Oracle Server

ORACLE SERVER:
Oracle Server is composed of two major parts
1) Oracle Instance.
2) Oracle Database.

Oracle Server = (Oracle Instance + Oracle Database)

An Oracle instance is a is composed of two major parts
1) Memory structure.
2) Background processes.

Oracle instance = (Memory structure + background processes.)

The typical background processes are

PMON, SMON , MMON ,DBWR ,LGWR ,CKPT ,ARCH etc ..

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(Note:
1>The Instance must be started to access the data in the database.

2>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 of them are used to improve the performance of the database, ensure that the database can be recovered in event of a software or hardware error, or perform other tasks necessary to maintain the database.

3>Every time an instance is started, a System Global Area (SGA) is allocated and Oracle background processes are started.

4>Always opens one and only one database

)