TidesDB is a storage engine, you use it to build databases. It's persistent and built on a log-structured-merge tree (LSM) with modern research incorporated such as
* Spooky - Granulating LSM-Tree Compactions Correctly
* WiscKey - Key-value seperation
TidesDB has cache layers such as a clock block cache but it's mainly a persistent layer and a library.
I also have a presentation on TidesDB you can find here:
https://www.youtube.com/watch?v=7HROlAaiGVQ
Memcached is a cache, TidesDB is an engine for building databases, think a game engine for building games like say Unity.
