12.2 A Sample Minimum Scenario
The procedures within a two-node cluster when one node fails and the various
types of standby systems that can take over as necessary are outlined below
(see Figure 12-1).
The two servers (primary and backup) are both connected to a SAN (storage
area network). Depending on the mode, this is only accessed by the active
node. The servers communicate with each other in such a way that they
regularly emit a sign of life
(heartbeat). The communication channels (or
heartbeat links) are also laid out in a redundant way,
so independent channels can be used by means of a variety of network
cards and cable channels. If one of the links fails, its backups
continue to report correctly that the relevant server is still
alive
. If there is no sign of life from the main system, the
standby system is activated, so it takes over the services of the
failed partner and removes it from the network completely (STONITH).
