RAC or Real Application Clusters is the Oracle database feature that allows2 or more instances to servicethe same database.

Prior to Oracle 9i, this option was called Oracle Parallel Server (OPS).

RAC = 1 database + 2 or more instances + 1 or more private interconnect(s) + 1 or more public network(s)

One reason that some databases migrate toRAC is to provide scalability by distributing the load across all instances. When additional scalability is required, additional nodes are added to the cluster and additional instances are started to provide more throughput thereby adding scalability.

The majority of RAC implementations are specifically to providefault tolerance that allows the application(s) to continue working even if one (or more) of the instances or servers in the cluster fail.

Fault tolerance is relatively easy to achieve due to Transparent Application Failover (TAF) features built in to RAC. When an instance crashes, all sessions connected to the failed instance can be reconnected (seamlessly) to asurviving instance. In some cases, a SQL SELECT in progress may be migrated without losing any records and finish running on the surviving instance after migration. Other features such as Fast Connection Failover (FCF) andFast Application Notification (FaN) also help provide very fast reaction in the event of a failure so that the application impact is minimized.

Since Oracle10g the use of Oracle Clusterware is mandatory. Additional third-party clusterware is supported (check the certification matrix), but even if another clusterware is used, Oracle Clusterware is still required. When a third-party clusterware is employed, Oracle Clusterware integrates with the existing clusterware to delegate certain responsibilities to the third-party clusterware. The integration varies based on the particular third-party vendor and Oracle's implementation on that platform.

It is worth noting that Oracle RAC is currently licensed as a separate add-on database option for Oracle Database Enterprise Edition. List prices at time of writing are US$20,000 per CPU for the add-on in addition to the EE list price of US$40,000. See the price list or Oracle Store for updated

RAC is also included with Oracle Database Standard Edition with certain restrictions. The most notable restrictions are:

• Up to 4 CPU sockets are allowed in the entire cluster (so either 2 nodes with 2 CPUs each, or 4 nodes with 1 CPU each, etc).
• Only Oracle Clusterware can be used for cluster management (no third-party clusterware allowed)
• ASM must be used to manage database storage

At the present time, there are no other configurations where RAC can be run (just SE and EE).

There is also a large user community for RAC. Most notably, the Oracle RAC SIG is a large, active user group. See the wiki page here and the RAC SIG website at http://www.oracleracsig.org/ for more information. It is free to join and offers many benefits like free live webcasts throughout the year (with on-demand playback for all the archives) and a valuable document library online.