Cluster

Hi, Clustering is with more than one application server. My qs:
a) How can we maintain single instance model / no duplicate instance across JVM (without terracota server)
b) Do we need any programatic code required for single instance (like keep the db locked and do not allow second server to create instance?
c) If my load balancer (app server level& hardware level) is perfect, still do we need any program/ code to handle?
thanks in adv.
Viswa.