Database Recovery

1
Database Recovery

• Lucas Finger

2
Overview

• Purpose of Recovery
• What Causes Database Failure?
• Being Prepared
• Techniques for Recovery

3
Purpose of Database Recovery

• To bring the database into the last consistent
  state, which existed prior to the failure.
• To preserve transaction properties
• Atomicity
• Consistency
• Isolation
• Durability

4
What Causes Database Failure?

• DB server computer or program crashes
• DB client computer or program crashes
• Network failure between client and server
• Data on a disk drive is corrupted
• A transaction executes an illegal operation
• Two or more transactions deadlock
• Transactions introduce errors into the DB

5
Being Prepared

• Planning ahead is the most important aspect of
  data recovery.
• What to think about
• How much can you afford to lose?
• How and where backups are stored
• Having a written plan
• Make sure it works!

6
Recovery Strategies

• Redundancy is the key to recovery
• Tools
• Backups, checkpoints, and transaction logs

Count Transaction Last Next Action Data Item Before After
1 T1 0 2 Begin      
2 T1 1 4 W X X100 X200
3 T2 0 9 Begin      
4 T1 2 5 W Y Y50 Y100
5 T1 4 7 R M M200 M200
6 T3 0 8 R N N400 N400
7 T1 5 null End      
7
Recovery Strategies

• Types of Transaction Logs
• Redo Log
• List of all changes in the order they were
  committed
• recovery and roll forward
• Mostly used for catastrophic failures
• Undo Log
• List of all changes in reverse order
• roll back
• Often more efficient than recovery and roll
  forward

8
Conclusion

• Data recovery is important in managing a database
  system.
• Having a good plan will help make a quick and
  successful recovery from a failure.
• Backups and checkpoints together with transaction
  logs help ensure your data is safe.

9
References

• Kumar, Vijay, and Meichun Hsu. Recovery
  Mechanisms in Database Systems. New Jersey
  Prentice Hall, 1998. 6-50.
• Riccardi, Greg. Principles of Database Systems
  with Internet and Java Applications. Boston
  Wesley Publishing, 2001. 386-391.
• Wynkoop, Stephen. Data Recovery Best
  Practices. Lumingent. 16 Dec 2004.
  lthttp//www.lumigent.com/files/whitepaper_datarec
  o very.pdfgt

10
QUESTIONS?