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http://docs.oracle.com/cd/E23943_01/web.1111/e13709/failover.htm (see the 'using replication groups' section)
By default, WebLogic Server attempts to create session state replicas on a different machine than the one that hosts the primary session state. You can further control where secondary states are placed using replication groups. A replication group is a preferred list of clustered servers to be used for storing session state replicas.
Using the WebLogic Server Administration Console, you can define unique machine names that will host individual server instances. These machine names can be associated with new WebLogic Server instances to identify where the servers reside in your system.
Machine names are generally used to indicate servers that run on the same machine. For example, you would assign the same machine name to all server instances that run on the same machine, or the same server hardware.
If you do not run multiple WebLogic Server instances on a single machine, you do not need to specify WebLogic Server machine names. Servers without a machine name are treated as though they reside on separate machines. For detailed instructions on setting machine names, see Configure Machine Names.
When you configure a clustered server instance, you can assign the server to a replication group, and a preferred secondary replication group for hosting replicas of the primary HTTP session states created on the server.
When a client attaches to a server in the cluster and creates a primary session state, the server hosting the primary state ranks other servers in the cluster to determine which server should host the secondary. Server ranks are assigned using a combination of the server's location (whether or not it resides on the same machine as the primary server) and its participation in the primary server's preferred replication group.