The Gateway exposes rich and persistent domain objects directly to the presentation logic. Because the domain objects are well encapsulated already - it is rather an advantage, than a shortcoming. Because of simplicity and built-in aspects, an EJB 3.1 happens to be the simplest and leanest candidate for a Gateway implementation.
Why local (to JSF 2, Wicket or a Fat Client)?
- Rich domain objects (JPA 2) contain business logic per definition. A call to method can change the state not only of the target entity, but of the whole connected graph. In the local case the EntityManager will correctly recognize the changes and compute deltas (change sets) and persist all changes at the end of the transaction. In remote case you will have to implement the same functionality - what is actually duplication.
- In local case you don't have to care about lazy loading. The entities gets just loaded on demand.
Why stateful (means: keeping the entities attached between requests)?
- In complex scenarios the data synchronization between layers becomes a challenge. If you keep you entities managed between requests, there is nothing left to do. You can even bind them directly to the UI - and it will still work.
- For every request you will have to build the domain graph again and again. This is not only complex, but it does not necessarily scale better, than a stateful solution
Local, stateful facades are not necessary the silver bullet, but the resulting architecture is very lean. It mainly consists of only domain objects, with a few Gateways.
Before you start prematurely "improving" the scalability and start with bloated applications, it is more reasonable to concentrate on the core business logic and stress test continually the application. In some cases a stateful architecture wont't scale. In this case you will have to provide the synchronization, delta-computation afterwards - with demand documented with stress test results. [See Gateway, page 101 in "Real World Java EE Patterns Rethinking Best Practices" book for more in-depth discussion]