Mongo appears to have a decent driver for it. I'd lean to that if i were looking for an alternative to SQL.

You shouldn't have any issues integrating it into Absinthe, because it doesn't have any ecto bindings out of the box. (There's a helper library, but even that is fairly small.)

I have heard of people using mnesia, but it's not really that common. I'd personally avoid it unless I had a good reason not to.