When you install update or delete an application the updates are automatically
distributed to all members in the cluster. In WebSphere Application Server V5 if
you updated an application on a cluster you had to stop the application on every
server in the cluster install the update and then restart the server. With
WebSphere Application Server V6 the Rollout Update option allows you to
update and restart the application servers on each node one node at a time.
This provides continuous availability of the application.