The previous sections described the process of starting up and shutting down a database. But the database is only part of a complete system—you also need a client process to access the database, even if that process is on the same physical machine as the database.
To access a database, a user connects to the instance that provides access to the desired database. A program that accesses a database is really composed of two distinct pieces—a client program and a server process—that connect to the Oracle instance. For example, running the Oracle character-mode utility SQL*Plus involves two processes:
The SQL*Plus process itself, acting as the client
The Oracle server process, sometimes referred to as a shadow process, that provides the connection to the Oracle instance
The Oracle server process always runs on the computer on which the instance is running. The server process attaches to the shared memory used for the SGA and can read from it and write to it.
As the name implies, the server process works for the client process—it reads and passes back the requested data, accepts and makes changes on behalf of the client, and so on. For example, when a client wants to read a row of data stored in a particular database block, the server process identifies the desired block and either retrieves it from the database buffer cache or reads it from the correct datafile and loads it into the database buffer cache. Then, if the user ...