Replication
Replication allows you to maintain separate databases, where the updates that are performed to one database are automatically propagated to the others. These redundant databases can be used both to maintain local copies of data (eliminating long network propagation delays) and to provide a second copy (which can continue in the event of failure of one of the databases). Once the failed database is brought back online, all updates that have occurred since the failure will be automatically applied to the database.
Replication does require that you completely duplicate the entire physical implementation, including multiple computer systems and storage. You will also have to implement a reasonably fast network communication link between the multiple computer systems. However, replication can be used to survive not only hardware or software problems, but also sitewide emergencies (assuming, of course, that the replicated database is running offsite).
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