rqlite
rqlite is an easy-to-use, lightweight, distributed relational database, which uses SQLite as its storage engine. rqlite is simple to deploy, operating it is very straightforward, and its clustering capabilities provide you with fault-tolerance and high-availability. rqlite is available for Linux, macOS, and Microsoft Windows.
Why?
rqlite gives you the functionality of a rock solid, fault-tolerant, replicated relational database, but with very easy installation, deployment, and operation. With it you've got a lightweight and reliable distributed relational data store. Think etcd or Consul, but with relational data modelling also available.
You could use rqlite as part of a larger system, as a central store for some critical relational data, without having to run larger, more complex distributed databases.
Finally, if you're interested in understanding how distributed systems actually work, rqlite is a good example to study. Much thought has gone into its design and implementation, with clear separation between the various components, including storage, distributed consensus, and API.
Uses sqlite as a source database