> So it's a simple mmap database like early versions of MongoDB?

Even simpler. I use mmap only for things that I build offline and that never change during the runtime. Everything else is just kept as your garden variety objects in memory.

(Well, although I guess you could say it is an mmap database, it's just that the mmaped file is the swap, and the OS manages it for me.)

> I like using atomic commands, but I imagine it would become tedious fast to do the extra work of doing the accounting of the transaction manually every time you want to do a non atomic transaction.

In my case it's not really tedious at all. Usually when I want to mutate something it looks something like this:

    let user_ref = state.require_login()?;
    let user = user_ref.write();
    user.set_field(value);
    drop(user);

Of course this depends on your requirements. If you'd have 20 different objects of 10 different types that you'd have to lock separately and you'd want to update all of them atomically it might become hairy (SQL would definitely be better in such a case), but I don't have such cases.