A RAC installation can provide extreme high availability for both planned and unplanned
outages. One instance can be restarted with new initialization parameters while the other instance
is still servicing requests against the database. If one of the hardware servers crashes due to a fault
of some type, the Oracle instance on the other server will continue to process transactions, even
from users who were connected to the crashed server, transparently and with minimal downtime.
RAC, however, is not a software-only solution: The hardware that implements RAC has special
requirements. The shared database should be on a RAID-enabled disk subsystem to ensure that
each component of the storage system is fault tolerant. In addition, RAC requires a high-speed
interconnect, or a private network, between the nodes in the cluster to support messaging and
transfer of blocks from one instance to another using the Cache Fusion mechanism.
The diagram in Figure 1-7 shows a two-node RAC installation. How to set up and configure
Real Application Clusters is discussed in depth in Chapter 10.
42 Oracle Database 11g DBA Handbook
Oracle Streams
As a component of Oracle Enterprise Edition, Oracle Streams is the higher-level component of
the Oracle infrastructure that complements Real Application Clusters. Oracle Streams allows the
smooth flow and sharing of both data and events within the same database or from one database
to another.
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