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From "Phill Moran" <pjmo...@rogers.com>
Subject RE: Help requested around rollback semantics
Date Fri, 20 Apr 2007 01:01:56 GMT
I thought that under any circumstance (except primary key messed up) that a
refresh would reload current database object image? If this is not the case how
does refresh work under any circumstances? Does it validate that the detached
object is in a valid state as a gate to refresh?

-----Original Message-----
From: Patrick Linskey [mailto:plinskey@bea.com] 
Sent: April 19, 2007 8:57 PM
To: open-jpa-dev@incubator.apache.org
Subject: RE: Help requested around rollback semantics

> In generic JPA, the only way to deal with the situation you describe 
> would be to manually call EntityManager.refresh() on each instance 
> that was involved in the transaction so as to ensure that it is up to 
> date.

Actually, I think it's even tougher than this. I think that after rollback,
entities are in an 'error' state, and you really can't do much of anything with
them. I don't think that there's even a guarantee that you can re-attach such
instances.

I believe that in either a transactional or extended PC, instances are detached
into this error state after a rollback. So, I don't think that you can actually
call refresh() on them, per the spec.

-Patrick

--
Patrick Linskey
BEA Systems, Inc.
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> -----Original Message-----
> From: Marc Prud'hommeaux [mailto:mprudhomapache@gmail.com] On Behalf 
> Of Marc Prud'hommeaux
> Sent: Thursday, April 19, 2007 5:20 PM
> To: open-jpa-dev@incubator.apache.org
> Subject: Re: Help requested around rollback semantics
> 
> David-
> 
> > So now what do I do? Is there a recommended approach for
> dealing with
> > this? How does my application detect this in some global way, and 
> > reloads from the persistence context all the instances it
> was keeping
> > around?
> 
> In my opinion, JPA is pretty weak when it comes to rollbacks. 
> This might be due to its legacy of being part of the EJB spec, where 
> individual entity instances typically aren't accessible after a 
> transaction has completed.
> 
> In generic JPA, the only way to deal with the situation you describe 
> would be to manually call EntityManager.refresh() on each instance 
> that was involved in the transaction so as to ensure that it is up to 
> date.
> 
> Fortunately, in OpenJPA you have a little more control over the post- 
> rollback behavior. You can use the openjpa.RestoreState property to 
> define what should be done with that state of persistent instances 
> after a rollback occurs. See:
> 
>    http://incubator.apache.org/openjpa/docs/latest/manual/
> manual.html#ref_guide_pc_scos_restore
> 
> 
> 
> 
> On Apr 19, 2007, at 4:40 PM, David Van Couvering wrote:
> 
> > Hi, all.  I am hoping you can help me out here.  You don't have a 
> > users list so I'm sending it to dev.
> >
> > I've been implementing some code against JPA and I was trying to 
> > figure out how to handle a transaction rollback. I wasn't sure if 
> > JPA is responsible for rolling back the state of any
> objects in the
> > persistence context.
> >
> > The answer appears to be no, and I found this in the spec:
> >
> > ===
> > 3.3.2 Transaction Rollback
> >
> > For both transaction-scoped and extended persistence contexts, 
> > transaction rollback causes all pre-existing managed instances and 
> > removed instances[15] to become detached. The instances'
> state will
> > be the state of the instances at the point at which the
> transaction
> > was rolled back. Transaction rollback typically causes the 
> > persistence context to be in an inconsistent state at the point of 
> > rollback. In particular, the state of version attributes and 
> > generated state (e.g., generated primary keys) may be
> inconsistent.  
> > Instances that were formerly managed by the persistence context 
> > (including new instances that were made persistent in that
> > transaction) may therefore not be reusable in the same manner as 
> > other detached objects-for example, they may fail when passed to the 
> > merge operation.
> > ====
> >
> > As an app writer, this statement is somewhat disconcerting.
> >
> > I was wondering if there is any guidance available for how I can 
> > write an app against JPA that handles rollback correctly.
> >
> > I'm writing a controller that interacts with entity objects, passing 
> > them around to various methods, storing them in member variables, 
> > and so on. I don't want to detach them, because (a) it is costly for 
> > me to copy all the data into a separate, detached instance, and then 
> > during the merge operation to have them copied back into the 
> > instance stored in the persistence context and (b) I never know if I 
> > can access a field or relationship because I don't know if the field 
> > or related instance has been loaded yet. When I keep them attached, 
> > JPA takes care of this for me.
> >
> > Then let's say I perform a persist() operation and it fails and 
> > rolls back. This means all my objects I've been passing around and 
> > storing in various places are suddenly detached and "may be" in an 
> > inconsistent state (which from the perspective of a program is the 
> > same as saying they *are* in an inconsistent state), and thus are 
> > invalid.
> >
> > So now what do I do? Is there a recommended approach for dealing 
> > with this? How does my application detect this in some global way, 
> > and reloads from the persistence context all the instances it was 
> > keeping around?
> >
> > The other concern I have is I'd like consumers of my controller to 
> > use entity objects as POJOs without having to know or care if they 
> > are entities. But the rollback semantics means my callers have to 
> > deal with handling a rollback. Alternately I could pay the copy cost 
> > of detaching my objects before passing them up to higher levels, but 
> > then my caller will still have to deal with the semantics of 
> > detached objects (e.g. lazy fields or associated instances may be 
> > null). I don't see any way to expose entities directly to higher 
> > levels of my app that are entity-unaware...
> > Which means I have to wrap my entities into proxy classes that 
> > handle all the JPA semantics internally and don't expose this to the 
> > caller.
> >
> > I am concerned that the rollback consequences can slip past 
> > developers, who will then build apps that behave very erratically 
> > after a transaction rollback. I know it's in the spec, but having 
> > worked with lots of developers, most of them don't read the spec, 
> > but instead cut-and-paste from examples. It would be great if we did 
> > a blog about this, and provided some code showing how to do it 
> > right... I could do this, but before I do I thought I'd check with 
> > you all first for any thoughts you have on this.
> >
> > Thanks!
> >
> > David
> 
> 

Notice:  This email message, together with any attachments, may contain
information  of  BEA Systems,  Inc.,  its subsidiaries  and  affiliated
entities,  that may be confidential,  proprietary,  copyrighted  and/or legally
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in this message. If you are not the intended recipient, and have received this
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