http://issues.apache.org/jira/browse/OPENJPA-24?page=comments#action_12432209 ] 
Kevin Sutter commented on OPENJPA-24:

I'm finally getting back to this exercise and I will have to admit that this is 
quite difficult not understanding the general architecture of thse 
ConfigurationProviders and ProductDerivations.  Instead of jumping right in and 
attempting to make Abe's suggested changes, I'm trying to understand the 
current architecture.  Here's what I am finding...

The application attempts to create an EMF via 
Persistence.createEntityManagerFactory().  This eventually gets us to the 
PersistenceProviderImpl.createEntityManagerFactory() methods, which creates the 
ConfigurationProviderImpl instance.  We then use this ConfigurationProviderImpl 
instance  to load the resources and create the PersistenceUnitInfo object.

The last thing we do in createEntityManagerFactory is to create the appropriate 
BrokerFactory.  We do this by calling Bootstrap.newBrokerFactory passing in the 
ConfigurationProvider instance and associated Loader.

Within this processing, I see where we are looping through the 
ProductDerivation services (sorted on Type).

This, in turn, creates the new JDBCBrokerFactory instance, which creates the 
corresponding JDBCConfigurationImpl.  After initializing the properties in the 
constructor, we attempt to load the global settings by calling 
Configurations.loadGlobals static method (via the ConfigurationImpl.loadGlobals 
method).  This is where it gets confusing.  Within this method, we create new 
instances of the ConfigurationProviderImpl and we use these instances to load 
resources (again?).

So, I am seeing where we loop through ConfigurationProviders, eventually 
creating and looping thorugh ProductDerivations, and then we create and process 
new instances of ConfigurationProviders.

Granted, I've only spent a couple of hours looking at this today, but I think I 
need some background on how these Services were working previously before 
attempting to correct it.  Maybe this was part of the reason why Abe wanted to 
get rid of the ConfigurationProvider service and drive everything through the 
ProductDerivations.  It's kind of confusing.


> Allow OpenJPA to be extensible
> ------------------------------
>                 Key: OPENJPA-24
>                 URL: http://issues.apache.org/jira/browse/OPENJPA-24
>             Project: OpenJPA
>          Issue Type: Bug
>          Components: kernel
>            Reporter: Kevin Sutter
>         Assigned To: Kevin Sutter
> The current OpenJPA architecture is not extendable to other implementations.  
> For example, if somebody wanted to provide their own PersistenceProvider 
> implementation, simply extending the 
> org.apache.openjpa.PersistenceProviderImpl would not suffice due to the 
> dependencies in the ConfigurationProviderImpl.  The discussion for this 
> improvement was started on the dev mailing list.  Once it was determined that 
> there was more to this request than a simple conditional or two, we decided 
> to open a JIRA report.
> The complete history of this request can be found in the OpenJPA dev mailing 
> list.  The first message was posted by me (Kevin Sutter) on August 14, titled 
> "Extending the OpenJPA Implementation".  I will attempt to paraphrase the 
> current state of the problem...
> We have three main players in this issue.  The PersistenceProvider, the 
> ConfigurationProvider, and the ProductDerivation (along with the various 
> implementations of these interfaces).  Currently, the ConfigurationProvider 
> is in the lib and is unaware of any specific persistence requirements.  The 
> ProductDerivation is in the kernel and, unfortunately, is aware of 
> persistence requirements, specifically the spec and store types.  Abe's 
> postings have indicated that we need to make these two interfaces more aware 
> of each other and work with each other.  We need to start with either making 
> ConfigurationProvider more persistence-aware and move it into kernel, or make 
> ProductDerivations less persistence-aware and move it into lib.  The latter 
> approach is preferred.
> After we get this re-organization of the base framework complete, we still 
> have a couple of other issues ot resolve:
>     *  Still need the ability to extend EMF's through a ProductDerivation.  
> This should be doable by adding a new PluginValue to indicate what class of 
> EMF to load.
>     *  There is still a question as to whether we will need to provide a 
> custom PersistenceProviderImpl and ConfigurationProviderImpl pair.  I still 
> think this will be necessary.   And, one of Abe's posts indicated that this 
> might help with class loading issues when multiple versions of OpenJPA-based 
> implementations are available in the same system.
> I also posted these questions last Friday.  (Abe has responded with some 
> answers, but I wanted to get this JIRA report created before trying to 
> paraphrase his answers.)
>     *  You mention in several places about separating away the notion of 
> specs and stores.  In a general sense, I understand what these are.  But, can 
> you elaborate on how these types are used in the ConfigurationProvider and 
> ProductDerivation interfaces?
>     * I've moved the ProductDerivation interface to the lib and added the 
> "load" methods from the ConfigurationProvider (as described in your previous 
> notes).  And, I've started to clean up the implementations that depend on 
> these interfaces.  But, concerning the implementation of the load methods...  
> Now that we need to return a ConfigurationProvider, would you expect that we 
> just new up a ConfigurationProviderImpl and then just call across to the 
> "load" methods on the implementation?  Since we want to keep the 
> ProductDerivations stateless, I'm not sure how else you were expecting to 
> create a ConfigurationProvider to return on these "load" methods.
>     * Now that ConfigurationProvider is bare, the 
> ConfigurationTestConfigurationProvider doesn't have much function.  I'll need 
> to take a look to see if this is even required any longer.
>     * Can you shed a bit more light on the Configurations class?  It doesn't 
> implement nor extend any interfaces or classes, but it seems to provide many 
> of the same methods as ConfigurationProvider, but as statics.  And, it's 
> dependent on having a Provider.  Can you explain the relationship of this 
> class in the bigger picture and how you think it might be affected by thes 
> changes?
> That's enough for the initial JIRA report.  We will now track this problem 
> here instead of the dev mailing list.  Thanks.
> Kevin

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