Copyright 2007 Digital Enterprise Research Institute. All rights reserved. SEMEDIA PARENTAL ADVISORY: Neither formulas nor inference rules in this presentation

Copyright 2007 Digital Enterprise Research Institute. All rights reserved. SEMEDIA Who the FOAF knows Alice? Towards Semantic Web Pipes Axel Polleres, DERI Galway Christian Morbidoni, SEMEDIA – University of Ancona, Italy Joint work with: Giovanni Tummarello, DERI Galway

3 Outline Web Pipes, Web Semantic Web Pipes Revocations in RDF MSG Theory DBin 2.0: a Semantic Web Client implementing RDF revocations Conclusions

4 Semantic Web as a quad space A semantic model (RDF) can be published on the web, at a specific web location (URL): Resolve( RDF/XMLhttp://polleres.net/foaf.rdf The collection of all the RDF graphs published on the web is today referred to by some as being the Semantic Web The Semantic Web can be therefore see as a huge quad store where: –Any graph is readable (in general, but HTTP access control possible) –It is possible to write, but only in controlled web spaces (e.g. ones own homepage) A big plus: it ties to the URL/DNS name ownership mechanism, ie. One could say: I own my RDF statements in some sense.

5 Additionally, plenty of RDF data on the Semantic Web already Personal FOAF, SIOC… etc. Databases : DBpedia, DBLP, Geonames, etc. Most of these use the Linked Open Data approach, e.g. Building the Semantic Web is about integrating data, that is already out there. (found on a blog)

6 Now how do I aggregate/process this RDF data? Available ingredients Services: SWSE (for full queries), Sindice (find the sources, query yourself) Browsers: Tabulator, Disco (browsers) etc.. Clients: Protégé (read/local edit), DBin 2.0 (read/write) Missing: Data processors for the Web a la Pipes!

7 Charles doesnt trust Alices information, He wants to provide a common view over his and Bobs information. But: He also wants to counter some of Bobs statements though, ie. Charles wants to say: I dont know Alice! Additionaly, he might want to do other stuff, like RDFS materialiation, a SPARQL query over this joint view,etc. An Example: Aggregating & Patching RDF data 3 FOAF files, which contain the personal information stored by Alice, Bob, and Charles:

8 Outline Web Pipes, Web Semantic Web Pipes Revocations in RDF MSG Theory DBin 2.0: a Semantic Web Client implementing RDF revocations (Demo) Conclusions & Outlook!

9 How to revoke statements? How can Bob state I dont know Alice! –Such that others know he revokes that statement? –Without ending up in overall inconsistency (explosive semantics of negation in classical logic?) –Lets see what means current Semantic Web languages provide…

10 How to state I dont know Alice? - Attempt 1: In RDF, using a new counter-property, extending the FOAF vocabulary + : Easy to write down, no overall inconsistency -: No semantics: –How would someone else know that doesntknow is the opposite of knows? –For any revocation, one would need to extend the vocabulary.

11 How to state I dont know Alice? - Attempt 2: In RDF, making revoked statements in a separate file, e.g., badstatements.rdf + : Easy to write down, no overall inconsistency -: No semantics: –how would a crawler be able to disambiguate good and bad graphs? –for actual revocation, one would need some pretty ugly SPARQL query:

12 How to state I dont know Alice? - Attempt 3: In OWL/RDF, I can express I dont know Alice !! + : clear semantics, W3C standard language -: Not necessarily what was intended: –Global inconsistency when combined with Bobs file. –We wanted to patch only thus only remove the single statement. –BTW, the RDF triples for this in OWL/RDF are quite verbose One could use para-consistent reasoning with OWL though, see [Z. Huang, F. van Harmelen, and A. ten Teije., 2005] –Still, needs full OWL DL reasoning, with nominals!

13 How to state I dont know Alice? - Attempt 4: In RDF, using reification and a special property a la N3: + : Easy to write down in N3 –Maybe still verbose though (by RDF reification vocabulary), but alternative forms of writing down reficied statements, e.g. as XMLLiteral possible (which has other nasty side effects.) -: Again, undesired semantics: –overall inconsistency –semantics only defined in terms of cwm, non-standard, not really written down properly

14 Outline Web Pipes, Web Semantic Web Pipes Revocations in RDF MSG Theory DBin 2.0: a Semantic Web Client implementing RDF revocations (Demo) Conclusions & Outlook!

15 Our solution: Describe revocations based on MSG theory! Practical approach Concise to write down Non (yet) standard semantics, admittedly Implemented in the Dbin 2.0 system for collaborative management and patching of RDF graphs

16 KEY CONCEPT: Minimum Self-contained Graph (MSG) MSG (Def). Given an RDF statement s and a graph G, the Minimum Self- contained Graph (MSG) containing that statement, written MSG(s,G), is the set of RDF statements comprised of the following: –The statement in question –Recursively, for all the blank nodes involved by statements included in the description so far, the MSG of all the statements involving such blank nodes Important Properties: –Each RDF Graph can be decomposed in a canonical set of MSGs –Each MSG has a unique (blank-node agnostic) hash sum For a deeper discussion see: G. Tummarello, C. Morbidoni, R. Bachmann-Gmur, O. Erling, "RDFSync: efficient remote synchronization of RDF models", Proceedings of the 6th International Semantic Web Conference 2007, Busan, Korea --> Idea: We use exactly this hash sum to revoke MSGs

17 RDF graph decomposition and identifiers MSG ID = Base64(MD5(Canonical(MSG))) = 45FA76B61FC0 Graph ID list = [MSG ID 1, MSG ID 2,..]

18 RDF modeling of MSG based revocations So, if Charles wants to revoke foaf:knows (stated in Bobs FOAF file) …he computes a base64 of the MSG hash 123JHG… …then adds to his own FOAF a triple: _:x pipes:revokesMSGHash "123JHG…"^^xsd:string. … plus optionally additional metadata about the revocation: _:x pipes:statedBy :me. _:x pipes:date " T16:20:00+9:00"^^xsd:dateTime. _:x pipes:revocationDescription "Who the FOAF said that I know Alice? I dont know her."^^xsd:string. _:x pipes:involvedResource :me. * In the paper, we called this slightly different, _:a 123JHG… but in between, we have refined our revocation vocabulary under the namespace

19 RDF revocation based on MSG hashes summary: In RDF, using our revocation vocabulary: _:x pipes:revokesMSGHash "123JHG…"^^xsd:string. + : –Concise to write down, –no overall inconsistency, specific revocations possible. –revocations on the level of MSGs –We have defined a well-defined semantics what revocation means, –allows revocation as a modular operator in a pipe. -: –not visible WHAT was revoked, before revocations are applied, but this may be viewed a a feature –MSG hashes need to be computed every time (in Dbin we do this anyway, and we suggest to avoid unnecessary computations by using additional metadata in the paper) –Theoretically hash-collision possible (proved not to be the case in current use case examples, can again be minimized by additional metadata)

20 Revocation in Dbin 2.0 Pipes In DBin, a pipe is currently a simple ordered sequence of RDF graphs, where the revocation operator is subsequently applied: Applying revocation in the order of the files, allows preferential views on RDF. A non-naïve version, allows additionally revocations of revocations, and arbitrary pipes, see: Christian Morbidoni, Axel Polleres, Giovanni Tummarello, Danh Le Phuoc. Semantic Web Pipes. Tech. Report. DERI-TR , 2007.

21 Outline Web Pipes, Web Semantic Web Pipes Revocations in RDF MSG Theory DBin 2.0: a Semantic Web Client implementing RDF revocations (Demo) Conclusions & Outlook!

22 DBin 2.0 overview Is a desktop client Provides a rich user interfaces which can be customized for specific domains (Brainlet model) As a basis, it Reads and Writes from/to the Semantic Web Implements Semantic Web Pipes: workflows which combine Semantic Web sources in specific ways 2.0

23 DBin 2.0, the Pipe Engine and the User Interface

24 …Lets show it as a Semantic Web Pipe Revocation operator Source 1 Source 2 Revocation operator Source 3 RDFS inference User Interface RDF Sources and operators piped togheter for processing RDF data in a specific way RDF

25 Outline Web Pipes, Web Semantic Web Pipes Revocations in RDF MSG Theory DBin 2.0: a Semantic Web Client implementing RDF revocations (Demo) Conclusions & Outlook!

26 Conclusions and Outlook: Novel concept of revocations in RDF takes into account that data on the web may be conflicting, but without introducing overall inconsistency Allows targeted application of revocations in any sep of a pipe. DBin 2.0 makes use of it for common views of RDF collaboratively maintained graphs. pipes.deri.org is the next step which will allow you to store, execute and save pipes involving operators like: RDF materialize, revoke, SPARQL, XSLT, merge, etc. 2.0 Discussion points for specifically this workshop: We believe that small, practical, small reasoning components such as provided by the revocations operator, work, for certain (not all) applications to get the Semantic Web going more quickly. Do we really need full OWL DL machinery for the aggregation and patching task? BTW: In our experiments so far, we experience that a large part of the aggregation time for pipes is taken for fetching data… distribution!