Application availability is often crucial when application updates are in progress. When we modify and deploy the J2EE enterprise application archive(EAR) files, we need to be starting and stopping the applications. How do we applying the updates by not interupting the availabilty of the application server.
you will need two set of servers and a load balancer. The first set is the current running version. The second set is the staging area. Updates will go on the staging servers and after everything is done loader balancer is configured to route the new incoming request to the staging ones. After no other user is on the first set their are the new staging area and they can be used for next updates.