From: Christian von Kutzleben <cvkutzleben_at_versant.com>
Date: Mon, 1 Jul 2013 09:49:32 +0200

Hi Kevin,

E.g. object navigation accesses and modifies the internal structures that
make up the object context through
internal interfaces (that are vendor specific) and you can't protect these
interfaces by having a thread-safe EM proxy.

Christian

On Thu, Jun 27, 2013 at 5:09 PM, Kevin Sutter <sutter_at_us.ibm.com> wrote:

> > If the container injected proxies were thread-safe, this just would
> > not be sufficient to protect the internal structures
> > of the persistence context implementation.
>
> But, isn't this the approach that is used by containers to ensure the
> injected EMs into Stateless Session Beans are "thread safe"? Either that,
> or the container ensures that unique instances are created for each
> injection. So, as long as the container is properly managing these
> injected instances, why can't we indicate/declare that use of injected EMs
> in Servlets is a safe practice?
>
>
> ----------------------------------------------------------------------------------------------------------------------------------------------------------------
> Kevin Sutter, Java EE and Java Persistence API (JPA) architect
> mail: sutter_at_us.ibm.com, Kevin Sutter/Rochester/IBM
> http://webspherepersistence.blogspot.com/
> phone: tl-553-3620 (office), 507-253-3620 (office)
> http://openjpa.apache.org/
>
>
> Christian von Kutzleben <cvkutzleben_at_versant.com> wrote on 06/27/2013
> 08:41:41 AM:
>
> > From: Christian von Kutzleben <cvkutzleben_at_versant.com>
> > To: Scott Marlow <smarlow_at_redhat.com>,
> > Cc: jsr338-experts_at_jpa-spec.java.net, Kevin Sutter/Rochester/IBM_at_IBMUS
> > Date: 06/27/2013 08:42 AM
> > Subject: Re: [jsr338-experts] Re: [jpa-spec users] Re: Thread safety
> > for injected EMs
> >
> > Hi Scott,
> >
> > My point is: if it does not work at all at a conceptual level, it
> > does not make sense to introduce such a feature.
> >
> > If the container injected proxies were thread-safe, this just would
> > not be sufficient to protect the internal structures
> > of the persistence context implementation.
> >
> > Christian
>
> > On Thu, Jun 27, 2013 at 3:14 PM, Scott Marlow <smarlow_at_redhat.com>
> wrote:
> > On 06/27/2013 04:10 AM, Christian von Kutzleben wrote:
> > Hi Kevin,
> >
> >
> > There seems to be a lot of conflicting information on the
> > web concerning
> > the thread-safety of injected EMs in runtime components.
> From a
> > WebSphere perspective, our injected EMs are thread-safe
> > regardless of
> > the runtime component -- EJBs or Servlets. But, there are
> > several
> > references that indicate injected EMs in a Servlet are not
> > thread-safe
> > [1, as an example]. Our JPA spec is not clear on this (or,
> > at least I
> > couldn't find a definitive answer). What's the consensus of
> > the group?
> > Should injected EMs in a Servlet be considered
> > thread-safe? Or, is
> > this an implementation detail left up to each application
> > server?
> >
> >
> > The injected instance implementing EntityManager could of course be made
> > thread-safe,
> > however, this only synchronizes threads accessing the underlying
> > persistence context through
> > the EntityManager interface and threads, accessing the persistence
> > context otherwise would
> > corrupt the data (specifically: field/property access of instances,
> > which cause delayed/lazy loading)
> >
> > Christian,
> >
> > IMO, this discussion is only for the container managed entity
> > manager that is injected. If we all agree here, in a future spec,
> > the container managed entity manager can be required to be thread-
> > safe. The underlying entity manager that is used for each entity
> > manager invocation, is not required to be thread-safe.
> >
> > If you implement a persistence provider, you can continue to enjoy
> > (mostly) not worrying about thread safety. Providers do need to
> > handle at least one concurrency case (in your tx sync callback), you
> > should handle transaction rollbacks from a background thread. From
> > section JPA 2.1 7.9.2:
> >
> > "
> > When the JTA transaction rolls back, the provider must detach all
> > managed entities if the persistence context is of type
> > SynchronizationType.SYNCHRONIZED or has otherwise been joined to the
> > transaction. Note that the JTA transaction may rollback in a
> > background thread (e.g., as a result of transaction timeout), in
> > which case the provider should arrange for the managed entities to
> > be detached from the persistence context but not concurrently while
> > the application is in an EntityManager invocation.
> > "
> >
> > Scott
>
> >
> > So IMO only the JPA implementation could provide means for
> > thread-safety, but this is not a required
> > feature, so from my point of view, unless a thread-safe feature would be
> > introduced to JPA and the
> > "injector" enables this, this can not work at all.
> >
> > (Personally, I'm in favor of not introducing such a feature; we had that
> > in the JDO spec and it just
> > creates unnecessary complexity and issues to only support some corner
> > usecases)
> >
> > Christian
> >
> > --
>
> > *Christian von Kutzleben*
> >
> > Chief Software Engineer
> > Versant GmbH, Subsidiary of Actian Corporation
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> > --
> > Christian von Kutzleben
> > Chief Software Engineer
> > Versant GmbH, Subsidiary of Actian Corporation
> > christian.vonkutzleben_at_actian.com
> > PHONE +49 40 60990-273
> > FAX +49 40 60990-113
> > www.actian.com
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-- 
*Christian von Kutzleben*
Chief Software Engineer
Versant GmbH, Subsidiary of Actian Corporation
christian.vonkutzleben_at_actian.com
PHONE +49 40 60990-273
FAX +49 40 60990-113
www.actian.com
Versant GmbH is incorporated in Germany. Company registration number: HRB
54723, Amtsgericht Hamburg. Registered Office: Halenreie 42, 22359 Hamburg,
Germany. Geschäftsührer: Marc Monahan, Fred Gallagher, Volker John
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