[aerogear-dev] TODO server app observations

[Marko Strukelj]

I’ve been looking into TODO app to get a better understanding of how aerogear.js works in a browser, and how we could abstract server-side into something we could call ‘aerogear-server’ as opposed to TODO app.

Here are some observations:


 - The *Endpoint classes are SLSB, but use extended persistence context [1]

Extended persistence context is not supported on Stateless session beans per JPA spec [2]. 

The reason is that stateless beans are not singletons or contextual objects (i.e. CDI scoped) but are pooled instances, so you can receive a different one for each call that you make. That would mean that you could get a different EntityManager instance each time, with possible uncommitted state from another method call by another client. It would be impossible to do consistent persistence across multiple invocations with that kind of random behaviour. Also if ten requests hit at the same time there would be ten instances of EntityManager, and ten separate persistence contexts, and since they are extended and ‘never’ cleared (EJB container decides, behaviour is unspecified) - we have ten memory leaks.

It looks like AS7 doesn’t much care if you use EXTENDED annotation in a SLSB - it will happily comply (and create a memory leak for you), whereas some other app servers will break on deployment [3], and Scott Marlow concedes it should probably be a deployment error [4], so maybe one day this would also break on AS7.



 - Method joinTransaction() only makes sense on application-managed entity manager

We are using container-managed entity manager - one that gets injected - as opposed to one retrieved by calling EntityManagerFactory.createEntityManager() method (that would be application-managed). Call to joinTransaction() is therefore a no-op, and should be removed [5][6][7].


 - Custom serializers look like plumbing
Would be good to have a way to autogenerate them during compile based on annotations. Also, for the purpose of autogenerating entities for the client based on server data definitions (metadata) we need to use those same definitions during serialization. This is the most obvious point where aerogear-server as a server-side framework comes to play.


[1] https://github.com/aerogear/TODO/tree/picketbox/server/src/main/java/org/aerogear/todo/server/rest
[2] http://stackoverflow.com/questions/2547817/what-is-the-difference-between-transaction-scoped-persistence-context-and-extend
[3] https://community.jboss.org/thread/196404
[4] https://community.jboss.org/message/721988#721988
[5] http://docs.oracle.com/javaee/6/api/javax/persistence/EntityManager.html#joinTransaction()
[6] http://docs.oracle.com/javaee/5/tutorial/doc/bnbqw.html
[7] http://docs.jboss.org/hibernate/orm/4.0/hem/en-US/html/architecture.html#d0e134


- marko