Shadow Process

Her er et klip fra Oracle 8i Concepts som beskriver \"dedicated server process\", der er det samme som \"shadow process\".

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Oracle creates server processes to handle the requests of user processes connected to the instance. In some situations when the application and Oracle operate on the same machine, it is possible to combine the user process and corresponding server process into a single process to reduce system overhead. However, when the application and Oracle operate on different machines, a user process always communicates with Oracle via a separate server process.

Server processes (or the server portion of combined user/server processes) created on behalf of each user\'s application may perform one or more of the following:
- Parse and execute SQL statements issued via the application.
- Read necessary data blocks from datafiles on disk into the shared database buffers of the SGA, if the blocks are not already present in the SGA.
- Return results in such a way that the application can process the information.

In a dedicated server configuration, a user process executes the database application on one machine and a server process executes the associated Oracle server on another machine.

The user and server processes are separate, distinct processes. The separate server process created on behalf of each user process is called a dedicated server process (or shadow process) because this server process acts only on behalf of the associated user process.

This configuration maintains a one-to-one ratio between the number of user processes and server processes. Even when the user is not actively making a database request, the dedicated server process remains (though it is inactive and may be paged out on some operating systems).

The dedicated server architecture is also used if the same computer executes both the client application and the Oracle server code but the host operating system could not maintain the separation of the two programs if they were run in a single process. (UNIX is a common example of such an operating system.)