Replication in Mobile Information Systems

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Replication in Mobile Information Systems

• Hagen Höpfner

Workshop Mobile Datenbanken und
Informationssysteme Dortmund, October 2nd 2002
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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

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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?

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Basics

• Additional logical concepts devices
• Database Server
• Mobile (Database) Clients

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The Replication Process
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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

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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

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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

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Our Pre-processing Approaches

• Using characteristics of mobile information
  systems for minimizing wireless data transmissions

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Semantic Replication DS

• Fragmentation of the database on DS
• FragmentSet of Data Server Extensions
• Queries may contain additional context-description
  s
• Only related fragments become replicated

Example
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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

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Semantic Replication MC (2)
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Conclusions

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

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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)

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Discussion
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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)
• fcmdselect Date, Admittance as Time, Band,
  Charge, Genre from FACTORY union select Date,
  Time, Event, NULL, NULL from AMO where event_type
  like 'concert'

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