This document summarizes a talk on common errors found in Linked Data. It discusses several types of errors discovered through analyzing over 150,000 RDF documents, including HTTP-level issues like URIs that don't return RDF descriptions, inaccurate content-type reporting, and duplicate content served at different URIs. It also describes reasoning issues such as undefined classes and properties, non-unique values for inverse-functional properties, malformed datatypes, and instances of disjoint classes. The document provides solutions like application workarounds, publishing validators, and the Pedantic Web Group for improving Linked Data quality.

1. Digital Enterprise Research Institute www.deri.ie
Weaving the Pedantic Web
LDOW 2010
Aidan Hogan, Andreas Harth, Alexandre Passant, Stefan
Decker, Axel Polleres
0:39:00
Copyright 2009 Digital Enterprise Research Institute. All rights reserved.
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2. Linked Data…
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3. Purpose of talk: Application developers…
how to not sink…
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4. Purpose of talk: RDF Publishers… how
to avoid common mistakes…
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5. Talking about errors in Linked Data…
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We’ll try not to ruin the party
…statistics based on crawl:
 April 2009
 5k domain limit
 150k URIS, 55k RDF docs
 12.5m triples (quads)
 Mentioning 1.6m URIs
 5,850 classes/9,507 props
 Accept: application/rdf+xml
…okay… so no RDFa
Statistics are *illustrative* not
exhaustive!
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6. Digital Enterprise Research Institute www.deri.ie
Chapter 1: HTTP-level issues…
…a good RDF description these days is hard to find
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7. Waldo URIs:
URIs with no dereferencable RDF
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Not a crawler’s idea of fun…
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8. Hmm not *so* many…
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 5.3% of HTTP URIs return 40x/50x
 Excluding redirects…
92.8% return 200 OK
 In return, only 45.4% of 200 Okay return report
application/rdf+xml
 34.8% return HTML… probably just HTML docs…
okay… maybe a *few* contain RDFa
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9. Lies… Damned Lies…
& Content-Type Reporting
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“Trust me, it’s RDF/XML”
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10. Okay… So he’s actually pretty honest
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 16.9% of valid RDF/XML documents
returned with an invalid/more generic
Content-type:
text/xml (9.5%)
application/xml (5.9%)
text/plain (1%)
text/html (0.4%)
 Of those returning
Content-type:application/rdf+xml
98.8% were valid RDF/XML
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11. Same triples, different document
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I wish they’d used a redirect…
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12. E.g., the Miracle at Calais:
turning 1,778 triples into ~∞ quads
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http://d.opencalais.com/1/type/em/r/SameTriplesDifferentDocument
(apologies to OpenCalais guys – it’s just a convenient example)
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13. Digital Enterprise Research Institute www.deri.ie
Chapter 2: Reasoning issues…
…or, how I learned to start worrying and stop loving OWL
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14. Undefined classes and properties…
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It looks important, but I’m afraid I don’t fully follow
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15. Quite common…
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 14.3% of triples use undeclared property
 8.1% of triples use undeclared class
 Three cases:
 Case 1: Namespace has no vocabulary/
is not deferencable
(e.g., rss:item)
 Case 2: Term invented in related namespace
(e.g., foaf:tagLine invented by LiveJournal)
 Case 3: Term is misspelt version of term defined in namespace
(e.g., foaf:image vs. foaf:img)
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16. Not-so-unique values for
Inverse-Functional Properties
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Despite what you claim,
not all of you can *actually be* Spartacus
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17. Spartacus relived…
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08445a31a78661b5c746feff39a9db6e4e2cc5cf
 sha1-sum of „mailto:‟
 common value for foaf:mbox_sha1sum
 An inverse-functional (uniquely identifying) property!!!
 Any person who shares the same value will be considered
the same
*I’m Spartacus!*
…and so’s my wife
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18. …unattended, can be pretty serious…
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foaf:mbox_sha1sum a owl:InverseFunctionalProperty .
?x foaf:mbox_sha1sum 08445a31a78661b5c746feff39a9db6e4e2cc5cf .
OWL 2 RL rule prp-ifp:
?p a owl:InverseFunctionalProperty . ?x1 ?p ?z . ?x2 ?p ?z .
⇒ ?x1 owl:sameAs ?x2 .
106 ?x1/?x2bindings in body
1012 inferred pair-wise and reflexive owl:sameAs statements
…or in simpler terms:
pow!
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19. Malformed/incompatible datatypes
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As he would undoubtedly be able to tell you,
“true” is not a valid xsd:int
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20. Not *too* bad…
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 4.7% of typed literals were “ill-typed” (lexically
invalid)…
 mostly xsd:dateTimes (26.4% of all date-time literals
were invalid; e.g., omitted the seconds field)
 Also, literals are sometimes incompatible with
the datatype-range of a property:
 E.g., 21.8% of ical:description triples used
language tags incompatible with the defined range of
xsd:string
 E.g., 100% of sl:creationDate triples use plain literal
values incompatible with defined range of xsd:date
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21. Mystical beings…
Members of disjoint classes
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Despite what FOAF says, it seems that
Persons can also be Documents
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22. Again, not *too* bad…
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 1,329 members of disjoint classes found
 Generally caused by naïve URI naming:
 Use of information resource URIs to name entities
(particularly foaf:Persons)
 E.g., <me> foaf:knows <jim/foaf.rdf> .
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23. Ontology hijacking…
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Anybody can say anything, anywhere, and unfortunately for everyone
else, have a good chance of being taken seriously
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24. Redefining Everything…
…and home in time for tea
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From http://www.eiao.net/rdf/1.0
<owl:Property rdf:about="http://www.w3.org/1999/02/22-rdf-syntax-ns#type">
<rdfs:label xml:lang="en">type</rdfs:label>
<rdfs:comment xml:lang="en">Type of resource</rdfs:comment>
<rdfs:domain rdf:resource="http://www.eiao.net/rdf/1.0#testRun"/>
<rdfs:domain rdf:resource="http://www.eiao.net/rdf/1.0#pageSurvey"/>
<rdfs:domain rdf:resource="http://www.eiao.net/rdf/1.0#siteSurvey"/>
<rdfs:domain rdf:resource="http://www.eiao.net/rdf/1.0#scenario"/>
<rdfs:domain rdf:resource="http://www.eiao.net/rdf/1.0#rangeLocation"/>
<rdfs:domain rdf:resource="http://www.eiao.net/rdf/1.0#startPointer"/>
<rdfs:domain rdf:resource="http://www.eiao.net/rdf/1.0#endPointer"/>
<rdfs:domain rdf:resource="http://www.eiao.net/rdf/1.0#header"/>
<rdfs:domain rdf:resource="http://www.eiao.net/rdf/1.0#runs"/>
</owl:Property>
Ontology hijacking!!
(apologies to EIAO guys – it’s just a convenient example)
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25. Solutions?
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26. Application side: workarounds
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 All presented issues have a suitable antidote, once
you know about them
 See paper for discussion…
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27. Publishing side: Validators!
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 Syntax errors quite rare, partly due to popularity of
W3C RDF/XML syntax validator
 Need an all-in-one validation service
 Should not only validate strict errors, but give
feedback on suspected issues
 We offer a prototypical service at:
http://swse.deri.org/RDFAlerts/
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28. Publishing side: Pedantic Web Group
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 Get the community to contact publishers about
errors/issues as they arise
 Get involved: http://pedantic-web.org/
 137 members!
 Acknowledgements to: Aidan Hogan, Alex Passant, Me, Antoine Zimmermann, Axel
Polleres, Michael Hausenblas, Richard Cyganiak, Stéphane Corlosquet
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