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From Craig L Russell <Craig.Russ...@Sun.COM>
Subject Re: makePersistent detached instance deleted on database
Date Fri, 10 Mar 2006 15:24:36 GMT
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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