Oracle Database

Oracle database: An Oracle database consists of files. 

·        Sometimes these are referred to as operating system files, but they are actually database files that store the database information that a firm or organization needs in order to operate. 

·        The redo log files are used to recover the database in the event of application program failures, instance failures and other minor failures.

·        The archived redo log files are used to recover the database if a disk fails. 

·        Other files not shown in the figure include:

o   The required parameter file that is used to specify parameters for configuring an Oracle instance when it starts up. 

o   The optional password file authenticates special users of the database – these are termed privileged users and include database administrators. 

o   Alert and Trace Log Files – these files store information about errors and actions taken that affect the configuration of the database.

Oracle Instance

The instance is a combination of memory Structure shared by all clients accessing the data, and a number of background processes that perform actions for the instance as a whole.


The instance is always associated with one and only one database. One of the main objectives of the instance is to ensure that connections by multiple users to access database data are handled as efficiently as possible.


Another reason that the instance is important is that, when multiple users access Oracle data, allowing more than one at a time to make changes to the same data can cause data corruption and cause the integrity of the data to become suspect. The instance facilitates locking and the ability for several users to access data at the sale time.


When a user process attempt to access data in database doesn’t connect to the database but to the instance. When specifying what to connect to from a user process, you always specify the name of the instance and not the same of the database. The instance in this way is the gatekeeper to the database. It provides the interface to the database without allowing a user to actually touch the various files that make up the database.