Some applications need a very fast response time for certain kinds of data; faster than what a DBMS can offer. Usually, a DBMS dedicated to one user would be fast enough, but for some applications (typically large-scale web applications) there are hundreds of concurrent queries and many concurrent updates. You can use many computers, but the data must be kept coherent among them, and keeping coherence can cause a bottleneck of performance.
A solution to this problem is to use a key-value store, which is a very simple database that can be replicated across a network. This keeps the data in memory to maximize the speed, but it also supports the option to save the data in a file. This avoids losing information if the server is stopped.
A key-value store is similar to the HashMap collection of the Rust standard library, but it is managed by a server process, which could possibly be running on a different computer. A query is a message exchanged...