%META:TOPICINFO{author="ChrisBartram" date="1170388816" format="1.1" version="1.2"}%
%META:TOPICPARENT{name="Hp3000DatabaseLogging"}%
---+ Roll Forward vs. Roll Backward Database Logging on the HP3000

The basic procedural difference is:

*Roll forward recovery:*

* A database is restored from a set of backup tapes

* Transactions from the log file (from the date/time the backup was taken until the present time) are applied to the database 

*Roll Backward recovery:*

* Transactions from a log file are "undone" (i.e. backed out) from a live database until a given "state" or point in time is reached 
Each method has it's advantages and disadvantages.

For instance, a database which is structurally "damaged" would likely need to be restored from a backup media, and transactions applied to it to "bring it up to date" (roll forward).

On the other hand, a database which is structurally intact but has been "damaged" by erroneous processing (perhaps someone accidentally deleted all of the week's orders) could use the log files to "roll back" the database, undoing the transaction(s) that corrupted the database, or back to a point in time before the error occurred.





-- Main.ChrisBartram - 18 May 2006