Databases Reference
In-Depth Information
As a clustered solution, RAC avoids the various activities involved in disk takeover:
mounting volumes, validating filesystem integrity, opening Oracle database files, and
so on. Not performing these activities significantly reduces the time required to achieve
full system availability. RAC gained in popularity partly because it eliminated the cre‐
ation and maintenance of the complex scripts typically used to control the activities for
hardware failover. For example, there is no need to script which disk volumes will be
taken over by a surviving node.
While RAC recovery is in progress, clients connected to surviving instances remain
connected and can continue working. In some cases users may experience a slight delay
in response times, but their sessions aren't terminated. Clients connected to the failed
instance can reconnect to a surviving instance and can resume working. Uncommitted
transactions will be rolled back and will have to be resubmitted. Queries that were active
will also be terminated, but a seamless transition can be provided by Transparent Ap‐
plication Failover, as we'll describe in the next section.
Oracle Transparent Application Failover
Transparent Application Failover (TAF) provides seamless migration of users' sessions
from one Oracle instance to another. You can use TAF to mask the failure of an instance
for transparent high availability or for migration of users from a highly active instance
to a less active one.
Figure 11-7
illustrates TAF with Real Application Clusters.
Figure 11-7. Failover with TAF and Real Application Clusters
