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From Justin Edelson <jus...@justinedelson.com>
Subject Re: AW: JPA Service: EntityManagerFactoryBuilder
Date Tue, 28 Sep 2010 14:40:37 GMT
On Tue, Sep 28, 2010 at 5:03 AM, Timothy Ward <timothyjward@apache.org> wrote:
>
> Hi Harald,
>
> The Java SE scenario you describe below is also not guaranteed to work. In Java SE many
> JPA providers use the persistence unit name as a unique identifier for the persistence
unit
> and cache the return value. It really is expected that you have exactly one
> EntityManagerFactory (and therefore database connection) per persistence unit.
This defeats the purpose of the two-argument version of
createEntityManagerFactory. Ignoring the map of parameters (assuming
that map contains valid parameters) is a spec violation:

"When any of these properties are specified in the Map
parameter, their values override the values of the corresponding
elements in the persistence.xml
file for the named persistence unit. They also override any defaults
that the provider might have applied."

Harald's need seems relatively straightforward. He needs to create a
unique EntityManagerFactory per database. This is foreseen by the OSGi
Enterprise spec and is *exactly* the use case for
EntityManagerFactoryBuilder. See page 408/409, section 127.3.4. The
spec reads:

"This can for example be used to provide
the database selection properties when the Persistence Unit is
incomplete or if the database
selection needs to be overridden."

It is correct that the data source of an already created
EntityManagerFactory cannot be changed, but that's what the
two-argument version of Persistence.createEntityManagerFactory and
EntityManagerFactoryBuilder are for.

Justin

>
> I'm a little confused as to the use case for this business intelligence application.
These new
> databases are being added dynamically at runtime, but use the same table structure? This
> sounds, to me, as though your application needs to cope with these dynamic changes.
> Perhaps if you stop your persistence bundle, re-bind the DataSource services to point
at the
> correct database, then bring the persistence bundle back up?
>
> Regards,
>
> Tim
>
> ----------------------------------------
>> From: Harald.Wellmann@multi-m.de
>> To: aries-user@incubator.apache.org
>> Date: Mon, 27 Sep 2010 13:57:19 +0200
>> Subject: AW: JPA Service: EntityManagerFactoryBuilder
>>
>> Hi Tim,
>>
>> it seems I misinterpreted the EntityManagerFactoryBuilder - thanks for pointing this
out.
>>
>> But I still don't see how to solve the situation with one persistence unit working
on multiple databases with the same structure, neither in OSGi nor in Java EE.
>>
>> In Java SE, I would simply call Persistence.createEntityManagerFactory(puName, props)
multiple times, with the same persistence unit name, but different property sets, specifying
different datasources.
>>
>> E.g. think of a business intelligence application doing analysis on customer databases,
where there is a separate database for each country. Each analysis request refers to given
country, and the application has to select the appropriate database at run-time. New countries
may be added at any time, so it would not work to hard-code n persistence units and/or EMFs
up front.
>>
>> Best regards,
>> Harald
>>
>

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