Log Shipping for High Availability

Log shipping for high availability is the process of keeping a secondary database on a standby server synchronized with the primary database by transferring (log shipping) and applying the database transaction logs from the primary to the secondary database. The idea behind log shipping is to take the work load that has already been applied to the primary database server and ship it over to a backup system, via the database transaction logs, where it can be applied to a standby database so that in the event of a failure to the primary system the database on the secondary system is just about in the same state as the primary database was right before the system failure.

This method is relatively simple to implement as all it requires is that the backup image of the database from the primary server be transferred to the secondary server. Next the backup is restored on the secondary server and left in a roll forward pending state. The user exit process that performs log archiving is modified so that it also transfers the database transaction logs over to the secondary server. This can also be done with any software designed to synchronize file system directories. There is another user exit process on the secondary database server that continuously applies the shipped logs to the database until such time as the secondary database is needed. At this point the database is removed from the roll forward pending state and activated so that it can accept incoming connections.



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