In order to communicate with oracle database, oracle users need a program such as SQL *Plus which can issue SQL statements and few processes which can execute these SQL statements. These processes are divided into User Process, Server Process and Background Processes. User process runs user application like SQL *Plus. Server process manages oracle user process’s requests. Server process executes SQL statements and returns result to user process. Background processes are the core of oracle database which handle over all database operations.
Each user process is associated to a single server process. The server process can be Dedicated or Shared. A dedicated process has one to one relationship with user process. A dedicated process occupies certain amount of memory. In oracle database 10g, a dedicated server process consumes about 5 MB of memory. In large scale application, memory requirements for dedicated server can be a serious issue. On the other hand, a shared server process is shared by multiple user processes. We do not need a dedicated server process for every user process, so less memory is required and performance increases. At a given time, Shared server process can serve only one user process. Multiple shared server processes are configured to handle multiple user processes.
Shared server architecture consists of Listener Process, Dispatcher Process, Request Queue, Shared server process and Response Queue. Network Listener process listens the user process request. If user process request requires a dedicated server process, listener process starts a dedicated server process. If the request can be assigned to a shared server process, then the request is forwarded to dispatcher process. Shared server configuration requires at least on dispatcher process. Dispatcher process places the request on request queue. Request Queue is created in SGA and shared by all dispatcher processes. On of free Shared server process picks up the request from request queue. After processing the request, shared server process places the result in response queue. Each dispatcher process has its own response queue in SGA. Dispatcher knows about the user process which placed the request, so the response is returned back to user process.
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