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Before I can answer this question, I should probably
clear up some Oracle terminology. This differs from
other RDBMS vendor's terminology.
In Oracle-speak, a database is basically the datafiles
on physical disk. An instance is a started database
connected to those datafiles. While the datafiles may
exist on the server, you have not instantiated the
database until you start it. You do not access the
datafiles directly, rather you connect to the database
instance to gain access to the data. So a database and
an instance are two different things.
For most of the Oracle databases that run in the
world, one instance connects to one database and
that's it. But in special configurations, you can have
multiple instances accessing the same database. In
other words, you can start Oracle running on multiple
servers accessing the same data. In order to
accomplish this in Oracle 9i, you need to use a
product called Real Application Clusters (RAC).
RAC is used for a number of reasons. First, you can
spread out the load on the database among multiple
servers. This is one of the ways that databases can
handle an extreme volume of users and transactions. So
better performance is one of the reasons. The second
reason to use RAC is for high availability. In normal
configurations (without RAC), if the instance goes
down, you no longer have access to your data. In RAC
environments, if an instance goes down other instances
can still access the data. User sessions can failover
to the remaining instances in these environments. If
you do not have a requirement for high availability
and/or high performance, then you do not need RAC.
Most Oracle databases in the world do not use RAC, but
that does not mean that RAC is never used. For More Information
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