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Shared servers in Oracle
A database server that is configured to allow many user processes to share few server processes. User processes connect to the dispatcher who directs them to a queue from which an idle shared server process picks a request.
This implies that a few server processes can server many clients.
Contrast this with dedicated server.
The request and response queues and other parts that are found in the pga as dedicated server are found in the sga when the server is a shared server.
In a shared server environment, a session is not bound to a single server process, rather, multiple processes can serve the session, although there is never more than one process for a server. This has some implications: The uga (user global area) cannot be stored in the pga (program global area) (aka process global area), so the uga is then found in the SGA.
Thanks to Sharif Siouri and to Dhaval Thakkar who both gave me feedback on this site and helped me improve it.