When a node fails, its mirror node takes over for it. If the failed node does not have a spare node, then at this point, the failed node will not have a mirror. A spare node will automatically replace a failed node’s mirror. Having a spare node reduces the time the system functions without a mirror node.
A spare node does not normally contain data, but constantly monitors for failure of active nodes in the DRU. When a node fails and does not recover within a specified timeout period, the spare node copies data from the mirror node and synchronizes with it. The time this takes depends on the amount of data copied and the system and network capacity. After synchronizing, the spare node automatically replaces the mirror node without manual intervention, thus relieveing the mirror node from overload, thus balancing load on the mirrors. This is known as failback or self-healing.
When a failed host is repaired (by shifting the hardware or upgrading the software) and restarted, the node or nodes running on it join the system as a spare nodes, since the original spare nodes are now active.
Spare nodes are not required, but they enable a system to maintain its overall level of service even if a machine fails. Spare nodes also make it easy to perform planned maintenance on machines hosting active nodes. Allocate one machine for each DRU to act as a spare machine, so that if one of the machines fails, the HADB system continues without adversely affecting performance and availability.
As a general rule, have a spare machine with enough Application Server instances and HADB nodes to replace any machine that becomes unavailable.