Process Model

The Oracle process model is organized around an instance, which consists of shared memory and a group of database system processes. Only one database can be opened by an instance, although an instance can be started initially without any database being mounted or opened at all. Real Application Clusters (RAC) technology (previously known as Oracle Parallel Server, or OPS) actually allows multiple instances to open the same database, allowing clustered systems to share a single database. Each instance has a unique instance ID, set with the ORACLE_SID environment variable, and the characteristics of an instance are determined by the contents of a parameter file, usually called init{$ORACLE_SID}.ora.

The shared memory belonging to ...

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