http://weblogic-wonders.com/weblogic/2010/12/17/comparison-between-cluster-multicast-messaging-and-unicast-messaging-mode/
"For the member servers in the cluster, group leaders are chosen and only those group leaders communicate with the servers among the group and these leaders notify each other about the availability of all the other servers.
For example : Suppose there are 6 servers in the cluster. 2 groups are made and there are 2 group leaders. Other 2 servers of the group will notify their leader that they are alive and the group leader will send this information to the leader of other group."
Basically there can be multiple leaders. On the "monitoring" tab of the cluster you see:
"Local Group Leader"
"Total Groups"
"Discovered Group Leaders"
"Groups"
Unicast prefers a "private" network channel
http://download.oracle.com/docs/cd/E12840_01/wls/docs103/config_wls/network.html
This in order to separate JNDI traffic from cluster traffic.
Here is says explicitly that you must set the listen address on the CLUSTER-BROADCAST channel.
The book Professional Oracle WebLogic Server says that WebLogic supports TCP/IP-based unicast or UDP-based multicast.
This book explains VERY well the Cluster Topologies and Unicast mode, at Chapter 12: Administering and Deploying Applications.
Why Groups are formed and change over time?
Here is what WebLogic documentation says http://download.oracle.com/docs/cd/E11035_01/wls100/cluster/features.html:
"Cluster members communicate to the group leader when they need to send a broadcast message which is usually the heartbeat message. When the cluster members detect the failure of a group leader, the next oldest member becomes the group leader."
"If the group leaders
become overloaded, this can lead to cluster members dropping out and rejoining the cluster frequently."
"we have found that if your connection between the data centers has a tendency to lose packets or the latency is over a few hundred milliseconds, this can
cause problems with WebLogic Server’s clustering mechanisms. Prefer architectures that do not require using WebLogic Server clusters that span data centers."
What is Unicast?
http://publib.boulder.ibm.com/infocenter/wllm/v2r0/index.jsp?topic=/com.ibm.wllm.doc/wllmover_multiuni.html
There is Unicast UDP and Unicast TCP.
This article explains the Groups formed when in Unicast
This article shows some Network communication topologies http://en.wikipedia.org/wiki/Unicast
"For the member servers in the cluster, group leaders are chosen and only those group leaders communicate with the servers among the group and these leaders notify each other about the availability of all the other servers.
For example : Suppose there are 6 servers in the cluster. 2 groups are made and there are 2 group leaders. Other 2 servers of the group will notify their leader that they are alive and the group leader will send this information to the leader of other group."
Basically there can be multiple leaders. On the "monitoring" tab of the cluster you see:
"Local Group Leader"
"Total Groups"
"Discovered Group Leaders"
"Groups"
Unicast prefers a "private" network channel
http://download.oracle.com/docs/cd/E12840_01/wls/docs103/config_wls/network.html
This in order to separate JNDI traffic from cluster traffic.
Here is says explicitly that you must set the listen address on the CLUSTER-BROADCAST channel.
The book Professional Oracle WebLogic Server says that WebLogic supports TCP/IP-based unicast or UDP-based multicast.
This book explains VERY well the Cluster Topologies and Unicast mode, at Chapter 12: Administering and Deploying Applications.
Why Groups are formed and change over time?
Here is what WebLogic documentation says http://download.oracle.com/docs/cd/E11035_01/wls100/cluster/features.html:
"Cluster members communicate to the group leader when they need to send a broadcast message which is usually the heartbeat message. When the cluster members detect the failure of a group leader, the next oldest member becomes the group leader."
"If the group leaders
become overloaded, this can lead to cluster members dropping out and rejoining the cluster frequently."
"we have found that if your connection between the data centers has a tendency to lose packets or the latency is over a few hundred milliseconds, this can
cause problems with WebLogic Server’s clustering mechanisms. Prefer architectures that do not require using WebLogic Server clusters that span data centers."
What is Unicast?
http://publib.boulder.ibm.com/infocenter/wllm/v2r0/index.jsp?topic=/com.ibm.wllm.doc/wllmover_multiuni.html
There is Unicast UDP and Unicast TCP.
This article explains the Groups formed when in Unicast
This article shows some Network communication topologies http://en.wikipedia.org/wiki/Unicast