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Oracle RAC (Real Application Clusters) is two or more instances running on two or
more servers connecting to the same database. The idea is to utilize the
additional resources of multiple machines to satisfy higher load demands as
well as provide a higher level of availability since connections can be
directed to any available instance. From a high level infrastructure-wise,
RAC uses cluster software (Oracle's or another parties' such as Sun), shared
disk storage, an interconnect between servers and specific configuration
parameters.
Data Guard is the configuration of at least one standby database associated
with a production or primary database. There are logical and physical
standby databases. Physical standbys are physical copies of the primary
database and are updated via a recovery operation. Logical standbys are
logical copies of your primary datbase and may vary greatly in physical
design. They are updated through SQL statements.
Data Guard is primarily a backup solution in the event of failure at the
primary database. A Data Guard database may also be "RAC'd."
The terms 'node' and 'instance' are often used interchangeably. An instance in a
RAC environment is pretty much the same instance as you know it from a
non-RAC environment. When I say node, I am referring to the server that the
instance is housed on.
I hope this helps.
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