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From "Patrick Linskey" <plins...@bea.com>
Subject RE: Flush all caches?
Date Mon, 11 Dec 2006 10:09:49 GMT
> -----Original Message-----
> From: Dain Sundstrom [mailto:dain@iq80.com] 
> Sent: Friday, December 08, 2006 1:12 PM
> To: open-jpa-dev@incubator.apache.org
> Subject: Re: Flush all caches?
> 
> On Dec 8, 2006, at 11:29 AM, Abe White wrote:
> 
> >> Other than the cost of not having a cache (which is what I want),  
> >> it is expensive to be creating a new EntityManager for each  
> >> transaction?
> >
> > No.
> 
> In general, is it a good strategy to use a new EntityManager 
> for each  
> transaction?

There's certainly nothing wrong with that approach. 

However, the situation is not as simple as it seems. When used in a JTA
environment, by default an EntityManager will use a transactional
persistence context. This means that each transaction essentially
automatically gets a new EntityManager under the covers, and each
operation that happens outside a transaction happens in its own
under-the-covers-EntityManager as well.

So, if you're in a JTA environment and haven't configured yourself to
use extended persistence contexts, then there's really no difference
between getting a new EM or reusing the existing one. Think of a
transactional-PC JTA EntityManager as a proxy, rather than a real EM.

> I think it is great for testing, but I am concerned about having to  
> write code two different ways, one for single node deployment and  
> another way for a clustered deployment.

It's worth noting that there are two different caches in play in OpenJPA
-- the EM-level cache (which is coincident with the persistence context
terminology in the spec, and I like to think of as a "working set"
instead of a "cache"), and the EMF-level cache. Modulo not having a way
to clear all EMF-level caches in a cluster, there should be no
difference between clustered behavior and non-clustered behavior in your
code. 

In any event, I kinda feel like there's a bit of a disconnect between my
assumptions and yours; lemme know if you've got the same feeling.

-Patrick
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