GAE and JPA

How does JPA work in a GAE environment?

The Google App Engine(GAE) supports JPA, but the persistence is not done in a relational database.

It uses a NoSQL-database, using BigTable technology.

So there have to be some restrictions:

1) Polymorph queries

2) Aggregation functions
3) Transactional behavior: in a transaction only objects of the entity group may be changed
4) ...

1) Especially in an object-oriented abstraction where the data model knows about inheritance relations between the entities and they get persisted accordingly e.g. each class will be saved into a separate table. 

On querying such a structure with JPQL GAE does not allow you the use of polymorph queries:

A extends B extends C

If you are not interested in retrieving a special entity type, it's very handy to retrieve all entities based on C, the top super class, and apply abstracted treatment in cases where common treatment can be done.

So a "from C where …." - JPQL query will be possible on a relational database, but unfortunately will fail in a GAE environment.

2) Aggregation functions like SUM, AVG, … are not usable on GAE.
The like -operator is limited in use - it can only be used on the end of a search - token e.g. ... like 'Adam R%' ....

3) The transactional behavior JPA offers is limited on the GAE platform.
In one single transaction A only objects of the same entity group may be changed.
Their changes will then be applied accordingly in the database.
An entity group is a collection or grouping of objects that creates a data structure. This data structure consists of root objects and dependent objects. Instances of an entity group are so called
- root objects (starting entity)
- and from that dependent objects.
On creation of an instance objects can point to a parent entity.
The entity without a parent entity are the so called root entity.
Datasets of this entity groups reside on different nodes of the cluster in the distributed data storage. But one single dataset is normally physically available on one node. So that during data processing communication overhead is reduced.  

One may question how the relationship inside an entity group is realized as GAE does not run on a relational database?
The relationship is not handled like on relational database by using attribute/s or in JPA language properties. The primary key of the entity is used to route through the hierarchy:
In such an entity group we have a parent having a primary key and all the children have a pk that contains the parent pk. The pk normally has the type and a certain id. Hence a child pk consists of the type + id of parent + own id.
For example: Invoice(5)/InvoiceItem(1)

Because of this @ManyToMany and joining is not usable as well.