Dedicated Server :-
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 can be paged out on some operating systems).

[b]
Dedicated server architecture can sometimes result in inefficiency. Consider an order entry system with dedicated server processes. A customer places an order as a clerk enters the order into the database. For most of the transaction, the clerk is talking to the customer while the server process dedicated to the clerk's user process remains idle. [I]The server process is not needed during most of the transaction, and the system is slower for other clerks entering orders. For applications such as this, the shared server architecture may be preferable.[/I]
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Dedicated and Shared Servers
Memory allocation depends, in some specifics, on whether the system uses dedicated or shared server architecture. Table 8-1 shows the differences.

Table 8-1 Differences in Memory Allocation Between Dedicated and Shared Servers

Memory Area Dedicated Server Shared Server 
Nature of session memory
 Private
 Shared
 
Location of the persistent area
 PGA
 SGA
 
Location of part of the runtime area for SELECT statements
 PGA
 PGA
 
Location of the runtime area for DML/DDL statements
 PGA
 PGA
 

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Shared Server Architecture
Shared server architecture eliminates the need for a dedicated server process for each connection. A dispatcher directs multiple incoming network session requests to a pool of shared server processes. An idle shared server process from a shared pool of server processes picks up a request from a common queue, which means a small number of shared servers can perform the same amount of processing as many dedicated servers. Also, because the amount of memory required for each user is relatively small, less memory and process management are required, and more users can be supported.

A number of different processes are needed in a shared server system:

A network listener process that connects the user processes to dispatchers or dedicated servers (the listener process is part of Oracle Net Services, not Oracle).

One or more dispatcher processes

One or more shared server processes

Shared server processes require Oracle Net Services or SQL*Net version 2.