Thaks for you reply.
The scene just like this:
deploy the app to instance in a cluster, set availablity = true.
In a page, i display the sessionid from server. when refresh the page quickly by IE's or Firefox's toolbar or menu, you'll find that sometimes the sessionid value is changed, sometimes it is unchanged.
And you may find glassfish's log like:
the sessionid's Expired is set true, and then the sessionid's Expired is set false.
http://forums.java.net/jive/thread.jspa?threadID=32742&tstart=15
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