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From Jörg von Frantzius <joerg.von.frantz...@artnology.com>
Subject Re: makePersistent detached instance deleted on database
Date Fri, 10 Mar 2006 17:22:57 GMT
Hi Craig,

sorry I didn't understand what you wanted to see here exactly. Maybe 
something like this:

Replication does work perfectly with detaching using an attachCopy() 
that inserts new instances, and it does so in a productive environment 
using an older JPOX version, however compliant with the spec that was.

With the current specification, there is one thinkable approach that 
might work (as pointed out by Matthew), and it has two drawbacks. Here's 
what it could look like:

    Object detachedGraph = pm1.detachCopy(root);
    try {
        pm2.makePersistent(detachedGraph);
    } catch (whatever Exception it is exactly) {
        Collection failedInstances = ... // failed are contained in exception
        // find out the original instances of the failed detached instances
        Collection originalFailedInstances; 
        for (Object failed: failedInstances) {
            Object originalFailed  = pm1.getObjectById(JDOHelper.getObjectId(failed));
    	originalFailedInstances.add(originalFailed);
        }
        pm1.makeTransientAll(originalFailedInstances);
        pm2.makePersistentAll(originalFailedInstances);
        // now that the new objects are made persistent, this should succeed:
        pm2.makePersistent(detachedGraph);
    }

The drawbacks are:

   1. it *won't work for datastore identity*, as the identities of the
      instances new to the second PM/datastore are lost upon
      makeTransient(), and there is no reliable way to assure that they
      will be assigned the same identities upon makePersistent() on the
      second PM,
   2. it certainly is *not very effective*.

The remedy could be the approach proposed by Marco: to have an 
additional method

    PersistenceManager.makePersistent(Object o, boolean insertNew)

and to have PersistenceManager.makePersistent(Object) default to 
PersistenceManager.makePersistent(o, false).

Regards,
Jörg

Craig L Russell schrieb:
> Hi Jörg,
>
> As I said earlier, I'd like to see the details of using this feature 
> for replication. What cases are covered, what cases are still 
> problematic, what modifications to the specification are needed in 
> order to accomplish the task?
>
> Craig
>
> On Mar 10, 2006, at 3:14 AM, Jörg von Frantzius wrote:
>
>> Hi Craig,
>>
>> replication really is lost in a specification gap: makePersistent() 
>> on transient instances won't update existing data, and on detached 
>> instances it won't insert new. For replication, you need both 
>> behaviours at the same time.
>>
>> That's really misfortunate for such a nice feature! Even more so as 
>> it is not just theory, but it proves to be working in production with 
>> JPOX' old implementation of attachCopy().
>>
>> Regards,
>> Jörg
>>
>> Craig L Russell schrieb:
>>> Hi Jörg,
>>>
>>> Using detachment for replication is an interesting use case, and I'd 
>>> like to see more in-depth analysis of the issues that you encounter 
>>> once you've done with it.
>>>
>>> The use-case for detachment is long-running optimistic transactions, 
>>> as you have noted below. We did add makeTransient(Object, 
>>> useFetchPlan) as a way to disconnect objects from one datastore that 
>>> could be used with another, but I really doubt that we are going to 
>>> be able to incorporate into the JDO API all the policy algorithms 
>>> needed by a general-purpose replication scheme.
>>>
>>> Craig
>>>
>>> On Mar 9, 2006, at 9:48 AM, Jörg von Frantzius wrote:
>>>
>>>> Craig L Russell schrieb:
>>>>> Hi Jörg,
>>>>>
>>>>> There are no tests planned for this behavior.
>>>> That's good ;-)
>>>>>
>>>>> The issue is that it violates the contract of detachment. 
>>>>> Detachment is intended to provide a "long-running optimistic 
>>>>> transaction" in which conflicts are detected in a subsequent 
>>>>> transaction.
>>>> I'd find it a little sad if a great feature like easy replication 
>>>> was sacrificed in favor of that. Unless replication should be 
>>>> reserved for JPOX (using a vendor extension), then maybe a future 
>>>> version of the spec could have something along the lines of the 
>>>> solution described by Marco in 
>>>> http://www.jpox.org/servlet/jira/browse/CORE-2741
>>>>
>>>> That would be great.
>>>>
>>>> Just for completeness, and maybe it's just me, but the only 
>>>> sentence about detaching in general that I could find is
>>>> "These methods provide a way for an application to identify 
>>>> persistent instances, obtain
>>>> copies of these persistent instances, modify the detached instances 
>>>> either in the same JVM
>>>> or in a different JVM, apply the changes to the same or different 
>>>> PersistenceManager,
>>>> and commit the changes."
>>>>
>>>> It's not really talking about an equivalent to long-running 
>>>> optimistic transactions, I find.
>>>>> If an instance is detached and then the underlying datastore 
>>>>> instance is deleted, this is a consistency violation that should 
>>>>> be detected by the transaction semantics. For example, in an order 
>>>>> system, if a customer is in a long-running transaction with 
>>>>> "groovy beads" in the shopping cart, and the administrators decide 
>>>>> that "groovy beads" are no longer to be sold, you want the order 
>>>>> that contains "groovy beads" to be rejected when the shopping cart 
>>>>> arrives at checkout. You don't want that order to reinsert "groovy 
>>>>> beads" into the database.
>>>> I agree that this surely must be catered for.
>>>>>
>>>>> Craig
>>>>>
>>>>> On Mar 9, 2006, at 8:40 AM, Jörg von Frantzius wrote:
>>>>>
>>>>>> Hi Craig,
>>>>>>
>>>>>> I was already afraid that "create a persistent instance" might 
>>>>>> only apply to the PM cache, not the datastore (but only after 
>>>>>> second read). However, would you say that JPOX is not JDO2 
>>>>>> compliant if it created missing instances in the datastore 
>>>>>> anyway? Will there be a test in the TCK2 that expects an 
>>>>>> exception to be thrown if a detached instances does not exist in

>>>>>> the datastore?
>>>>>>
>>>>>> And, most of all, what sense would it make to forbid the creation

>>>>>> of missing detached instances in the datastore? There is lots of

>>>>>> application for that behaviour, and at least I don't know of any

>>>>>> problem with it.
>>>>>>
>>>>>> Regards,
>>>>>> Jörg
>>>>>>
>>>>>> Craig L Russell schrieb:
>>>>>>> Hi Jörg,
>>>>>>>
>>>>>>> On Mar 9, 2006, at 1:43 AM, Jörg von Frantzius wrote:
>>>>>>>
>>>>>>>> Craig L Russell schrieb:
>>>>>>>>>> Also I find it confusing that the method most prominently

>>>>>>>>>> used for inserting new objects shouldn't do so for
detached 
>>>>>>>>>> instances.
>>>>>>>>>
>>>>>>>>> There is a bunch of history that you should look at,
most of 
>>>>>>>>> which is in the jdo-dev archives. Bottom line, we used
to have 
>>>>>>>>> a different API, attachCopy, but we looked at what it
had to 
>>>>>>>>> do for transient and detached instances and decided that
it 
>>>>>>>>> wasn't worth making a different API for attaching detached

>>>>>>>>> instances.
>>>>>>>> That particular behaviour of attachCopy() wasn't really 
>>>>>>>> specified, but it was pleasant JPOX-specific behaviour, if
I 
>>>>>>>> remember correctly. I saw the discussion and I didn't see
where 
>>>>>>>> inserting the instances would be forbidden by the spec, and

>>>>>>>> still I don't see where it says that, especially in the light

>>>>>>>> of 12.6.7. Please excuse my ignorance, where does it say
that?
>>>>>>>
>>>>>>> <spec>
>>>>>>> These methods make transient instances persistent and apply 
>>>>>>> detached instance changes
>>>>>>> to the cache.
>>>>>>> ...
>>>>>>> For a detached instance, they locate or create a persistent
>>>>>>> instance with the same JDO identity as the detached instance,

>>>>>>> and merge the persistent
>>>>>>> state of the detached instance into the persistent instance.

>>>>>>> Only the state of persistent fields
>>>>>>> is merged.
>>>>>>> </spec>
>>>>>>>
>>>>>>> This means that if there is already a persistent instance in
the 
>>>>>>> cache with the same object id as the detached instance, the 
>>>>>>> detached state will be merged. If there is not a persistent 
>>>>>>> instance in the cache, a cache instance is created and the 
>>>>>>> detached state is merged with the persistent instance.
>>>>>>>
>>>>>>> But there is no creation aspect of makePersistent on a detached

>>>>>>> instance.
>>>>>>>
>>>>>>> Craig
>>>>>>>
>>>>>>>>>
>>>>>>>>> Regards,
>>>>>>>>>
>>>>>>>>> Craig
>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> Craig
>>>>>>>>>>>
>>>>>>>>>>> On Mar 8, 2006, at 7:14 AM, Erik Bengtson wrote:
>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> Hi,
>>>>>>>>>>>>
>>>>>>>>>>>> What happens when we invoke makePersistent
on a detached 
>>>>>>>>>>>> instance that was
>>>>>>>>>>>> deleted by another isolated process? I suspect
that we 
>>>>>>>>>>>> raise an exception
>>>>>>>>>>>> instead of reinserting it for a second time.
Is that right?
>>>>>>>>>>>>
>>>>>>>>>>>> Maybe this can be clarified in the spec.
>>>>>>>>>>>>
>>>>>>>>>>>> Regards,
>>>>>>>>>>>
>>>>>>>>>>> Craig Russell
>>>>>>>>>>> Architect, Sun Java Enterprise System 
>>>>>>>>>>> http://java.sun.com/products/jdo
>>>>>>>>>>> 408 276-5638 mailto:Craig.Russell@sun.com
>>>>>>>>>>> P.S. A good JDO? O, Gasp!
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>> Craig Russell
>>>>>>>>> Architect, Sun Java Enterprise System 
>>>>>>>>> http://java.sun.com/products/jdo
>>>>>>>>> 408 276-5638 mailto:Craig.Russell@sun.com
>>>>>>>>> P.S. A good JDO? O, Gasp!
>>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>> Craig Russell
>>>>>>> Architect, Sun Java Enterprise System 
>>>>>>> http://java.sun.com/products/jdo
>>>>>>> 408 276-5638 mailto:Craig.Russell@sun.com
>>>>>>> P.S. A good JDO? O, Gasp!
>>>>>>>
>>>>>>
>>>>>
>>>>> Craig Russell
>>>>> Architect, Sun Java Enterprise System 
>>>>> http://java.sun.com/products/jdo
>>>>> 408 276-5638 mailto:Craig.Russell@sun.com
>>>>> P.S. A good JDO? O, Gasp!
>>>>>
>>>>
>>>
>>> Craig Russell
>>> Architect, Sun Java Enterprise System http://java.sun.com/products/jdo
>>> 408 276-5638 mailto:Craig.Russell@sun.com
>>> P.S. A good JDO? O, Gasp!
>>>
>>
>
> Craig Russell
> Architect, Sun Java Enterprise System http://java.sun.com/products/jdo
> 408 276-5638 mailto:Craig.Russell@sun.com
> P.S. A good JDO? O, Gasp!
>


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