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I have a Glassfish (3.1.2) cluster over 2 computers as nodes, with a mod_jk load balancer.

Are servlet instance variables replicated perfectly? If not, how do I make sure it is?

Are all actions, including method calls and disk writes, replicated perfectly? If not, how do I make sure they are?

These may seem like stupid questions, but I'm not seeking "load balancing" as much as I am seeking exact replication to enable future upgrading without any service interruption. How do I achieve this if it is not already the case?

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What is replicated is session state. Method calls and disk writes, definitely not. The user has a session with one member of the cluster, and that continues until it fails, in which case another cluster member takes over the session in the same state. It is assumed you are using a common, maybe clustered, database, LDAP, etc. The cluster members do not work in lockstep.

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