This chapter focused on the need for application severs in any multitier Web application architecture. Application servers contain business logic functionality that's at the heart of your application's purpose. Engineers have found that flexibility and scalability can be gained by building these servers as sets of distributed application objects.
The J2EE solution for this is Enterprise JavaBeans. EJBs offer designers a multitude of options to easily model synchronous and asynchronous client sessions and provide an object-oriented “live” representation of the underlying data persisted by the application.
In the context of application servers and EJBs, we covered
Why serving application logic is necessary
How EJBs provide the opportunity ...