Most failover solutions include software that runs on the machine to monitor specific
processes, such as the background processes of the Oracle instance. If the primary
node itself has not failed but some process has, the monitoring software will
detect the failure of the process and take some action based on scripts set up by the
system administrator. For example, if the Oracle instance fails, the monitoring software
may attempt to restart the Oracle instance three times. If all three attempts are
unsuccessful, the software may initiate physical hardware failover, transferring control
to the alternate node in the cluster.
Figures 11-4 and 11-5 illustrate the process of implementing a simple failover.
Protecting Against System Failure | 267
Figure 11-4. Before failover
Figure 11-5. After failover
Oracle Instance
Primary
Server
Heartbeat Alternative
Server
Oracle Database
Oracle Instance
Heartbeat
Alternative
Server
Oracle Database
Primary
Server
268 | Chapter 11: Oracle and High Availability
Outage duration for hardware failover
The time for failover to take effect, and therefore the length of the associated database
downtime, depends on the following intervals:
Time for the alternate node to detect the failure of the primary node
The alternate node monitors the primary node using a heartbeat mechanism.
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