questions of Cluster

[柴 小柴]

hello everyone! i have some questions:
Question 1
I have read this from tomcat-doc(5.0.24):
.............
For large clusters, the next release will support a primary-secondary 
session replication 
where the session will only be stored at one or maybe two backup servers
.............
Can you explain more about "primary-secondary session replication" and how 
it works?
Thank you very much :)

Question 2
I have read this from tomcat-doc:
..............
In order to keep the network traffic down in an all-to-all environment, 
you can split your cluster into smaller groups. 
This can be easily achieved by using different multicast addresses for the 
different groups. 
A very simple setup would look like this: 

  
        DNS Round Robin
              |
        Load Balancer
         /           \
     Cluster1      Cluster2
     /     \        /     \
 Tomcat1 Tomcat2  Tomcat3 Tomcat4
...............
Do you mean that:
if "Tomcat1" is crashed,only "Tomcat2"(if it's alive) can continue the 
services and replace "Tomcat1",other instances in "Cluster2" can not do 
that,
because different groups have different multicast addresses?
If my guess is true,i thank it may make a reduction in the capability of 
"Cluster".
Am I correct?
Thank you for any comment!

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