Chapter 10. Seam and Object/Relational Mapping

Seam was created because of frustration with the statelessness typical of the previous generation of Java application architectures. Seam's state management architecture was originally designed to solve problems relating to persistence, particularly problems associated with optimistic transaction processing. Scalable online applications always use optimistic transactions. An atomic (database/JTA) level transaction should not span a user interaction unless the application is designed to support only a very small number of concurrent clients. But almost all work involves first displaying data to a user, and then updating that data. Hibernate was designed to support a persistence context that spanned an optimistic transaction.

Unfortunately, the "stateless" architectures that preceded Seam and EJB3 had no construct to represent an optimistic transaction. Instead, these architectures provided persistence contexts scoped to the atomic transaction. This resulted in many problems for users, and causes the number one user complaint: Hibernate's LazyInitializationException. A construct was required to represent an optimistic transaction in the application tier.

EJB3 recognizes this problem, and introduces the idea of a stateful component (a stateful session bean) with an extended persistence context scoped to the lifetime of the component. This is a partial solution to the problem (and is a useful construct in and of itself), but there are still two issues with this approach:

Seam solves the first problem by providing conversations, and scoping stateful session bean components to the conversation. (Most conversations actually represent optimistic transactions in the data layer.) This is sufficient for many simple applications where persistence context propagation is not required, such as the Seam booking example application. For more complex applications, with many loosely-interacting components in each conversation, propagation of the persistence context across components becomes an important issue. Therefore, Seam extends the persistence context management model of EJB3, to provide conversation-scoped extended persistence contexts.