Well, as you see the concept of a relational database exists most of the
time either sync(1,2) or async(3). Although these solutions  are great in
their case (esp. Prevayler), but they are not excatly what I'm looking
for. Among them db4o seems, well, closer to what I want and I'll give a
try in next days. Thanks for your response guru, and still looking for
other alternatives.