Portable JDO/JPA Design for App Engine Datastore and Relational Database

Question

I'm working on a project that need to run on App Engine and other Java Application Server. In App Engine we use datastore, and in other environment we will use traditional relational database (mostly MySQL).

I want know if it's possible that "have one JDO/JPA model that works on both".

If it's possible. How? Specifically, how do we handle the Key? Datastore required us to use it's own Key object or using "Key as encoded string", how do we port those keys to relational database.

If not, what would be the best practice? The idea we have right now is define abstract DAO, and have two set of DAO implementations. I believe the best way is using Objectify for datastore and JPA for relational database. But that way we could not leverage GWT RequestFactory (another technology we are using). Or can we?

Answer 1

Clearly JDO is designed to work on all datastores, whether RDBMS, ODBMS, document, map-based, web-based, document-based, file-based ... blah blah. Yes such portability is realistic. If you don't want portability you could use Objectify, but you say you want portability so that's not an option (so no idea why you think its the "best way"). You can use a String as PK in all datastores.

Answer 2

I don't know about GAE but I know JDO should be datastore independent so you can map your classes using JDO annotations and make sure while you are doing that, you aren't using any RDBMS based extensions (i.e. Datanucleus), i'm not sure if there are such extensions in the first place.

For keys, well obviously you shouldn't use GAE's but again, I'm not sure if it's a must or not.

Answer 3

I find it really hard to match the same "persistence" model on both a relational database and hierarchical database (the datastore here) since most of the time it requires thinking/structuring your data in a different way.

For example, you might need to duplicate data accross many entities in order to be able to run queries on it with the datastore.

From the few you said about your project, if you need to have it both in Google App Engine AND traditional servers (tomcat, JBOSS, WebSphere, whatever...) I would use Google Cloud SQL to keep my data model the same...

Or if you need a hierarchical database in both cases, install an open source one with your "traditional" servers...

What kind of projects are we talking about in the first place ? :)