Getting Started DB 11g - Oracle Instance Overview

Oracle Instance Overview

An Oracle database system consists of an Oracle database and an Oracle instance.


A database consists of a set of disk files that store user data and metadata.
Metadata consists of structural, configuration, and control information about the database.

An Oracle instance (or database instance) contains the set of Oracle background processes and a shared memory for them (System Global Area SGA)


Oracle background processes operate on the stored data using the shared allocated memory that those processes use to do their work (SGA).

Each instance has an instance ID, also known as a system ID (SID).

Because there can be multiple Oracle databases on a host computer, each with its own set of data files, you must identify the instance to which you want to connect. For a local connection, you identify the instance by setting operating system environment variables ORACLE_SID and ORACLE_HOME. For a remote connection, you identify the instance by specifying a network address and a database service name.

An Oracle instance must be started to read and write information to the database.

The Oracle instance creates the database upon receipt of instructions from the Oracle Database Configuration Assistant (DBCA) utility or the CREATE DATABASE SQL statement.

When the Oracle instance is not available, your data is safe in the database, but it cannot be accessed by any user or application.

The properties of an Oracle instance are specified using instance initialization parameters. When the instance is started, an initialization parameter file is read, and the instance is configured accordingly.

Reference: Oracle 11g 2Day DBA

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