A typical system has two or more HSMs connected as ‘live’ units. This provides increased capability where the processing requires more than one HSM, and provision for backup in the event of an HSM failure.
Each HSM is normally connected to the Host via a separate Host port, although a port-sharing unit can be used if the number of Host ports available is limited. The sharing configuration is not capable of providing backup if the port or the port-sharing unit becomes faulty.
Optionally it is possible to have a backup unit not connected to the Host but ready for connection in place of a faulty unit. This is not the preferred practice because the unit may remain idle for a long time and may itself have developed a fault.
In addition to the ‘live’ units, a typical system contains at least one HSM connected to a test or development computer system. This allows changes in the environment to be tested, without disturbing the live system.