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Subject: RE: Can I reuse instances?
Date: Tue, 10 Apr 2007 14:57:06 -0700
Message-ID: <7D856CDFE035FF45A0420ACBD71BDD6303E40022@repbex02.amer.bea.com>
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Thread-Topic: Can I reuse instances?
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From: "Patrick Linskey" <plinskey@bea.com>
To: <open-jpa-dev@incubator.apache.org>

> Your implementation uses JPA Entities to hold the state. These JPA  
> Entities are not at all specified by CMP Entity Beans. JPA Entities  
> as cheap to create so all you need to do is hold on to the ids and  
> when you need state from the database, ask JPA EntityManager for the  
> state. If the state is already in the second level cache, this is  
> very cheap to access.

I imagine that it's also possible that Dain's CMP entity beans are
themselves the JPA entities, rather than beans that delegate on to a JPA
entity.

-Patrick

-- 
Patrick Linskey
BEA Systems, Inc. 

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> -----Original Message-----
> From: Craig.Russell@Sun.COM [mailto:Craig.Russell@Sun.COM] 
> Sent: Monday, April 09, 2007 5:45 PM
> To: open-jpa-dev@incubator.apache.org
> Subject: Re: Can I reuse instances?
> 
> Hi Dain,
> 
> On Apr 9, 2007, at 11:10 AM, Dain Sundstrom wrote:
> 
> > On Apr 8, 2007, at 1:56 PM, Craig L Russell wrote:
> >
> >> Hi Dain,
> >>
> >> I haven't looked in detail at the life cycle of CMP beans in a  
> >> couple of years, but in general you can't simply keep the 
> state of  
> >> the underlying Entities through the life cycle. CMP beans are  
> >> pooled and reused in transaction contexts and you have to 
> load the  
> >> state at specific points in the life cycle.
> >
> > It depends on the commit option the container is using.  In commit  
> > option A, you assume the cmp engine is the sole user of the  
> > database, so you don't need to load.  Normally people use commit  
> > option B where you keep instances activated with a specific 
> primary  
> > key and reload the data in each tx.
> >
> >> Using the primary key stored in the CMP bean to do em.find at the  
> >> appropriate time is the obvious way to take advantage of the em  
> >> second level cache. To the extent that this is not efficient, we  
> >> should fix it in the JPA layer not the CMP layer.
> >
> > I would prefer to keep as much of the CMP stuff on the CMP side as  
> > possible so the JPA implementation can focus on JPA issues.   One  
> > of the assumption of JPA is that entities are light weight and  
> > cheap to create, so you take the safe route and construct a 
> new one  
> > when every you need an instance.  In CMP the assumption is that  
> > entity instances are expensive to create, so less safe route and  
> > you pool them.  Reusing instances is really a CMP problem, but I  
> > don't think it can be implemented without the help of the 
> JPA engine.
> 
> I'm afraid we're getting wrapped around the axle on definitions.  
> Here's what I'm trying to get across:
> 
> CMP Entity Beans are expensive to create and there's a lot of  
> required behavior to manage them. You pretty much have to implement  
> the life cycle in the spec. It's your choice how to implement the  
> "state" part of the beans. You can use wrappers around various kinds  
> of state objects like ResultSet or generated classes that contain  
> fields with the state.
> 
> Your implementation uses JPA Entities to hold the state. These JPA  
> Entities are not at all specified by CMP Entity Beans. JPA Entities  
> as cheap to create so all you need to do is hold on to the ids and  
> when you need state from the database, ask JPA EntityManager for the  
> state. If the state is already in the second level cache, this is  
> very cheap to access.
> 
> Craig
> >
> > -dain
> >
> >
> 
> 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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