||Oracle Tips by Burleson
10g Grid Computing
Real Application Cluster Architecture
Oracle RAC Instance can be configured both as
shared server and as 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
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 a RAC may
benefit from using shared servers.
As seen in Fig. 4.10, 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 4.10 Shared Server Architecture
Prior to the release of Oracle Database 10g,
you needed to set up at least one dispatcher for the shared server
configuration to be enabled. You normally needed to set the
dispatchers initialization parameter to configure the information
With Oracle Database 10g, even without
specifying a dispatcher with the dispatcher’s parameter, you can
enable shared server 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:
When you need to use shared servers while the
system is running, you can simply set the dynamic shared_servers
initialization parameter to a value greater than zero with an
ALTER SYSTEM command.
As with other parameters, you can change just
the current instance with this command and, if you are using an
SPFILE, you can change the parameter for future instances as well.
For example, to activate three shared servers in the current
instance and the SPFILE, enter this command:
SYSTEM SET SHARED_SERVERS=3 SCOPE=BOTH;
There are several other parameters that can be
set in the shared server environment, but they are not required.
Once you set shared_servers, your system will be running in
shared server mode.
When you need 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 that if you try to start an instance using these parameters,
you will receive the following error: “ORA-25138: <parameter>
initialization parameter has been made obsolete
Even if you try to set mts_servers
during the runtime of an instance:
SYSTEM SET MTS_SERVERS = 2;
ALTER SYSTEM SET
MTS_SERVERS = 2
ERROR at line 1:
MTS_SERVERS initialization parameter has been made obsolete
All the replacement parameters listed in the
table are dynamic, meaning that you can change the values while the
instance is running. The following Table shows the replaced parameters.
REPLACED BY PARAMETER
Table 4.3 Oracle 10g Replacement Parameters
In the case of the dispatcher’s parameter, the results of the
change will depend on which attributes you modify. 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.
You can dynamically modify the other attributes
(listener and service) and affect 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 you to make more informed decisions on what attributes to
modify and helps determine if you need to add or remove dispatchers.
For example, to get service and other details
about dispatchers, use the following query:
name, dispatchers, substr(service,1,20) service, idle, busy
CODE DEPOT FOR FULL SCRIPT
SERVICE IDLE BUSY
------------- ---------- --------
LONDBXDB 1641097 8
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