- A Dedicated Server Process, which serves only one user or one user connection
- A Shared Server Process, which server multiple users or multiple user connections
Oracle databases are by default configured for dedicated server process and if you need to enable Shared server process you need to configure it manually by setting one or more initialization parameters. However a Dedicated server process is always required for:
- To submit a batch job (for example, when a job can allow little or no idle time for the server process)
- To use Recovery Manager (RMAN) to back up, restore, or recover a database
Figure 1: Oracle Database Dedicated Server Processes
Shared Server Process:
Figure 2: Oracle Database Shared Server Processes
In Shared server configuration a client user process connect a dispatcher and the dispatcher then communicates with the oracle database for client request processing once the processing is done oracle then sends back the output or solution back to the dispatcher which in turn communicates it to the client user process. One dispatcher can serve multiple client user processes by connection pooling which can be enabled for shared server process. Further, shared server can be configured for session multiplexing, which combines multiple sessions for transmission over a single network connection in order to conserve the operating system's resources.
Initialization Parameters for Shared Server:
The following initialization parameters control shared server operation:
- SHARED_SERVERS: Specifies the initial number of shared servers to start and the minimum number of shared servers to keep. This is the only required parameter for using shared servers.
- MAX_SHARED_SERVERS: Specifies the maximum number of shared servers that can run simultaneously.
- SHARED_SERVER_SESSIONS: Specifies the total number of shared server user sessions that can run simultaneously. Setting this parameter enables you to reserve user sessions for dedicated servers.
- DISPATCHERS: Configures dispatcher processes in the shared server architecture.
- MAX_DISPATCHERS: Specifies the maximum number of dispatcher processes that can run simultaneously. This parameter can be ignored for now. It will only be useful in a future release when the number of dispatchers is auto-tuned according to the number of concurrent connections.
- CIRCUITS: Specifies the total number of virtual circuits that are available for inbound and outbound network sessions.
Shared server is enabled by setting the SHARED_SERVERS initialization parameter to a value greater than 0. The other shared server initialization parameters need not be set. Because shared server requires at least one dispatcher in order to work, a dispatcher is brought up even if no dispatcher has been configured.
Determining a Value for SHARED_SERVERS and others:
The SHARED_SERVERS initialization parameter specifies the minimum number of shared servers that you want created when the instance is started. After instance startup, Oracle Database can dynamically adjust the number of shared servers based on how busy existing shared servers are and the length of the request queue.
You can limit the number of shared servers that can be created by setting MAX_SHARED_SERVERS parameter specifies the maximum number of shared servers that can be automatically created by PMON. After you set the SHARED_SERVERS parameter oracle decides the number of shares servers that needs to be created based on how busy the system is.
The SHARED_SERVER_SESSIONS initialization parameter specifies the maximum number of concurrent shared server user sessions. Setting this parameter, which is a dynamic parameter, lets you reserve database sessions for dedicated servers.
Configuring Dispatchers:
Ex: This is a typical example of setting the DISPATCHERS initialization parameter.
DISPATCHERS="(PROTOCOL=TCP)(DISPATCHERS=2)"
You can use multiple different protocols for different dispatchers within the same initialization parameter.
Ex: DISPATCHERS='(PROT=tcp)(DISP=5)', '(PROT-tcps)(DISP=3)'
Monitor the following views to determine the load on the dispatcher processes:
- V$QUEUE
- V$DISPATCHER
- V$DISPATCHER_RATE
To identify the name of the specific dispatcher process to shut down, use the V$DISPATCHER dynamic performance view.
SQL> SELECT NAME, NETWORK FROM V$DISPATCHER;
Each dispatcher is uniquely identified by a name of the form Dnnn.To shut down dispatcher D002, issue the following statement:
SQL>ALTER SYSTEM SHUTDOWN IMMEDIATE 'D002';
Monitoring Shared Server:
The following views are useful for obtaining information about your shared server configuration and for monitoring performance.
- V$DISPATCHER: Provides information on the dispatcher processes, including name, network address, status, various usage statistics, and index number.
- V$DISPATCHER_CONFIG: Provides configuration information about the dispatchers.
- V$DISPATCHER_RATE: Provides rate statistics for the dispatcher processes.
- V$QUEUE: Contains information on the shared server message queues.
- V$SHARED_SERVER: Contains information on the shared servers.
- V$CIRCUIT: Contains information about virtual circuits, which are user connections to the database through dispatchers and servers.
- V$SHARED_SERVER_MONITOR: Contains information for tuning shared server.
- V$SGA: Contains size information about various system global area (SGA) groups. May be useful when tuning shared server.
- V$SGASTAT: Contains detailed statistical information about the SGA, useful for tuning.
- V$SHARED_POOL_RESERVED: Lists statistics to help tune the reserved pool and space within the shared pool.
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