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An Oracle database system—identified by analphanumeric system identifier or SID—comprisesat least one instance of the application, along withdata storage. An instance—identified persistentlyby an instantiation number (or activation id:SYS.V_$DATABASE.ACTIVATION#)—comprises a setof operating-system processes and memory-structures that interact with the storage. (Typicalprocesses include PMON (the process monitor)and SMON (the system monitor).) Oracledocumentation can refer to an active databaseinstance as a "shared memory realm".

Users of Oracle databases refer to the server-side memory-structure as the SGA (System Global Area). The SGA typicallyholds cache information such as data-buffers, SQLcommands, and user information. In addition to storage, thedatabase consists of online redo logs (or logs), whichhold transactional history. Processes can in turn archive theonline redo logs into archive logs (offline redo logs), whichprovide the basis (if necessary) for data recovery and for thephysical-standby forms of data replication using Oracle DataGuard.

Users of Oracle databases refer to the server-side memory-structure as the SGA (System Global Area). The SGA typicallyholds cache information such as data-buffers, SQLcommands, and user information. In addition to storage, thedatabase consists of online redo logs (or logs), whichhold transactional history. Processes can in turn archive theonline redo logs into archive logs (offline redo logs), whichprovide the basis (if necessary) for data recovery and for thephysical-standby forms of data replication using Oracle DataGuard.

If the Oracle database administrator hasimplemented Oracle RAC (Real ApplicationClusters), then multiple instances, usually ondifferent servers, attach to a central storage array.This scenario offers advantages such as betterperformance, scalability and redundancy. However,support becomes more complex, and many sites donot use RAC. In version 10g, gridcomputing introduced shared resources where aninstance can use (for example) CPU resources fromanother node (computer) in the grid.

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