DB Issues - Recovery

A DBMS must be able to restore the database (or return it to a known condition) in the event of a system failure. This may include; operator error, incorrect or invalid data, program errors (soft crash) and hard disk crashes, natural catastrophes (hard crash).

Large DBMSs may have the ability to "Rollback" transactions to a specified point (enables rollback and roll forward), or backout to a specified point (actually nullifies transactions)

A distributed database has additional problems to deal with than a centralised database. This includes communications link failure (lost messages). Hence recovery here may be once a message is deemed lost, asking for the message to be resent.

Commonly used techniques

Restore/rerun - reprocessing the day's transactions against the backup copy of the database.
Backward recovery - (rollback). Eg customer A has $100 transferred to customer B, and the process fails after $100 has been deducted from Customer A. Rollback will "undo the Customer A" transaction.
Forward recovery - (rollforward). Starts with an earlier copy of the database and applies after images (the results of good transactions).

Most PC-DBMSs have few if any constructs to assist in recovering corrupt data bases or in rollback of partial transactions.

Users of a PC-DBMS should periodically backup or copy critical database components.

McFadden, Hoffer (1994)

dBASE

When a record is deleted in a database, initially it is just "Marked for deletion". this means that a record may be recovered by undeleting the record.
To permenantly delete a record the relational table containing the record is "PACKED".
If the index or file is corrupted a "PACK and REINDEX" may be issued to update the table and rebuild the indexes.
In general if a file error occurs you should recover from backup.

MS-Access

Repairing a Database
- You may need to repair a database if Microsoft Access quits unexpectedly: for example, if there is a power outage or your computer experiences a hardware problem. The database won't become corrupted if you quit Microsoft Access normally.
- To repair a database, from the File menu in the Microsoft Access Startup window, choose Repair Database.
Notes
- When you open, compact, encrypt, or decrypt a corrupted database, Microsoft Access asks if you want to repair the database. If you choose [OK], Microsoft Access repairs the database.
- In some situations, Microsoft Access may not detect that a database has become corrupted. If a database behaves unpredictably, use the Repair Database command to repair it.