1. 11.06.2004Raluca Paiu1 Semantic Web Search By Raluca PAIU [paiu@l3s.de]

2. 11.06.2004Raluca Paiu2 Overview The TAP System Edutella Edutella Wrapper

3. 11.06.2004Raluca Paiu3 The TAP System Goal: create a single schematically unified global knowledge base by knitting together data from disparate web services into a coherent whole.

4. 11.06.2004Raluca Paiu4 TAP Architecture [1] TAP provides: A facility for publishing data A library which implements an application programming interface for consuming this data A registry

5. 11.06.2004Raluca Paiu5 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

6. 11.06.2004Raluca Paiu6 TAP Architecture [3] Consuming data - through a minimalist query interface called GetData

7. 11.06.2004Raluca Paiu7 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

8. 11.06.2004Raluca Paiu8 GetData [1] Simple query interface to network accessible data presented as directed labeled graphs. Requirements: Simplicity Predictability

9. 11.06.2004Raluca Paiu9 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.

10. 11.06.2004Raluca Paiu10 GetData [3] The abstract syntax of a GetData query: GetData(, ) -> GetData(,, “inverse=yes”) -> GetData(S,P)  O GetData(O,P,”inverse=yes”)  S S O P

11. 11.06.2004Raluca Paiu11 GetData [4] Example: GetData(, birthplace) => GetData(, Author, inverse=yes) =>,

12. 11.06.2004Raluca Paiu12 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

13. 11.06.2004Raluca Paiu13 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

14. 11.06.2004Raluca Paiu14 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

15. 11.06.2004Raluca Paiu15 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

16. 11.06.2004Raluca Paiu16 Edutella Wrapper [1] The process that every wrapper must perform is the following: 1.Receives a QEL as a string that uses the Elena Common Ontology 2.Understands the QEL query 3.Maps the Elena Common Ontology to the local ontology 4.Converts the QEL to the local query language 5.Sends the transformed query to the repository 6.Receives the results from the repository 7.Transforms the results to a variable binding table 8.Returns the results

17. 11.06.2004Raluca Paiu17 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

18. 11.06.2004Raluca Paiu18 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 Y1Y2Yn … P1 P2 Pn

19. 11.06.2004Raluca Paiu19 Edutella Wrapper [4] Example: ?- qel:s(X,,Y), qel:s(Y,, ). The corresponding tree: Name: X Type: variable Restrictions: null Name: Y Type: variable Restrictions: null Name: Artificial_Intelligence Type: resource Restrictions: null

20. 11.06.2004Raluca Paiu20 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,, inverse=yes) For each binding of Y X,, inverse=yes) Return the results as RDF graph answers

21. 11.06.2004Raluca Paiu21 Thank You !