Primary Components
The Oracle architecture includes a number of primary components:
Oracle server:There are several files, processes, and memory structures in an Oracle server; however, not all of them are used when processing a SQL statement. Some are used to improve the 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. The Oracle server consists of an Oracle instance and an Oracle database. ------(oracle server=instance+database)
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. They consolidate functions that would otherwise be handled by multiple Oracle programs running for each user. The background processes perform. input/output (I/O) and monitor other Oracle processes to provide increased parallelism for better performance and reliability. ----(=memory+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 instance. ---(=data files+redo log +control files)
Other key files:Nondatabase files are used to configure the instance, authenticate privileged users, and recover the database in the event of a disk failure.
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.
Other processes:Many other processes exist that are used by other options, such as Advanced Queuing, Real Application Clusters, Shared Server, Advanced Replication, and so on. These processes are discussed within their respective courses.
The SGA consists of several memory structures:
– Shared Pool
-Library Cache(often used executed SQL statements)
-Data Dictionary Cache(used data definitions)
– Database Buffer Cache
– Redo Log Buffer
– Other structures (for example, lock and latch management, statistical data)
There are two additional memory structures that can be configured within the SGA:
– Large Pool(Session memory (UGA) for the Shared Server;I/O server processes;Backup and restore operations or RMAN;Parallel execution message buffers
– Java Pool
