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From: Andrus Adamchik <andrus@objectstyle.org>
Subject: Re: Some jpa questions
Date: Fri, 25 Aug 2006 00:50:33 +0400
To: dev@geronimo.apache.org

Hi David,

On Aug 24, 2006, at 9:41 PM, David Jencks wrote:
> I've been looking at the ejb 3 persistence spec and it looks to me  
> as if there are some poorly explained bits.

That is certainly the case.

> Anyone have any clues about these or any clue about where I could  
> get an answer?
>
> 1. The @PersistenceContext annotation appears to be per jee  
> component (web app or ejb).  You can specify a map of properties to  
> pass in to the EMF.createPersistenceContext method.  Lets suppose  
> you have tx scoped PC, and you have 2 ejbs with tx Required and one  
> calls the other, and one specifies a map of properties and the  
> other doesn't (or they specify different maps).  What's supposed to  
> happen?

I guess propagation of context within a transaction takes precedence  
over property overrides:

Section 5.8.2:  "If a persistence context is already associated with  
a JTA transaction, the container uses that persistence
context for subsequent invocations within the scope of that  
transaction, according to the semantics for
persistence context propagation defined in section 5.6.3."

> 2. What's supposed to happen when you try to use a CM tx scoped EM  
> outside a JTA tx?  Section 5.6.1 seems to imply we aren't supposed  
> to just throw an exception, but section 5.9.1 seems to imply there  
> is no provider EM in back of the CM EM to do any work without a JTA  
> tx.  Are we supposed to create and close a provider EM for each  
> call outside a tx?

Good question... I have no idea.

> - How heavyweight are EntityManagers?

I guess this could be implementation specific, but they are  
lightweight in Cayenne, and I assume they are lightweight in other  
providers as well.

> Is it more efficient to create a new one or pool them?

While the spec provides some leeway (see below), container with  
pluggable provider has no way of knowing the best strategy. My vote  
is to avoid pooling EntityManagers by default.

Section 5.8.2: "It is undefined whether a new entity manager instance  
is created for every persistence context, or whether entity manager  
instances are sometimes reused. Exactly how the container maintains  
the association between persistence context and JTA transaction is  
not defined."

> Are they cheap enough so every CM tx-scoped EM can have a "outside  
> tx" provider EM attached to it in case anyone is dumb enough to do  
> work outside a tx?  Is this more or less resource efficient than  
> creating a new EM for each out-of-tx use?

Not sure I understand the question.

Andrus