What is Oracle instance

What is Oracle instance?

An Oracle server is made up of an Oracle instance and an Oracle database.

An Oracle database can only be accessed through one instance. See our tutorial on managing an oracle instance to learn how to manipulate the associated database.

An Oracle server instance is made up of a SGA (Global Area System) shared memory structure and several background Oracle processes each with a specific role.


An instance can only open one database at a time.

A database can be opened by multiple instances using the RAC (Oracle Real Application Clusters) option for high availability and load balancing. (see diagram below)


In such a configuration, users waiting to be connected can be directed to the least loaded instance and in addition, if one instance fails, the other takes over for better database availability.

N.B: In Windows environment, each instance is associated with a service named OracleService “InstanceName”. This service must be started in order to start the instance.

When it starts up, the instance uses an initialization settings file.

An instance has a SID. This determines the parameter file used at STARTUP (spfile ”SID” .ora or spfile.ora or init ”SID” .ora in this order of priority, if Oracle does not find one it goes to the next).

The components of the instance (SGA + background processes) are described in the following sections.

To learn more about the components and basic concepts of the Oracle architecture, take look at our Expert DBA Team Club blog and more advance topics are available at this source.

Automatic Oracle Instance Registration

A feature called automatic Oracle instance registration is the ability of an instance to register itself automatically with the listener at startup. Before Oracle 8i, an instance had to be configured via the listener.ora file. Now Oracle provides automated instance registration with the associated listener. Database instance registration enables load balancing and failover features as well.

When started, the database will register itself with the default listener without any additional configuration requirements. If you use multiple listener configurations, then you will need to add some additional parameters to the parameter file or spfile file for that instance. Configuration for listeners other than the default listener will require the inclusion of the SERVICE_NAME and LOCAL_LISTENER optional parameters in the init.ora file of the instance you want registered. Note that the LOCAL_LISTENER parameter also overrides other parameters of Shared server configuration. This is most important thing and as Oracle DBA you should need to aware of this.

If you use multiple listeners, you will need to modify your parameter file or you will find your database registering itself with both the listener you have defined for it and the default listener.