Accessing the GAE Datastore: Use JDO, JPA or the low-level API?

Question

Any recommendations on how to best access the Google App Engine Datastore? Via JDO, JPA or the native API?

The obvious advantages of JDO/JPA are portability to other database engines, but apart from that, any reason not to work directly with the Datastore API?

Answer 1

I don't know much about JPA, but I went with JDO, and if you're new to it I can say that it has a pretty steep learning curve and a lot of extraneous stuff that doesn't apply to GAE. What you do win is owned relationships, which allow you to have classes with actual references to each other instead of just datastore IDs. There are also some useful things JDO does via annotations, such as the @Element(dependent = "true") annotation, which saves you quite a bit of work as it allows you to delete a parent object and JDO will delete all its children. In general, the GAE docs miss a lot of things that you need to know to use JDO effectively, so I would say that its crucial to read the datanucleus docs, and pay particular attention to fetch groups.

You can also find a large collection of succinct examples for JDO and JPA that address almost every conceivable scenario here.

Finally I would look at the Objectify and Twig, two apparently popular alternative frameworks, which were mentioned in a question I asked when I was also trying to make this decision.

On a side note, as for portability to other databases, I think worrying about portability on GAE is a bit misguided. As much as Google wants us to think that GAE code is portable, I think its a pipe dream. You will end up coding to target the particular mix of APIs that Google offers, a mix that you probably won't see anywhere else, and also coding around GAE's many limitations and idiosyncracies, so I would forget about portability as a factor for settling on a data-access API. In fact, if I could remake my decision on this matter, I think I would use a data-access framework that's built specifically for GAE, such as objectify.