RAC ARCHITECTURE

The Oracle Real Application Clusters (RAC) option was first released as part of Oracle 9.0.1 in the summer of 2001. It is considered by many within the Oracle community to be the most significant feature introduced in Oracle 9i.

Oracle Real Application clusters allows multiple instances to access a single data base, the instances will be running on multiple nodes. In an standard Oracle configuration a database can only be mounted by one instance but in a RAC environment many instances can access a single database.

The table below describes the difference of a standard oracle database (single instance) an a RAC environment

Component	Single Instance Environment	RAC Environment
SGA	Instance has its own SGA	Each instance has its own SGA
Background processes	Instance has its own set of background processes	Each instance has its own set of background processes
Datafiles	Accessed by only one instance	Shared by all instances (shared storage)
Control Files	Accessed by only one instance	Shared by all instances (shared storage)
Online Redo Logfile	Dedicated for write/read to only one instance	Only one instance can write but other instances can read during recovery and archiving. If an instance is shutdown, log switches by other instances can force the idle instance redo logs to be archived
Archived Redo Logfile	Dedicated to the instance	Private to the instance but other instances will need access to all required archive logs during media recovery
Flash Recovery Log	Accessed by only one instance	Shared by all instances (shared storage)
Alert Log and Trace Files	Dedicated to the instance	Private to each instance, other instances never read or write to those files.
ORACLE_HOME	Multiple instances on the same server accessing different databases ca use the same executable files	Same as single instance plus can be placed on shared file system allowing a common ORACLE_HOME for all instances in a RAC environment.