How an Oracle Instance Is Started

When Oracle starts an instance, it reads the server parameter file (SPFILE) or initialization parameter file to determine the values of initialization parameters. Then, it allocates an SGA, which is a shared area of memory used for database information, and creates background processes. At this point, no database is associated with these memory structures and processes.

Restricted Mode of Instance Startup

You can start an instance in restricted mode (or later alter an existing instance to be in restricted mode). This restricts connections to only those users who have been granted the RESTRICTED SESSION system privilege.

Forced Startup in Abnormal Situations

In unusual circumstances, a previous instance might not have been shut down cleanly. For example, one of the instance's processes might not have terminated properly. In such situations, the database can return an error during normal instance startup. To resolve this problem, you must terminate all remnant Oracle processes of the previous instance before starting the new instance.

How a Database Is Mounted

The instance mounts a database to associate the database with that instance. To mount the database, the instance finds the database control files and opens them. Control files are specified in the CONTROL_FILES initialization parameter in the parameter file used to start the instance. Oracle then reads the control files to get the names of the database's datafiles and redo log files.

At this point, the database is still closed and is accessible only to the database administrator. The database administrator can keep the database closed while completing specific maintenance operations. However, the database is not yet available for normal operations.

How a Database Is Mounted with Real Application Clusters

If Oracle allows multiple instances to mount the same database concurrently, then the database administrator can use the CLUSTER_DATABASE initialization parameter to make the database available to multiple instances. The default value of the CLUSTER_DATABASE parameter is false. Versions of Oracle that do not support Real Application Clusters only allow CLUSTER_DATABASE to be false.

If CLUSTER_DATABASE is false for the first instance that mounts a database, then only that instance can mount the database. If CLUSTER_DATABASE is set to true on the first instance, then other instances can mount the database if their CLUSTER_DATABASE parameters are set to true. The number of instances that can mount the database is subject to a predetermined maximum, which you can specify when creating the database.