WebLogic Server Communication in a Cluster:-

WebLogic Server instances in a cluster communicate with one another using two basic network technologies:

IP multicast, which server instances use to broadcast availability of services and heartbeats that indicate continued availability.

IP sockets, which are the conduits for peer-to-peer communication between clustered server instances.
The way in which WebLogic Server uses IP multicast and socket communication affects the way you configure your cluster.

One-to-Many Communication Using IP Multicast
IP multicast is a simple broadcast technology that enables multiple applications to "subscribe" to a given IP address and port number and listen for messages. A multicast address is an IP address in the range from 224.0.0.0 to 239.255.255.255.

IP multicast broadcasts messages to applications, but it does not guarantee that messages are actually received. If an application's local multicast buffer is full, new multicast messages cannot be written to the buffer and the application is not notified when messages are "dropped." Because of this limitation, WebLogic Server instances allow for the possibility that they may occasionally miss messages that were broadcast over IP multicast.

WebLogic Server uses IP multicast for all one-to-many communications among server instances in a cluster. This communication includes:

Cluster-wide JNDI updates—Each WebLogic Server instance in a cluster uses multicast to announce the availability of clustered objects that are deployed or removed locally. Each server instance in the cluster monitors these announcements and updates its local JNDI tree to reflect current deployments of clustered objects. For more details, see Cluster-Wide JNDI Naming Service.

Cluster heartbeats— Each WebLogic Server instance in a cluster uses multicast to broadcast regular "heartbeat" messages that advertise its availability. By monitoring heartbeat messages, server instances in a cluster determine when a server instance has failed. (Clustered server instances also monitor IP sockets as a more immediate method of determining when a server instance has failed.)

How to Monitor the servers ??

# 1) Through Console:-
go to the AdminConsole click on Domains->servers-monitor-
>Health


# 2) Using the weblogic.Admin utiliy:-
java weblogic.Admin -url t3://hostname:port -username
weblogic -password weblogic ms1 GETSTATE


# 3) One more way is form WLST:-

For Ex the following shows how to find the status of
AdminServer in the same way we can find out the state of
other managed servers also:-

wls:/WLS10/serverConfig> state('AdminServer')
Current state of 'AdminServer' : RUNNING
wls:/WLS10/serverConfig>