1. 6/26/2006ICPS'06 Tracking Nearest Surrounders in Moving Object Environments * Ken C. K. Lee 1 Josh Schiffman 1 Baihua Zheng 2 Wang-Chien Lee 1 Hong Va Leong 3 1 Pennsylvania State University 2 Singapore Management University 3 Hong Kong Polytechnic University * International Conference on Pervasive Service, Lyon, France, June 2006

2. 6/26/2006 2 ICPS'06 Outline Introduction Nearest Surrounder Query Proposed Techniques Performance Evaluation Conclusion

3. 6/26/2006 3 ICPS'06 Traffic info Navigation Local weather Emergency service Logistics Location-Based Services Geographical Information System (GIS) Tracking Mobile Commerce

4. 6/26/2006 4 ICPS'06 Moving Object Tracking One of the essential LBSs Moving object tracking Perform a spatial query on moving objects continuously Existing studies Continuous Window Query Continuous Nearest Neighbor Query Above queries are based on (Euclidean) distance. Angle is also an important and interesting factor in spatial queries. Our focus: Nearest Surrounder Query

5. 6/26/2006 5 ICPS'06 What is Nearest Surrounder (NS) Query? A query which finds nearest objects to a query point at every distinct angle. Recently reported at ICDE06. Applications Robot football: How a robot decide where to pass the ball? Digital battlefield: How a soldier finds a fire clearing path? Nearest Surrounder Query

6. 6/26/2006 6 ICPS'06 Given a query point, q, NS(q) searches the database to return a result set: { } where object is the nearest to q within the specified angular range. Example: NS(q) = {,,,, <NULL,[ f, g ) } NULL means no object found Objects o 2, o 4, o 5, o 10, are not NS. scenario NS Query - Definition

7. 6/26/2006 7 ICPS'06 Previous work on NS Query is based on static objects. What about the moving object environments? Scenario o 1 moved down a bit o 2 (previously blocked by o 1 ) is now included in the result set. Part of the empty angular range is now covered by o 1. scenario Continuous NS Query

8. 6/26/2006 8 ICPS'06 This work presents a framework for continuously tracking NS query results. Two major costs in keeping NS query results up-to- date in a moving object environment. Query lookup Query result update Safe region is used to address the issue of query lookup Partial query evaluation rather than query reevaluation is used to reduce the cost of query result update. Through experimentation, we show the efficiency of these two techniques adopted in our framework. Our Contribution

9. 6/26/2006 9 ICPS'06 Users (query clients) register their queries at the database server. Moving objects update the server periodically. Base station relays messages between wireless and wired network Database server matches updates to NS queries. System Framework

10. 6/26/2006 10 ICPS'06 Request queue Pending requests Data storage Object index (R-tree) Query index (R-tree) Processes Query processor 1. Query evaluartion 2. Determining Safe Regions for Queries Location updater 1. Query Lookup 2. Partial Query Evaluation Database Architecture

11. 6/26/2006 11 ICPS'06 Used to alleviate query lookups Two types of safe regions Safe regions for closed angular ranges Safe regions for open angular ranges Safe regions are indexed in query index. Two indices: Closed Angular Range/Open Angular Range If a query whose safe region is not touched by moved objects (old/new positions), query evaluation is not needed. Issues The more precise a safe region, the less false hits Safe Regions

12. 6/26/2006 12 ICPS'06 Safe region for closed angular range Bounding Circle Quadrant-Based Bounding Box Safe Region Approximation

13. 6/26/2006 13 ICPS'06 Safe region for open angular range Bounded region of empty angular range inside the service area. Cover a lot of unnecessary space. Approximated safe region Safe Region Approximation

14. 6/26/2006 14 ICPS'06 Full query revaluation Evaluate a query whenever a moved object touches its safe region. Partial query re-evaluation Evaluate only the portion of a NS query result affected by moved object. Object movement considered as deletion of the old position followed by insertion of the new position Query Re-Evaluation

15. 6/26/2006 15 ICPS'06 Deletion of an object position may introduce other objects (previously hidden by the removed object) to the result set; may introduce open angular range. Example: Deleting o 1 Remove from existing result. Add and to the result. Partial Evaluation - Deletion

16. 6/26/2006 16 ICPS'06 Insertion of a new object position may remove some existing objects from the resultt set; may remove an open angular range. Example: Adding o 1 Remove and from existing result. Add,, and to the result. Partial Evaluation - Insertion

17. 6/26/2006 17 ICPS'06 Experiment Settings Candidates: Brute Force (BF) (search queries and reevaluate them when objects move) Safe Region (SR) lookup queries with safe regions as filters Partial Evaluation (PE) determine the partial change to the NS result. Parameter settings: Runtime environment: RedHat Linux Enterprise 3.0 on Xeon 2.6 computers Performance Evaluation

18. 6/26/2006 18 ICPS'06 Processing time (log) – elapse time for a query to be identified and updated. SR+PE outperform all others, SR is the second best. BF, the baseline case, does the worst. Real Datasets 0.01 0.1 1 10 100 1000 RIPA BF SR SR+PE Processing time (sec)

19. 6/26/2006 19 ICPS'06 QB Box outperforms BCircle because of more precise safe regions. Safe Region Approximation

20. 6/26/2006 20 ICPS'06 This paper studies the issues of tracking nearest surrounder query in moving object environment Based on a centralized approach Safe region and partial evaluation are effective for query lookup and query result updates Future work Moving query point. Distributed approach Conclusion & Future Work