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From Marc Prud'hommeaux <mprud...@apache.org>
Subject Re: Help requested around rollback semantics
Date Fri, 20 Apr 2007 00:20:08 GMT
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


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