FANN stands for "Fennel Approx Nearest Neighbor" and is a tiny (~200 LOC) Rust library for doing approx nearest neighbor search. It is meant to be for educational purposes. You're free to use it in production at your risk (though since this is only ~200 lines of Rust, you can literally read/inspect the code before using it in prod).
Fennel offers realtime feature engineering platform as fully managed service. You can read Fennel docs here.