redis-server 5:4.0.9-1ubuntu0.2 (amd64 binary) in ubuntu bionic

 Redis is a key-value database in a similar vein to memcache but the dataset
 is non-volatile. Redis additionally provides native support for atomically
 manipulating and querying data structures such as lists and sets.
 .
 The dataset is stored entirely in memory and periodically flushed to disk.