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Install Oracle Shared Server

Oracle RAC Cluster Tips by Burleson Consulting

This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters.  To get immediate access to the code depot of working RAC scripts, buy it directly from the publisher and save more than 30%.

Oracle RAC Instance can be configured either as shared server and dedicated server. In shared server architecture, the listener assigns each new client session to one of the dispatchers. As the user makes requests, the dispatcher sends the request to the shared server. It is also possible that a different set of shared servers are utilized for a given user session. The dispatchers act as the coordinating agents between the user sessions and the shared servers.

A dispatcher is capable of supporting multiple client connections concurrently. Each client connection is bound to a virtual circuit. A virtual circuit is a piece of shared memory used by the dispatcher for the client connection requests and replies.

An idle shared server process picks up the virtual circuit from the common queue, services the request, and relinquishes the virtual circuit before attempting to retrieve another virtual circuit from the common queue. In this way, a small number of server processes are able to service a large number of clients or users. This method also supports an increased number of users with less system resources.

Note that not all applications are certified to use shared servers, but that server-side load balancing in RAC may benefit from using shared servers.

As seen in Figure 4.9, the listener communicates with the dispatchers on behalf of the user or client sessions. Once the user sessions establish connectivity with dispatchers, the shared servers service them.

Figure 4.9: Shared Server Architecture

Prior to the release of Oracle Database 10g, at least one dispatcher needs to be setup for the shared server configuration to be enabled. Normally the dispatcher?s initialization parameter needed to set to configure the information about dispatchers.

With Oracle Database 10g, even without specifying a dispatcher with the dispatcher?s parameter, shared server can be enabled by setting shared_servers to a nonzero value. The default behavior is that Oracle creates one dispatcher for the TCP protocol automatically. This way, it is easier to configure a shared server environment. The equivalent dispatcher?s initialization parameter for this configuration would be:


The dynamic shared_servers initialization parameter needs to be set to a value greater than zero with an ALTER SYSTEM command in order to use shared servers while the system is running

As with other parameters, this command can be used to change the current instance. If an SPFILE is in use, the parameter can be changed for future instances as well. For example, to activate three shared servers in the current instance and the SPFILE, enter this command:


There are several other parameters that can be set in the shared server environment, but they are not required. Once shared_serversis set, the system will be running in shared server mode.

When needing to configure another protocol other than TCP/IP, configure a protocol address with one of the following attributes: ADDRESS, DESCRIPTION, or PROTOCOL.

Parameters with the prefix MTS are now obsolete. This means if these parameters are used to start an instance, the following error will be received: ?ORA-25138: <parameter> initialization parameter has been made obsolete.? This occurs even if trying to set mts_servers during the runtime of an instance:


ERROR at line 1:
ORA-25138: MTS_SERVERS initialization parameter has been made obsolete

All the replacement parameters listed in the table are dynamic, meaning that the values can be changed while the instance is running.  Table 4.4 shows the replaced parameters.


















Table 4.4:  Oracle 10g Replacement Parameters

In the case of the dispatchers parameter, the results of the change will depend on which attributes are modified. Since several of the attributes affect the network session layer when a dispatcher is started, they cannot be changed for dispatchers already started. These attributes are: protocol, address, description, presentation, connections, sessions, ticks, and multiplex.

Other attributes, such as listener and service, can be dynamically modified consequently affecting existing as well as new dispatchers of the same configuration.

There is a new view, v$dispatcher_config, that shows more information about existing dispatchers. This view displays information about the dispatcher configurations, including attributes that were not specified and were given a default value. The column CONF_INDX in v$dispatcher_config can be joined to the CONF_INDX column in v$dispatcher to see all of the detailed information about a given dispatcher. This information helps the DBA make more informed decisions on what attributes need to be modified and helps determine if dispatchers need to be added or removed.

For example, to get service and other details about dispatchers, use the following query:

SQL> select name, dispatchers, substr(service,1,20) service, idle, busy

from v$dispatcher,v$dispatcher_config

where v$dispatcher.conf_indx =    

      v$dispatcher_config.conf_indx ;


---- ----------- ------------- ---------- --------
D000           1 LONDBXDB         1641097        8


This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters, Rampant TechPress, by Mike Ault and Madhu Tumma.

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.


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