Clustering can be structured asymmetrically or symmetrically. In asymmetric
clustering, one machine is in hot-standby mode while the other is
running the applications. The hot-standby host machine does nothing but
monitor the active server. If that server fails, the hot-standby host becomes
the active server. In symmetric clustering, two or more hosts are running
applications and are monitoring each other. This structure is obviously more
efficient, as it uses all of the available hardware. However it does require that
more than one application be available to run.