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Semantic Web Search

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  1. Semantic Web Search By Raluca PAIU [paiu@l3s.de] Raluca Paiu

  2. Overview • The TAP System • Edutella • Edutella Wrapper Raluca Paiu

  3. The TAP System • Goal: create a single schematically unified global knowledge base by knitting together data from disparate web services into a coherent whole. Raluca Paiu

  4. TAP Architecture[1] • TAP provides: • A facility for publishing data • A library which implements an application programming interface for consuming this data • A registry Raluca Paiu

  5. TAP Architecture [2] • Publishing Data TAPache • Functions as a module for Apache HTTP server • Provides the GetData interface • Offers a mechanism for aggregating the data in multiple RDF files Raluca Paiu

  6. TAP Architecture [3] • Consuming data - through a minimalist query interface called GetData Raluca Paiu

  7. TAP Architecture [4] • The registry: • Available as a separate server • Can be abstracted as a lookup table • Redirects the queries to the appropriate sites • Caching Raluca Paiu

  8. GetData [1] • Simple query interface to network accessible data presented as directed labeled graphs. • Requirements: • Simplicity • Predictability Raluca Paiu

  9. GetData [2] • Allows a client program to access the values of one or more properties (or their inverse) of a resource from a graph • Each GetData query is a SOAP message • A message specifies two arguments: • The resource whose properties are being accessed • Properties that are being accessed • Optional arguments: the client wants the inverse of properties, the number of answers desired • The answer of a GetData query is a graph which contains the resource (whose properties are being queried) along with the properties specified in the query and their respective targets / sources. Raluca Paiu

  10. GetData [3] • The abstract syntax of a GetData query: • GetData(<resource>, <property>) -> <value> • GetData(<resource>, <property>, “inverse=yes”) -> <value> • GetData(S,P)  O • GetData(O,P,”inverse=yes”)  S S P O Raluca Paiu

  11. Example: GetData(<Yo-Yo Ma>, birthplace) => <Paris> GetData(<Yo-Yo Ma>, Author, inverse=yes) => <Appalachian Journey>, <Taverner> GetData [4] Raluca Paiu

  12. Edutella • P2P networking infrastructure based on RDF • Offers the following services: • Query Service – standardized query and retrieval of RDF metadata • Replication Service – for availability, balancing and data persistence • Mapping Service – translation between different metadata vocabularies • Mediation Service – mediate access between different services • Clustering Service – set up the semantic routing and semantic clusters Raluca Paiu

  13. Edutella Query Service • Standardized query exchange mechanism for RDF metadata stored in distributed RDF repositories • The Edutella network uses the query exchange language family RDF-QEL-i (based on Datalog semantics) as standardized query exchange language format which is transmitted in an RDF/XML-format. • The query languages levels are defined as follows: • RDF-QEL-1 – restricted to conjunctive formulas only • RDF-QEL-2 – extends RDF-QEL-1 with disjunction • RDF-QEL-3 – contains the full Datalog Semantics (conjunction, disjunction, negation) • Further levels allow different models of recursion Raluca Paiu

  14. Datalog Semantics [1] • A Datalog program can be expressed as: • A set of rules/implications: • Head – one positive literal in the consequent of the rule • Body – conjunction of one or more literals in the antecedent of the rule, including conditions on variables • A set of facts – single positive literals • The actual query literals (a rule without head) Literals – predicates expressions describing relations between any combination of variables and constants Raluca Paiu

  15. Datalog Semantics [2] • Disjunction – expressed as a set of rules with identical head • A Datalog query is formed by: • Conjunction of query literals • A possibly empty set of rules Raluca Paiu

  16. Edutella Wrapper [1] • The process that every wrapper must perform is the following: • Receives a QEL as a string that uses the Elena Common Ontology • Understands the QEL query • Maps the Elena Common Ontology to the local ontology • Converts the QEL to the local query language • Sends the transformed query to the repository • Receives the results from the repository • Transforms the results to a variable binding table • Returns the results Raluca Paiu

  17. Edutella Wrapper [2] • Wrapping QEL to GetData: • Map the QEL query to a N-Tree • Every node corresponds to a variable or a resource • A node (corresponding to a variable) might have associated some restrictions • Traverse the N-Tree to find the order in which the GetData queries have to be sent • Top-down – for direct search • Bottom-up – for inverse search • Bind the results to the variables Raluca Paiu

  18. Edutella Wrapper [3] • For a node corresponding to a variable, which has more than one child, intersect the results obtained on each branch • Apply the restrictions (if any) to the node corresponding to a variable • If the query is made of rules, we have an N-Tree for each rule  we have to make an union between the results corresponding to a variable from each tree. • Return the results as RDF graph answers X Pn P1 P2 … Y1 Y2 Yn Raluca Paiu

  19. Edutella Wrapper [4] • Example: • ?- qel:s(X,<http://localhost/data/tap.rdf/teamMember>,Y), • qel:s(Y,<http://localhost/data/tap.rdf/hasResearchArea>, <http://localhost/data/tap.rdf/Artificial_Intelligence>). • The corresponding tree: Name: X Type: variable Restrictions: null <teamMember> Name: Y Type: variable Restrictions: null Name: Artificial_Intelligence Type: resource Restrictions: null <hasReseachArea> Raluca Paiu

  20. Edutella Wrapper [5] • The tree corresponds to a direct search -> bottom-up traversal (first all the children of a node, than the node itself) • Y <- GetData(<Artificial_Intelligence>, • <hasResearchArea>, inverse=yes) • For each binding of Y X <- GetData (<binding_Yi>, <teamMember>, inverse=yes) • Return the results as RDF graph answers Raluca Paiu

  21. Thank You ! Raluca Paiu