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From Robert McIntosh <rob...@bull-enterprises.com>
Subject Re: [persistence] Some thoughts regarding CMP and JDO
Date Sun, 10 Aug 2003 06:22:34 GMT
If you guys don't mind me jumping in here, this is exactly why I added 
the to the wiki wishlist that the CMP be pluggable. Every one of our 
customers could not use CMP right now, or even Hibernate, Castor, JDO 
and the others, because of their limitations when it comes or O.R. 
mapping and the query limitations of OCL. Having a really pluggable CMP 
could allow someone like us, who has a layered, pluggable persistence 
engine, to plug into a container and provide the end user with the 
ability to have really complex persistence requirements yet gain the 
capabilities of an EJB container (transactions, security, clustering, etc.).

RObert

Thomas Mahler wrote:

> Hi Dain,
>
> thanks for the quick reply and for explaining your plans.
> I'd like to explain some central concepts of OJB to increase the mutual
> understanding and to start a discussion how to get together.
>
> Dain Sundstrom wrote:
>
>> Hello Thomas (and the rest of the OJB team),
>>
>> Jeremy Boynes and I (and a few others) wrote the CMP 2.0 
>> implementation in JBoss, and we have been working on the persistence 
>> code in the initial Geronimo code base.
>>
>> There is some code right now (a compiler and sql generator) and a 
>> fairly extensive design, but it looks like we have similar designs.  
>> The design is fairly simple from the high level.  We will support 
>> several front end layers simultaneously at runtime (CMP, JDO, maybe 
>> Hibernate, heck maybe SQL).
>
>
> Front end layers means APIs that user code uses for persistence
> operations, right?
> OJB currently provides support for ODMG, JDO and SODA as standardized
> user APIs.
>
> I have some doubts about Hibernate in this context.
> 1. Hibernate does not expose a standard API, but its own proprietary 
> API. Why supporting proprietary APIs?
> 2. AFAIK Hibernate does not provide any SPI to plug in a user defined 
> persistence service. So how do you plan to plug in your central 
> persistence service into Hibernate?
>
>>   The job of the front end layer is to handle the life-cycle and 
>> callbacks required by the related specification, but all real work 
>> will be delegated to a centralized persistence service.  This 
>> persistence service handles caching, locking, versioning, clustering 
>> and so on.  When persistence service actually needs to manipulate 
>> data it delegates to a store manager service.  
>
>
> We have a similar design in OJB. The JDO front end layer is currently
> being implemented on top of a transaction manager called OTM (Object
> Transaction Manager). The OTM layer is reponsible for
> management of distributed object level transactions (incl. JTA and 
> JCA), lockmanagement (our lockmanagement is working accross clusters 
> already!) and cache management (We provide wrappers to JCS, Tangosol 
> Coherence, etc.).
>
> The OTM layer itself is written on top our persistence microkernel 
> called PersistenceBroker. It seems to correspond to your StoreManager. 
> The PersistenceBroker is a pluggable component. That is the existing 
> implementation for RDBMS persistence can be replaced at runtime by 
> other implementations targeting LDAP, Filebased, XML or whatsoever.
>
> Our ODMG implementation is currently built directly on top of the 
> PersistenceBroker, but we plan to refactor it to be also based on OTM.
>
> The resulting layering is shown in the attached images.
>
>> The target initial store managers include SQL 92, SQL 99, Oracle 
>> (which is not really SQL), file based (XML maybe), and we have plans 
>> to add LDAP, clustered database layer and some legacy systems. 
>
>
> OJB PersistenceBroker already provides support for more than 10 of the 
> most popular RDBMS platforms. I'm working on a file based solution 
> (based on Prevayler), others have been playing around with LDAP.
>
>  The following ASCI picture sums this up (if it comes
>
>> through):
>>
>>                 ---------------
>> CMP ----------> |             | ------> SQL
>> JDP ----------> | persistence | ------> Oracle
>> Hibernate ----> | manager     | ------> LDAP
>>                 |             | ------> CICS (whatever)
>>                 ---------------
>>
>> Now the persistence manager has a huge job, so it is broken down into 
>> plugins for caching, locking and so on, which effectively  makes the 
>> persistence manager just a coordinator of the plugins.
>
>
> I mentioned it before: this is exactly how we do it in OJB already. We 
> are using a pluggable component concept. We are thinking of enabling 
> our components for Avalon and/or JMX.
>
>>
>> Anyway, this is getting a little too technical for right now, 
>> considering the initial code doesn't even have Entity beans.  From 
>> what I have seen, we have a similar vision, and I think we should 
>> talk about merging our efforts into a common persistence engine 
>> (maybe we can even get Gavin and the Hibernate team to sync up with us).
>
>
> I don't know how much of your plans are production level code already?
> For OJB the answer is quite clear. It's a working and mature codebase.
>
> It provides a robust and scalable architecture that matches the 
> requirements you mention to a high degree.
>
> So in my eyes it would save geronimo *a lot of work* to base the 
> persistence engine on OJB.
> Writing generic persistence engines is a tricky business. We are doing 
> it with OJB for about 3 years now...
>
>>  I think it would be really positive for Java to at least have all of 
>> us at least talking so our systems can play well together, but if we 
>> joined forces....  :D
>
>
> I agree. I'd like to see this happen.
> I believe that OJB already implements 80 - 90 % of the scope you 
> describe. It would make a lot of sense to use OJB as the base 
> persistence framework and to simply add the yet missing pieces like 
> StoreManagers for LDAP or CICS and a CMP code generator.
>
> So before embarking on the adventure of writing yet another generic 
> persistence engine, I think we should look at the things that 
> db.apache.org already offers today.
> We should define the gaps and should come to a solid make or buy 
> decision.
>
> Dain, please get me right. I'm not trying to sell something. I simply 
> want to avoid double work and to push integration of Apache projects.
>
> We won't think about implementing our own servlet engine when we 
> simply can use Tomcat, would we?
>
> cheers,
> Thomas
>
> PS: I'm offline for the next 2 weeks, so I won't be able to reply to 
> you in this time. But other OJB developers are available.
>
>>
>> -dain
>>
>>
>> On Thursday, August 7, 2003, at 07:02 AM, Mahler Thomas wrote:
>>
>>> Hello all,
>>>
>>> I'm contacting you on behalf of the Apache OJB development team
>>> (http://db.apache.org/ojb).
>>> OJB is part of the Apache DB subproject and aims at providing first 
>>> class
>>> standards based object relational mapping technology. We are currently
>>> finalizing our 1.0 release.
>>>
>>> Our team is excited to have a complete J2EE implementation at Apache 
>>> and we
>>> are willing to contribute to your project.
>>>
>>> OJB is heavily used in Tomcat, JBOSS and other application server
>>> environments and supports JTA and JCA.
>>> OJB provides special support for implementing BMP solutions easily.
>>> It provides ODMG and JDO compliant APIs.
>>>
>>> That's why we feel that OJB is a natural choice if you are thinking 
>>> about a
>>> persistence engine to implement CMP (and maybe JDO). We are willing to
>>> integrate all necessary changes into our codebase.
>>> Who is working on persistence concepts? Whom could we contact to get
>>> involved into the respective discussions?
>>> If you have any questions don't hesitate to contact me or the ojb 
>>> developer
>>> mailing list.
>>>
>>> cheers and all the best for this new project,
>>> Thomas Mahler
>>>
>>> OJB developer
>>> mailto:thma@apache.org
>>
>>
>>
>>
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>>
>
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