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On Triple Dissemination, Forward-Chaining and Load Balancing in DHT Based RDF Stores

Computer Networks and Telematics Prof. Christian Schindelhaue. Albert-Ludwigs-University Freiburg SS 2009 Department of Computer Science. On Triple Dissemination, Forward-Chaining and Load Balancing in DHT Based RDF Stores.

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On Triple Dissemination, Forward-Chaining and Load Balancing in DHT Based RDF Stores

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  1. Computer Networks and Telematics Prof. Christian Schindelhaue Albert-Ludwigs-University Freiburg SS 2009 Department of Computer Science On Triple Dissemination, Forward-Chaining and Load Balancing in DHT Based RDF Stores Dominic Battre, Felix Heine, Andre Höing, and Odej Kao Presented by Aldarwich Yaser

  2. Overview • Motivation • Introduction • RDF • DHT • Pastry • Triples dissemination • Reasoning • Load Balancing • References 1

  3. Motivation • Centralized database Shortcomings • Incapable to handle load • Capacities limitation like in (Seasame,Jena) • Decentralized database • Example: Babelpeers,RDFpeers and Edutella • Provides scalibility,effeciency and capacity • Reasoning • Infer new data from existing information • Load balancing

  4. RDF Introduction • Resource Description Framework (RDF) • Used for representing information on the Web • RDFs provides a powerful model for storing and inferencing knowledge . • In RDF everything is represented by triples of the form(S,P,O) Example: Germanyhas CapitalBerlin S P O 2

  5. DHT Introduction • Solve the item location problem in a distributed network of nodes • Use a key k to calculate the ID ID=hash(k) • Operations: • Put(k, x) • Get(k) 3

  6. Triple dissemination Triple T=(s,p,o) Responsible node for s identifier = (hash(s)) identifier = (hash(p)) identifier = (hash(o)) Responsible node for p Query q = (s, p, o) Responsible node for o identifier = (hash(p)) 4 http://videolectures.net/iswc08_kaoudi_rdfs/

  7. Pastry Protocol • Each peer has a 128-bit ID: nodeID • Unique and uniformly distributed • Use cryptographic function applied to IP-address • Message takes O(log N) steps to destination • Node state contains: • Leaf Set • Routing table explain • Neighborhood Set

  8. Pastry (prefix-matching) • Route(m, 323310)? 323310 323211 103231 322021 313221 Node-id Key

  9. RDf Reasoning • The query is formulated gernerally • RDFs extract data even if the description does not exactly match the query • Example: Christian fatherof Schindelhauer Father subpropertyof relatives => Christian relative of Schindelhauer

  10. RDFS Rules 6

  11. Node Architecture • Each node hosts multiple RDf databases • local triples database • Received triples database • Replica database • Generated triples Generated Triples Replica Received Triples Local Triples 5 Node

  12. Triple dissemination in DHT Node1 Node2 Node3 Node4 Generated Triples Generated Triples Generated Triples Generated Triples Replica Replica Replica Replica Received Triples Received Triples Received Triples Received Triples Local Triples Local Triples Local Triples Local Triples 7

  13. Triples life-cycle • Triples are subjected to different events like (Joining, Departure) • Triples life-time • long life time triples has few refreshes refreshes • short life time triples(generated triples) • Update triples update inferred triples • Soft-state

  14. Node Departure • Node substitution • Correction of routing table • Replica duty • Decreasing number of replicas n9 n1 n2 n4 n3 8

  15. Node Arrival • More complicated • Query recieving • Task of replica nodes • Time reduction n9 n1 n6 n2 n4 n3 9

  16. Load balancing • Major criticism against DHT based RDF strores • Many collisions are unavoidable • Example: • DHT stores many triples with predicate rdf:type “ rdfs:subClassOf“ create many triples with Predicate rdf:type • Overlay Tree Builds for discrete DHT positions like the one stores triples with rdf:type 10

  17. Local Node1Node2 Node3 Node4 Exte references Remote Triples Local Local Local Local references references Exte Exte Remote Triples Remote Triples Remote Triples Remote Triples references Generated Triples Generated Triples Generated Triples Generated Triples Received Triples Received Triples Received Triples Received Triples Local Triples Local Triples Local Triples Local Triples 11 Load-balancing with remote triples database

  18. Replicated overlay tree Root Rank2 Rank1 12

  19. Query routing in overlay tree Rank2 Rank1 Root Qeury Result 13

  20. Handling RDFs rules in load balancing • Problem of RDF rules • As node is overloaded, the triples are splited into other nodes • Example: a, rdfs:domain, x u, a, v Node1 Node2 Node3 a, rdfs:domain, x u,a,v u,a,v a, rdfs:domain, x

  21. Handling RDFs rules in load balancing • Solution • Make copy of most common rdfs schema into each node in overlay tree Node1 Node2 Node3 Node4 a, rdfs:domain, x u,a,v a, rdfs:domain, x u, a, v a, rdfs:domain, x a, rdfs:domain, x

  22. Conclusion • P2p based distributed database offer better scalability and source integration • Real power of RDF is stems from possibility to derive new data from explicit knwoledge • Overlay tree is the solution for overloading problem

  23. References • http://www.videolectures.net • http://cone.informatik.uni-freiburg.de • http://www.w3schools.com • http://www.w3.org/TR/rdf-schema/ • http://peersim.sourceforge.net/ • http://infolab.stanford.edu • http://www.edutella.org/edutella.shtml • Battre,heine,Kao:Top k RDF query evaluation in p2p 14

  24. Thanks for your Attention

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