Aside from RDB and key-value, one other model I haven't heard mentioned much is Terracotta (currently the only example of its kind, AFAIK). That gives you basic

Aside from RDB and key-value, one other model I haven't heard mentioned much is Terracotta (currently the only example of its kind, AFAIK). That gives you basically all the advantages of a database, including replicated ACID transactions, but with data that behaves like live objects. Better than live objects, in fact, because of lazy loading and unloading, which let objects grow beyond the limits of RAM. Better than a database, too, in that it only moves diffs, and it knows exactly where they're needed. If you are the only one currently touching a data structure, the only traffic is "may I?" "carry on until I say otherwise".