Oracle Architecture

Oracle Server: There are several files, processes and memory structures in an Oracle server. Some are used to improve performance of the database, to ensure that the database can be recovered in the event of a software or hardware error, or to perform other tasks necessary to maintain the database.

• Is a database management system that provides an open, comprehensive integrated approach to information management?

Oracle Instance: An Oracle instance is the combination of the background processes and memory structures. The instance must be started to access the data in the database. Every time an instance is started, a System Global Area (SGA) is allocated and Oracle background processes are started. Background processes perform functions on behalf of the invoking process.

Key Features:

• Is a means to access an Oracle Database?
• Always opens one and only one database.
• Consists of memory and background process.

Oracle Database: An Oracle database consists of operating system files, also known as database files, that provide the actual physical storage for database information. The database files are used to ensure that the data is kept consistent and can be recovered in the event of a failure of the database. An oracle database consists of 3 file types:

• Data files, containing the actual data in the database.
• Online Redo log files, containing a record of changes made to the database to enable recovery of the data in the case of failures.
• Control Files, contaminant information necessary to maintain and verify database integrity.

Key Features:

• Is a collection of data that is treated as a unit.
• Consists of 3 file types.

Other Key Files : Non database files are used to configure the instance, authenticate privileged users, and recover the database in the event of a disk failure.

• The Parameter File ( init.ora ), defines the characteristics of an Oracle instance. For example, it contains parameters that size some of the memory structures in the SGA.
• The Password File, authenticates users privileged to start up and shut down an Oracle instance.
• Archived Redo Log Files ( archives ), are offline copies of the online redo log files that may be necessary to recover from media failures.

User and Server Processes : The user and server processes are the primary processes involved when a SQL statement is executed; however, other processes may help the server complete the processing of the SQL statement.