4ei - Discuss Distributed Databases

4e - Discuss Distributed Databases

1. Data availability

   • Partitioned
     • Local data is always availability
     • Global data is dependent upon communication link
     • Losing a local site is minor problem in comparison
   • Replicated
     • All data is always available
     • However, data may not be up to date, so not suitable for real-time system
       • Appropriate where immediate updates are not crucial
   • Centralised
     • All data is dependent on communication link
     • Minor problem if only one link goes down
     • Data is always up to date
     • Integrity of data is maintained, so there are no conflicts
     • Appropriate for real-time systems

2. Backups

   • Partitioned
     • No central backups
     • Each site must change backups
       • Changing tapes
       • Security
       • Testing
       • Off-site storage
   • Replicated
     • Natural backup for each replica
     • Still need at least one backup in case corruption
   • Centralised
     • Needs a fail safe backup solution
       • Changing tapes
       • Security
       • Testing
       • Off-site storage