I would recommend to look into NoSQL and Distributed Cache to achieve scalability that you are looking for.
The truth of the matter is: Games, Users, Dogs, Cats.. are not really relational, they are what they are, and deserve to be represented with rich data structures rather than flattened and dumped to tables.
There is a growing movement to support NoSQL and Distributed Cache naively in Spring ( check out Spring Data, Spring Gemfire, etc.. projects ). But most of the solutions are easy enough to integrate with Spring on your own.
Humans are stateful and mutable beings that have no problems processing many things concurrently and share state with others + they are usually "coupled"