To be fair, Spark is way too slow to be used as a back-end database system (there's a big bap between "lightning speed computation", as they put them, and a common workload for a database serving data to a user-facing application).

Now of course Spark makes up for it with its great flexibility and scalability, but I do not really see the two technologies as competing ones.

This even without getting into the other parts of the data model (insert, update, delete) that do not exist in Spark (or "kind of" exist), by design.