Shared memory resources for dispatchers, virtual circuits, and shared servers are preconfigured enable shared servers at runtime. Database administrators can start dispatchers and shared servers with the SQL ALTER SYSTEM statement without having to restart the instance. When shared server mode is turned on, a dispatcher is started automatically on the TCP/IP protocol even if the DISPATCHERS parameter has not been set. Using a shared server is equivalent to setting the DISPATCHERS parameter in the database initialization parameter file as follows:

DISPATCHERS="(PROTOCOL=tcp)"

Configure the DISPATCHERS parameter directly if either of the following conditions apply:

• You need to configure a protocol other than TCP/IP.

• You want to configure one or more of the optional dispatcher attributes, such as multiplexing.

You can specify the following attributes for the DISPATCHERS parameter. The PROTOCOL attribute is required, and the others are optional. The ADDRESS attribute is used to set a specify port number, such as when using a firewall.

• ADDRESS

• CONNECTIONS

• DESCRIPTION

• DISPATCHERS

• LISTENER

• MULTIPLEX

• PROTOCOL (required)

• SERVICE

• SESSIONS

You change the dispatcher configuration using the SQL statement ALTER SYSTEM. You do not need to restart the instance after setting the parameters.

• Grouping Services by Dispatcher
  
• Monitoring Dispatchers
  

SERVICE_NAMES=sales.us.example.com
INSTANCE_NAME=sales
DISPATCHERS="(PROTOCOL=tcp)" 
DISPATCHERS="(PROTOCOL=tcp)(SERVICE=adminsales.us.example.com)" 

Session multiplexing, available with Oracle Connection Manager, enables multiple client sessions to funnel through a single protocol connection. For example, several client processes can connect to one dispatcher by way of a single connection from Oracle Connection Manager.

Oracle Connection Manager allows communication by users to the dispatcher by way of a shared connection. At any one time, users might need the connection, while other client processes linked to the dispatcher by way of the connection manager process are idle. Session multiplexing is beneficial because it maximizes use of the dispatcher process connections.

Session multiplexing is also useful for database link connections between dispatchers. The limit on the number of sessions for each dispatcher is operating system specific.

To enable session multiplexing, set the attribute MULTIPLEX in the DISPATCHERS parameter to on or an equivalent value.

DISPATCHERS="(PROTOCOL=tcp)(MULTIPLEX=on)"

If a shared server is configured on the server side, and a client connection request arrives when no dispatchers are registered, then the request is processed by a dedicated server process. If you want a particular client always to use a dispatcher, then configure (SERVER=shared) in the CONNECT_DATA section of the connect descriptor. For example:

sales= 
(DESCRIPTION= 
  (ADDRESS=(PROTOCOL=tcp)(HOST=sales-server)(PORT=1521))
  (CONNECT_DATA=
     (SERVICE_NAME=sales.us.example.com)
     (SERVER=shared)))

If the (SERVER=shared) attribute is configured and a dispatcher is not available, then the client connection request is rejected, and a message is sent to the client.

If the database is configured for a shared server and a particular client requires a dedicated server, then you can configure the client to use a dedicated server in one of the following ways:

• You can configure a network service name with a connect descriptor that contains (SERVER=dedicated) in the CONNECT_DATA section. For example:

  sales= 
  (DESCRIPTION= 
    (ADDRESS=(PROTOCOL=tcp)(HOST=sales-server)(PORT=1521))
    (CONNECT_DATA=
       (SERVICE_NAME=sales.us.example.com)
       (SERVER=dedicated)))
  

• You can configure the client profile file, sqlnet.ora, with USE_DEDICATED_SERVER=on. This adds (SERVER=dedicated) to the CONNECT_DATA section of the connect descriptor the client uses.

  Note:

  If USE_DEDICATED_SERVER is set to ON, then existing (SERVER=value) entries in connect descriptors are overwritten with (SERVER=dedicated).