I already got one? My employer mostly has these requirements. We ended up writing our own query language because it is a graph DB, but all the bits are there.

I already got one? My employer mostly has these requirements. We ended up writing our own query language because it is a graph DB, but all the bits are there. LMDB + a journal + a query interpreter is really all you need. We don't have off-line mirroring yet, but that's on the list. You could start with Synapse [] and rip out the bits you don't need. It is open source. [] https://github.com/vertexproject/synapse https://github.com/vertexproject/synapse