Extended Persistence Context

An Extended Persistence Context is useful in managing conversational web applications by providing the ability to the programmer to keep the persistent objects "live" till the conversation with the user is over. The persistent actions performed during the interaction can be flushed to the database or kept queued in the persistence context using transaction attribute settings.

A very good blog by Sahoo is available that lists the rules of propagation of an extended persistence context to called J2EE components. 

It also shows how to use EJB3 annotations on servlets to manage extended persistence contexts.