In a clustered environment,
the Session Manager requires an affinity mechanism
so that requests for a particular session are always sent to the same server.
This satisfies the Servlet 2.2 requirement
that a session be active in only one JVM at a time.
It also provides for better performance, because the sessions are cached in memory.
Session affinity is provided in a server group by the Web server plug-ins.
In clustered environments other than server groups,
an affinity mechanism (for example, IBM WebSphere Edge Server affinity) must be used.