You have persistent processes which you can send and receive messages from which can be treated like a data structure store, as well as ETS if you need more advanced querying and Mnesia if you need embedded persistence.

I'm not sure how these compare to Redis in terms of performance/latency but Mongo is more of a "proper" database than any of these.

Ok. I've used Elixir and read Thomas' Programming Elixir, and from the little exposure I got I knew it was normal to use it as a kv store but still use something like postgres for a traditional db. I did a double take when I saw Mongo in the table.

We definitely use databases such as Postgres, MySQL, Riak, etc. It is just that when it comes to ephemeral data, the tools that ship with the Elixir and OTP may be enough. See how Phoenix does the whole pubsub and presence without Redis or similar.