• The Semantic Web Publishing Vocabulary (SWP)
  provides the necessary terms
• http//www.w3.org/2004/03/trix/swp-2/

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The Semantic Web Publishing Vocabulary
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Graph containing provenance information about
itself
lturnuuid8c845860-dce7-11d9-b9c0-00112ff60c7fgt
exPeterSmith a foafPerson foafname
"Peter Smith" foafmbox ltmailtopeter.smith
_at_petersmith.comgt . lturnuuid8c845860-dce7-11d9-b
9c0-00112ff60c7fgt swpassertedBy
lturnuuid8c845860-dce7-11d9-b9c0-00112ff60c7fgt
swpauthority lthttp//www.bizer.degt
dcdate "2005-06-14T1718100200"
swpsavedFrom lthttp//www.bizer.de/myFriends.htmgt
.
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TPL - Trust Policy Language

• A Trust Policies consists of a
• Policy Name
• Policy Description
• Query Template
• Explanation Templates
• A Query Template consists of
• Graph patterns
• which are matched against the set of Named Graphs
  in the browsers repository.
• Constraints
• which further restrict the resulting variable
  bindings.

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Example Policy

• Trust only information that has been asserted by
  at least two different sources.

Policy6 rdftype tplTrustPolicy
tplpolicyName "Two different sources"
tplpolicyDescription "Trust only information
that has been asserted by at least two
different sources."
tpltextExplanation "it was stated by at least
two different sources. The sources are"
tplgraphPattern tplpattern
"(?GRAPH swpassertedBy ?warrant .
?warrant swpauthority ?authority)"
tpltextExplanation "_at__at_?authority_at__at_"
tplconstraint "COUNT(?authority) gt 2" .
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Applying the Policy
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Justification Bindings

• The TriQL.P query engine caches the variable
  bindings resulting from policy patterns.
• They may be used later to generate explanations.

Triple
Justification Bindings
exChris dcname Chris
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Generating Explanations
Triple
Justification Bindings
exChris dcname Chris
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The METRIC() Interface

• The browser provides and open interface for
  integrating application domain specific
  assessment metrics.
• Metrics currently implemented
• eBay, TidalTrust, Appleseed
• PageRank (foafknows, rdfseeAlso)
• Metrics generate custom explanationsabout their
  calculation process.

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Summary

• Semantic Web applications need flexible trust
  architectures supporting subjective,
  task-specific trust policies.
• Context- and content-based assessment methods are
  applicable in situations where ratings are not
  available.
• The Named Graphs data model proved suitable for
  representing information together with
  trust-related meta-information.
• Expressing policies as query templates is an
  alternative to expressing them as rules.
• Next step Redesign TPL syntax to be closer to
  SPARQL.

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Thanks -)

• TriQL.P Browser
• http//www.wiwiss.fu-berlin.de/suhl/bizer/TriQLP/b
  rowser
• Named Graphs Paper
• http//www.websemanticsjournal.org/ps/pub/2005-23
• NG4J - Named Graphs API for Jena
• http//www.wiwiss.fu-berlin.de/suhl/bizer/ng4j/