A request from a user is a single program interface call that is part of the user’s SQL
statement. When a user makes a call, its dispatcher places the request on the request
queue, where it is picked up by the next available shared server process.
The request queue is in the SGA and is common to all dispatcher processes of an
instance. The shared server processes check the common request queue for new
requests, picking up new requests on a first-in-first-out basis. One shared server
process picks up one request in the queue and makes all necessary calls to the database
to complete that request.
When the server completes the request, it places the response on the calling
dispatcher’s response queue. Each dispatcher has its own response queue in the SGA.
The dispatcher then returns the completed request to the appropriate user process.
For example, in an order entry system each clerk’s user process connects to a
dispatcher and each request made by the clerk is sent to that dispatcher, which places
the request in the request queue. The next available shared server process picks up the
request, services it, and puts the response in the response queue. When a clerk’s
request is completed, the clerk remains connected to the dispatcher, but the shared
server process that processed the request is released and available for other requests.
While one clerk is talking to a customer, another clerk can use the same shared server
process.

适配器请求和响应队列
一图胜千言 , Concepts上相应的图就可以概况这段话了