DATABASE CONTROL ISSUES: SECURITY, BACKUP AND RECOVERY, CONCURRENCY - Fundamentals of Database Management Systems - page 306

through the transaction log, automatically rerunning all of the transactions from the
point at which the data error occurred.
Another note about backward recovery: some systems are capable of
automatically initiating a roll-backward operation to undo the changes made to
the database by a partially completed and then halted or failed transaction. This is
called '' dynamic backout .'' There are situations in which it is helpful to restore the
database to the point at which there is confidence that all changes to the database up
to that point are accurate. Some systems are capable of writing a special record to
the log, known as a '' checkpoint ,'' that specifies this kind of stable state.

Duplicate or ''Mirrored'' Databases
A backup and recovery technique of a very different nature is known as duplicate
or '' mirrored '' databases . Two copies of the entire database are maintained and
both are updated simultaneously, Figure 11.6. If one is destroyed, the applications
that use the database can just keep on running with the duplicate database. This is
a relatively expensive proposition, but allows continuous operation in the event of
a disk failure, which may justify the cost for some applications. By the way, this
arrangement is of no help in the case of erroneous data entry (see backward recovery
above) because the erroneous data will be entered in both copies of the database!
The greater the ''distance'' between the two mirrored copies of the database,
the greater the security. If both are on the same disk (not a good idea!) and the disk
fails or is destroyed, both copies of the database are lost. If the two copies are on
different disks but are in the same room and a fire hits the room, both might be
destroyed. If they are on disks in two different buildings in the same city, that's
much better, but a natural disaster such as a hurricane could affect both. Thus, some
companies have kept duplicate databases hundreds of miles apart to avoid such
natural disasters.

Disaster Recovery
Speaking of natural disasters, the author lived through Hurricane Andrew in Miami,
FL, in August, 1992 and learned about disaster recovery first-hand! The information

Physical separation

Database
Copy
1

Database
Copy
2

Database
Application

F IGURE 11.6
Mirrored databases

Computer