[nkallen]

I wonder if you could handle backpressure this way, somewhat analogously to a thread-per-connection with a bounded number of threads. I don't think node.js API exposes the necessary functionality, but the point is that it's possible with callbacks:

  var MAX = 100,
      connections = 0;

  function continue() {
    server.once('connection', function(c) {
      if (++connections < MAX) continue();
      process(c, function() {
        if (--connections == MAX - 1) continue();
      });
    });
  }

  continue();

Node.js doesn't have threads so you wouldn't need atomics or locking. I don't think it's fair to say this is spaghetti, but it has some complexity, yes.