I forked Redis years ago providing threads and SQL support, though I can't remember if transactions were supported now. This was done mostly for fun and as an experiment, learned a lot about locking and database internals in the process:
I read your code a couple years ago after finding your repo through a search for Redis and SQLite - what a fun project! Learned a good deal from your changes, so thanks for open-sourcing your experiment.
Main concept: http://thredis.org/
Implementation details: https://github.com/grisha/thredis/blob/master/README-THREDIS