Neo4j for Internal Storage

So far, Core persistence has been based on JPA, and through this, several DBMS and flavors are supported, including variants featuring JSON-based columns.

Following Spring Data JPA being introduced by SYNCOPE-1799, it seems feasible to leverage Spring Data Neo4j to provide an alternative, Neo4j-based, persistence layer.

Note on licensing: while Neo4j Community itself is released under GPL 3.0, the Neo4j Java driver - which will be the only dependency to be actually added besides Spring's - is released under AL 2.0.