To be fair, a quick google search for "open source datalog database" does not return much that is usable out-of-the-box, other than datomic: mostly academic projects or adapters for other databases with some different querying API, like SPARQL.
Datomic seems to be one of the best ones in terms of execution of the ideas behind it and usage in real world projects, to say the least.
Datomic seems to be one of the best ones in terms of execution of the ideas behind it and usage in real world projects, to say the least.