For most Oracle users, the terms database and instance are used synonymously. However, there are differences that become especially important if you are using the Oracle Parallel Server (OPS) Option. The database is the data on disk, stored in operating system files (or possibly, under Unix, in raw files). The instance is composed of the System Global Area (SGA) memory and the background processes. The instance is STARTed using Oracle Server Manager or Oracle Enterprise Manager (OEM). The database is then MOUNTed on the instance and is finally OPENed. The users CONNECT to the instance in order to access the data in the database. Figure 1.1 shows the basic components of the Oracle database and instance.

Figure 1-1. The Oracle database and the Oracle instance

Except in an OPS environment, there is a one-to-one correspondence between instance and database. In the OPS world, the database can be MOUNTed on multiple instances.