The API is broken down into two independent layers plus one for various utility stuff like cross platform mmaped files, RC4 random numbers generation and so on. The first layer serve as a general purpose key/value store for the host application blobs (i.e binary or text data). You can store whole disk files inside a single database to generate a cross platform TAR archive for example.
The second one is the document store layer similar in concept to what MongoDB offer but without the networking layer since everything run in the same process of the host application. Both layers are transactional and are able to recover after some external failure thanks to the SQLite journaling mechanism from which UnQLite is based on.
Since its release in 2013, there was four major bugs including two critical that involves data loss under certain load. All known data corruption bugs were fixed in the last release and no major bugs were discovered a year or so.
The library enjoy particular success among Python and C/C++ developers especially Chinese that used to bombard the original developers with various internal aspect of the library.