Replication in Mobile Information Systems

1. Replication in Mobile Information Systems Hagen Höpfner Workshop Mobile Datenbanken und Informationssysteme Dortmund, October 2nd 2002

2. Agenda • Introduction and motivation • Basics • Replication Process • Pre-processing • Copying the data • Updating the data and synchronisation • Our approaches • Database server support for semantic replication • Mobile client support for semantic replication • Conclusions and outlook

3. Introduction and Motivation • Replication in distributed databases well known • Better performance • Better availability • Replication in mobile information systems • Mobility and lightweight system • New demands • What does Replication in Mobile IS mean?

5. Basics • Additional logical concepts devices • Database Server • Mobile (Database) Clients

6. The Replication Process

7. Pre-processing • ... On the Database Server: • Definition of the replica set • Subset of the whole Database • Content-based vs. Non-content-based • Allowing or Denying Updates • ... On a Base Station near the MH • Caching  Only new Data from DS • Profiling • ... On the Mobile Client • Caching  Reuse of already replicated Data • Classical Approaches vs. Semantic Caching • ... On the Database Server: • Definition of the replica set • Subset of the whole Database • Content-based vs. Non-content-based • Allowing or Denying Updates • ... On a Base Station near the MH • Caching  Only new Data from DS • Profiling • ... On the Mobile Client • Caching  Reuse of already replicated Data • Classical Approaches vs. Semantic Caching • ... On the Database Server: • Definition of the replica set • Subset of the whole Database • Content-based vs. Non-content-based • Allowing or Denying Updates • ... On a Base Station near the MH • Caching  Only new Data from DS • Profiling • ... On the Mobile Client • Caching  Reuse of already replicated Data • Classical Approaches vs. Semantic Caching

8. Copying the Data • Disk-File-Based • Requires additional hardware  Not usable in most cases • Agent-Based • Network-Protocol-Based • Can also support files • Broadcast • Requires separate uplink channel for querying

9. Updating and Synchronisation • Data centric • Holding different versions of data • E.g. Multiversion Reconciliation • Transaction centric • Tentative transactions vs. Base updates • E.g. Two-Tier Replication • E.g. Bayou: • Per-write dependency checks • Per-write merge procedures

10. Our Pre-processing Approaches Using characteristics of mobile information systems for minimizing wireless data transmissions

11. Semantic Replication: DS • Fragmentation of the database on DS • Fragment=Set of Data + Server Extensions • Queries may contain additional context-descriptions • Only related fragments become replicated Example

12. Semantic Replication: MC (1) • Usage of typical query structure for semantic caching on mobile clients • Content-based queries • A lot of small queries with small but exact answers • Usage of a Trie for indexing cached data • Edges represent predicates

13. Semantic Replication: MC (2)

14. Conclusions • Replication in mobile information systems differs from replication in distributed databases in: • Intensions • Requirements • Techniques • Excurs: Our actual works on mobile information systems

15. Outlook • Support for local joins • Detailed specification of semantic comparison of queries • Representation of constraints etc. (The View Update Problem) • Integration of update mechanisms(The Offline Update Problem)

16. Discussion

17. Example: Server Site • AMO (date, time, event, event_type) • CINEMAXX (id, room#, fsk, date, time, movie, genre) • FACTORY (date, time, admittance, band, charge, genre) • Fragmentation function: • Fcmd=(fcmd, vlmd) • fcmd=select Date, Admittance as Time, Band, Charge, Genre from FACTORY unionselect Date, Time, Event, NULL, NULL from AMO where event_type like '%concert%'; back