1) Is it possible to start a transaction in an entity bean, If not then why?
2) Why is the SessionSynchroniztion Interface not defined for the entity bean?
Basically Entity beans are designed to represent database data. It is just a O/R mapping object. They are not supposed to implement any business logic. You need transaction when you implement a business logic. To implement ejb has Session Beans. So there is no need to create any transaction in entity bean or to implement SessionSynchronization interface.
Hope this help,