Oracle Essentials [Electronic resources] : Oracle Database 10g, 3rd Edition نسخه متنی

2.1 Instances and Databases

Many Oracle practitioners use the terms
"instance" and
"database" interchangeably. In
fact, an instance and a database are different (but related)
entities. This distinction is important because it provides insight
into Oracle's architecture.

In Oracle,
the term database
refers to the physical storage of information, and the term
instance
refers to the software executing on the server that provides access
to the information in the database. 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.

Figure 2-1. An instance and a database

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). (The SGA is discussed further later in
this chapter and both the SGA and PGA are further discussed in Chapter 6.) An instance can connect to one and only one
database, although multiple instances can connect to 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.

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. If the bridge is closed, the island exists but no traffic
flow is possible. In Oracle terms, if the instance is up, data can
flow in and out of the database. The physical state of the database
is changing. If the instance is down, users cannot access the
database even though it still exists physically. The database is
static: no changes can occur to it. When the instance comes back into
service, the data will be there waiting for it.