Question : Improper RAC Load Balancing

Oracle 9.2.0.8 on Windows 2003 Server.  We are experiencing problems with the database connectivity from the production web servers for the last 2-3 weeks. We are using Oracle RAC and there are 2 database servers. The load seems to shift until most of the connections are going to the same Oracle node i.e. second node. We are using Windows load balancing and all microsoft drivers on the web servers. Does anybody have any idea what might cause this?

LISTENERS_xPC =
  (ADDRESS_LIST =
    (ADDRESS = (PROTOCOL = TCP)(HOST = XXX.XXX.XX.XX)(PORT = 1521))
    (ADDRESS = (PROTOCOL = TCP)(HOST = XXX.XXX.XX.XX)(PORT = 1521))
  )
 
LISTENER_xPC1 =
  (ADDRESS = (PROTOCOL = TCP)(HOST = XXX.XXX.XX.XX)(PORT = 1521))
 
LISTENER_xPC2 =
  (ADDRESS = (PROTOCOL = TCP)(HOST = XXX.XXX.XX.XX)(PORT = 1521))
 

xPC =
  (DESCRIPTION =
    (LOAD_BALANCE = on)
    (FAILOVER = on)
    (ADDRESS_LIST =
     (ADDRESS = (PROTOCOL = tcp)(HOST = XXX.XXX.XX.XX)(PORT = 1521))
     (ADDRESS = (PROTOCOL = tcp)(HOST = XXX.XXX.XX.XX)(PORT = 1521))
    )
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = PC)
      (FAILOVER_MODE =
        (TYPE = session)
        (METHOD = basic)
      )
    )
  )
 
PC1 =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = XXX.XXX.XX.XX)(PORT = 1521))
    (CONNECT_DATA =
      (SERVICE_NAME = pc)
      (INSTANCE_NAME = pc1)
      (SERVER=DEDICATED)
    )
  )
 
PC2 =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = XXX.XXX.XX.XX)(PORT = 1521))
    (CONNECT_DATA =
      (SERVICE_NAME = pc)
      (INSTANCE_NAME = pc2)
    )
  )

Answer : Improper RAC Load Balancing

Server side load balancing can also be used to balance the number of connections to each instance. Session count balancing is method used when you set a listener parameter, prefer_least_loaded_node_listener-name=off. Note listener name is the actual name of the listener which is different on each node in your cluster and by default is listener_nodename.
Session based load balancing takes into account the number of sessions connected to each node and then distributes the connections to balance the number of sessions across the different nodes.


Reference:
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       RAC: Frequently Asked Questions
        Doc ID:       Note:220970.1