Redis

Redis is an open-source, in-memory key-value data store. Because it keeps data in RAM instead of on disk, it can read and write in well under a millisecond, which makes it one of the most popular tools for caching database results, storing user sessions and counting things in real time.

Besides simple strings, Redis supports data structures such as lists, sets, sorted sets (used for leaderboards), hashes and streams, and it offers publish/subscribe messaging and key expiry. Data can be saved to disk for durability, and Redis can be replicated and clustered for high availability.