FineDB is a multi-threaded NoSQL DBMS written in C, founded by Amaury Bouchard in 2013. It is based on Nanomsg and LMDB(Lightning Memory-Mapped Database) and is currently open-sourced and available on Github. It handles all basic queries. It uses LMDB as its internal storage manager, so all detailed information on the storage model can be found on LMDB's page. The way that the database is set up is that there are three main threads to handle the connections, writing and communication between the threads. For the communication between the threads, there are two nanomsg communication streams that have fanin and fanout streams. With these streams and threads, the DBMS connects to the LMDB storage model. This DB has not been updated since September 2013 so it is considered defunct.
FineDB was created by Amaury Bouchard of France in June of 2013, listed on his website as a project that he could use to learn more about network programming, multi-threading, replication and clustering. There have been no major updates to the code since September of 2013. Based on blog posts, however, Bouchard has said that he is currently focused on his startup and will continue to work on the DB soon.
FineDB makes use of Google's Snappy (previously known as Zippy), which is open-source and written in C++. Snappy uses byte-oriented encoding during the compression process.
Again, since FineDB is derived from LMDB, we see that the concurrency control other than that of the system as a whole is directly handled by the LMDB storage engine. Thus, we has the use of MVCC so that readers and writers do not block each other.
There are three main types of threads in the server. The main thread creates the other threads while listening for connections. The write thread writes into the LMDB storage engine asynchronously. The communication threads read in the requests and communicate with the client-side.