LemonGraph is a log-based transactional graph (nodes/edges/properties) database engine that is backed by a single file. The primary use case is to support streaming seed set expansion. The core of the graph library is written in C, and the Python (2.x) layer adds friendly bindings, a query language, and a REST service. LemonGraph rides on top of (and inherits a lot of awesome from) Symas LMDB - a transactional key/value store that the OpenLDAP project developed to replace BerkeleyDB.