Hot Backup is a configuration where one machine is in a standby mode, ready to take over the load from a failing system. Starting with D3 9.0, Hot Backup now tests to confirm that a license has been purchased.
It is often required to have a system configuration where downtime due to a hardware or software failure cannot be tolerated, or must be reduced to a very short time. A solution more affordable than fault tolerance is to double all necessary hardware resources and maintain the database on two normal systems.
For more information, see the Hot Backup section in the D3 Linux System Administration Guide or D3 AIX System Administration Guide.