The availability of an application server can be measured by comparing the actual amount of time it is operational against the total time it could possibly be available. Highly available systems may need to operate 100% of the time. To accomplish that level of availability, you must address every possible single point of failure. In addition to hardware and software failures, you must have a way to continue operations while both the hardware and software are being upgraded or replaced.
Oracle Application Server solves these high-availability problems with clusters and with two different failover solutions for the OracleAS Infrastructure.
We introduced the concept of clusters earlier in this chapter. Although both a farm and a cluster are made up of multiple instances of Oracle Application Server components, a cluster is a more integrated grouping than a farm.
In a cluster, the individual instances of a component are aware of each other, and they cooperate to provide service. The OPMN server keeps the members of a cluster informed as to the status of other members in that cluster, so if one member fails, the others pick up its load. The members of a cluster are designed to be interchangeable, providing a higher level of availability than an individual instance or a group of instances in a farm.
You can create clusters of OracleAS Web Cache instances or Oracle Application Servers. As mentioned earlier, a cluster requires some type of ...