1. Universität Innsbruck Leopold Franzens Copyright 2006 DERI Innsbruck www.deri.at LarCK Workshop, ISWC/ASWC 2007 - Busan, Korea 16-Feb-14 Towards Scalable Information Spaces Reto Krummenacher, Elena Simperl, Dieter Fensel

2. 2 LarCK Workshop, ISWC/ASWC 2007 - Busan, Korea 16-Feb-14 Background: Triple Space Computing Semantic Space rd out

3. 3 LarCK Workshop, ISWC/ASWC 2007 - Busan, Korea 16-Feb-14 Background: Triple Space Computing Host / Kernel Information Space Information Space Information Space Information Space Information Space

4. 4 LarCK Workshop, ISWC/ASWC 2007 - Busan, Korea 16-Feb-14 Triple Space Computing at Web Scale The Web is distributed, heterogeneous, dynamic, and open. Required is a Triple Space platform which copes with these principles at Web scale. Classical guarantees encountered in closed environments cannot longer hold. Adaptation of the functional properties publish and retrieve is required. Feasible non-functional guarantees must be determined from a scalability perspective

5. 5 LarCK Workshop, ISWC/ASWC 2007 - Busan, Korea 16-Feb-14 Outline 1.(Non-) Functional Properties and Their Impact on Scalability of Information Spaces 2.Scalability-driven Trade-Offs 3.Levels of Scalability for Semantic Space Middleware

6. 6 LarCK Workshop, ISWC/ASWC 2007 - Busan, Korea 16-Feb-14 Two Dimensions There are two dimensions to a middleware that influence its scalability –The exposed functionality - the service delivered to the clients - the functional properties. –The internal behavior of the system that is not obvious or visible to clients at interaction time - the non-functional properties (NFP).

7. 7 LarCK Workshop, ISWC/ASWC 2007 - Busan, Korea 16-Feb-14 Functionals (1/2) Publishing information –Writing of one or more RDF triples into an identifiable space. –No guarantee about when the triple is available and persistently stored. Retrieving information –Reading triples by template/query. –No guarantees about when and if a match is returned. –An empty result set does not imply that no matching triple is available. –Retrieval is possible from an identifiable space, or from any available spaces.

8. 8 LarCK Workshop, ISWC/ASWC 2007 - Busan, Korea 16-Feb-14 Functionals (2/2) Query complexity –Ranging from triple patterns to arbitrarily complex queries. –Query resolution (reasoning). Explicit vs. inferred data –Publication at data level – no reasoning, multiple copies. –Retrieval at knowledge level – inference, materialization, only one copy. Local reasoning vs. distributed reasoning. Transactions –E.g. when publishing sets of triples within an atomic operation.

9. 9 LarCK Workshop, ISWC/ASWC 2007 - Busan, Korea 16-Feb-14 Non-Functionals Security– retrieve,publish Fault-tolerance – retrieve,publish Availability – retrieve,publish Reliability – retrieve,publish Completeness – retrieve Correctness – retrieve Consistency – retrieve Durability – publish Response time – retrieve,publish Scalability – retrieve,publish data/query related TS related dependability

10. Technical Means for NFP Redundancy –duplication of processes (n-out-of- m, n < m). Recovery –Re-installation of previous system state. Load balancing –decentralization of processes by replicating data. by distributing data (partitioning, clustering). 10 LarCK Workshop, ISWC/ASWC 2007 - Busan, Korea 16-Feb-14

11. 11 LarCK Workshop, ISWC/ASWC 2007 - Busan, Korea 16-Feb-14 NFP and their Realization (1/3) Fault-Tolerance –Redundancy: prevention, 1-out-of-2 –Recovery Availability –Load balancing: 1-out-of-N Reliability –Redundancy: prevention, 2-out-of-3 Completeness –No distribution of data

12. 12 LarCK Workshop, ISWC/ASWC 2007 - Busan, Korea 16-Feb-14 NFP and their Realization (2/3) Consistency –No replication: no system-caused inconsistency Correctness –No false-positives, a query engine issue, completeness Durability –Recovery: persistency is given by storage Response time –Load balancing

13. NFP and their Realization (3/3) Scalability –The trade-offs between non-functional properties and between functionals and non-functionals need to be analyzed in order to to achieve a feasible level of scalability. –The appropriate configurations for the desired behavioral and structural realization and the corresponding scalability level has to be selected. 13 LarCK Workshop, ISWC/ASWC 2007 - Busan, Korea 16-Feb-14

14. 14 LarCK Workshop, ISWC/ASWC 2007 - Busan, Korea 16-Feb-14 Scalability Trade-Offs Correctness Completeness Precision vs. recall Completeness Availability Load balancing by distribution Consistency Availability Load balancing by replication Consistency Fault Tolerance Replication could be hidden Reliability Response Time Completeness Response Time Consistency Response Time Durability Response Time Non-local writing results in latency

15. 15 LarCK Workshop, ISWC/ASWC 2007 - Busan, Korea 16-Feb-14 Scalability Levels Depending on the desired functionality and non- functionality, the space middleware has different possibilities to provide a scalable infrastructure. The richer functionality the middleware provides, the less can be guaranteed about the non-functional properties and scalability.

16. 16 LarCK Workshop, ISWC/ASWC 2007 - Busan, Korea 16-Feb-14 Scalability Levels: Discussion (1/3) Simplest approach - proved and tested with respect to scalability –The target space is indicated by a resolvable URL –Distribution is client-driven –Discovery by use of a DNS-like procedure –Ensures at least local scalability

17. 17 LarCK Workshop, ISWC/ASWC 2007 - Busan, Korea 16-Feb-14 Scalability Levels: Discussion (2/3) Most complex approach – likely not scalable –Access to spaces without indication of a URL –(Knowledge-driven) retrieval on the entire Semantic Web –Query forwarding, distributed discovery of relevant spaces –Distributed reasoning to integrate distributed information sources/spaces or large scale (local) reasoning after transfer of triples

18. 18 LarCK Workshop, ISWC/ASWC 2007 - Busan, Korea 16-Feb-14 Scalability Levels: Discussion (3/3) Loosening classical IR guarantees is of benefit to achieve scalability –Access to space(s) without indication of a URL –Best effort retrieval – no completeness –No guarantees about consistency –Rather data than knowledge-driven Performance –Search scope –Discovery strategy

19. 19 LarCK Workshop, ISWC/ASWC 2007 - Busan, Korea 16-Feb-14

20. Scalability Levels: Discussion (4/4) Further issues –Publication of sets of triples as atomic operation –Transactionality Local transactions: data is visible to other hosts after commit Tansactions in case of distributed writting, i.e. publication at remote hosts Locking an information space is unfeasible for distributed, decentralized settings 20 LarCK Workshop, ISWC/ASWC 2007 - Busan, Korea 16-Feb-14

21. 21 LarCK Workshop, ISWC/ASWC 2007 - Busan, Korea 16-Feb-14 Conclusions Semantic spaces offer at least local scalability and local completeness. Scalability at global scale requires the management of scalability trade-offs. However… –The times of 100% complete and correct solutions are gone. [D. Fensel, STI International 2007] –incomplete, incorrect but requested! [D. Fensel, IEEE Intelligent Systems 22(6) 2007]

22. 22 LarCK Workshop, ISWC/ASWC 2007 - Busan, Korea 16-Feb-14 Thank you. Universität Innsbruck Leopold Franzens LarCK Workshop, ISWC/ASWC 2007 - Busan, Korea Copyright 2006 DERI Innsbruck www.deri.at 16-Feb-14