Oracle Database Administration: The Essential Refe

Server Processes

There are actually two types of server processes that may be created by Oracle, depending on whether the Multi-Threaded Server is configured. The Shared Server process, described earlier, is used whenever a Dispatcher process requests a connection for a user process. An MTS configuration allows a configurable number of user processes to share a single server process. When Multi-Threaded Server is not being used, either because it has not been configured or because no Dispatcher process is available, a dedicated server process is started. This server process is used to connect the user process (client) to the Oracle instance via the SGA. In this scenario, there is a one-to-one correspondence between user process and server process.

Note that on some systems, when running in dedicated server configuration on the same machine, the user process and the server process are combined into a single service.