OPS clusters were often deployed to
assure a highly available database. Real Application Clusters followed, in Oracle9i,
and was first made generally available in 2001.
At first glance, Real Application Clusters (RAC) may look similar to the clustered
solutions described earlier in the ???Simple Hardware Failover??? section. Both failover
and Real Application Clusters involve clustered hardware with access to disks from
multiple nodes. The key difference is that with simple hardware failover only one
node is an active instance. With RAC, the Oracle database is spread across multiple
nodes and each node has an active Oracle instance. Clients can connect to any of the
instances to access the same database.
Because each Oracle instance runs on its own node, if a node fails, the instance on
that node also fails. The overall Oracle database remains available since surviving
instances are still running on other working nodes.
Figure 11-6 illustrates Real Application Clusters on a cluster.
Real Application Clusters and hardware failover
The Real Application Clusters option can typically provide higher levels of availability
than simple hardware failover.
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