Ok, so there is some terminology mismatch here, sorry but this is my fault I think since when in the article I speak of RDBMS I think to Oracle, MySQL, ..., relational SQL databases.

Btw in Wikipedia I read: "A short definition of an RDBMS may be a DBMS in which data is stored in the form of tables and the relationship among the data is also stored in the form of tables."

Basically once you have the idea of tables, a lot of things about SQL databases are automagically needed: queries and indexes for instance.

Also MongoDB supports _id fields specifically. And tables, so you can build tables that have as unique goal to describe relations among other tables. So MongoDB is an RDBMS following the definition of Wikipedia, that may be wrong of course (and if so should be fixed).

With Redis Sets probably it is possible to build the storage system of an RDBMS just with software on the client library side, but it can't be considered an RDBMS AFAIK because of the lack of tables, and Sets can't be considered tables since they don't have the field:value idea at all, but are just collection of elements.

Btw in the future I'll try to use the term RDBMS better, and will refer to Oracle, MySQL, Firebird, DB2, and so forth, as SQL databases.