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The instance runs on the computer or server; the
database is stored on the disks attached to the server. Figure 2-1 illustrates this
relationship.
The database is physical: it consists of files stored on disks. The instance is logical: it
consists of in-memory structures and processes on the server. For example, Oracle
uses an area of shared memory called the System Global Area (SGA) and a private
memory area for each process called the Program Global Area (PGA). An instance
can be part of one and only one database, although multiple instances can be part of
the same database. Instances are temporal, but databases, with proper maintenance,
last forever.
Users do not directly access the information in an Oracle database. Instead, they pass
requests for information to an Oracle instance.
34 | Chapter 2: Oracle Architecture
The real world provides a useful analogy for instances and databases. An instance
can be thought of as a bridge to the database, which can be thought of as an island.
Traffic flows on and off the island via the bridge.


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