> Server-less doesn't mean pricing-less

Well of course not, although that would be wonderful.

> Your can't perform operations to get active users

Something like this would do it, right:

  var refUserOnline= db.ref('/.info/connected');
  refUserOnline.on('value', function(snapshot) {
                    if (snapshot.val()) {
                        var sessionRef = refUserSessions.push();
                        sessionRef.child('began').set(firebase.database.ServerValue.TIMESTAMP); // set UUID too
                        sessionRef.child('began').onDisconnect().remove();
                    }

});

> Paying 100$ per month for something you can do on a 5$ DigitalOcean server is something that make you thinking twice about server-less magic-less.

I pay Firebase a handful of dollars every month and they handle user auth, hosting, CDN and data. Three or four overheads I no longer need to worry about. Having used Digitalocean and Stormpath (yikes) in the past, I'm happy to have my eggs in one rather than multiple baskets.

Yeah it cost me a few hours on Stackoverflow after bumping into some of the problems you describe but the path from idea to working app is so short with Firebase - and the cognitive load typically inherent with managing multiple services, so small - that it's worth learning the 'Firebase way' of dealing with data (which is mostly just the NoSQL way).

Crisp is sweet btw, I use it daily. Good job.