[jboss-user] [JBoss Seam] - Re: EntityManger-per-user-session...

[iradix]

It does seem that we aren't quite understanding each other.  Let me clarify a few points that I think might help you.

1) So I don't confuse things (including myself) I'm going to pretend that Persistence Context and Entity Manager mean the same thing.  @PersistenceContext(type=EXTENDED) does not provide an EM scoped to the conversation, because it is not aware that conversations exist.  This confused me at first too because there is a lot of new information to take in.  First you have the EJB3 spec and the EM propagation rules there, and then you have your Seam Managed EM with it's own rules.  What you get by using @PersistenceContext(type=EXTENDED) is an EM that will survive from call to call on the session bean where it's defined, even if those calls span requests.  It will also be propagated through to other session beans called by the original.  That might be an oversimplification but I think you get the gist.  What you get with a seam managed persistence context is a single EM per conversation which can be injected using @In.

2) Once you've got your Seam Managed EM set up it will act as an object cache over the course of the conversation, only hitting the database for object retrieval when it has to.  So if you retrieve a User as soon as the conversation is started, every time you call em.find(User.class, userId) afterwards you get the cached user back.  No extraneous queries.  The EM will also check to see which objects have been updated and generated the appropriate sql once it is flushed.

3) I see 2 reasons off the top of my head not to use a session scoped EM.  The first is that you are caching data that the user has accessed throughout their session when much of it is quite possibly no longer relevant.  Why take up the extra memory?  I see no real benefits to this approach and even if your app is small enough that it doesn't make a difference now, it's not going to be very scalable.  The second is a matter of providing accurate data.  In my opinion it's a good thing to drop your entity manager at a well defined point (when the data in it is no longer being used) so that if the user chooses another operation that relies on that data a fresh, up to date version of it will be retrieved.  If you use the same entity manager throughout a user's session they will only see any changes that they have made (which are retrieved from their EM cache).



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