At this point I really have to wonder how many people have written graph engines on top of lmdb! In the past month I've seen two from the same bank built on top of lmdbjava. Instead of reinventing this same wheel over and over it'd probably make sense for somebody to sitdown with lmdb and tinkerpop [1] and bang out one decent implementation.

...actually this has been done [2] but the project looks abandoned. So NSA guys you should get right on this.

[1] http://tinkerpop.apache.org/docs/current/reference/

[2] https://github.com/pietermartin/thundergraph