code: function session(req, res, next) {
      //
      // The session id is stored in the cookies.
      // `req.signedCookies` is assigned by the `cookie-parser` middleware.
      //
      var sid = req.signedCookies[key];
  
      //
      // Default to an empty session.
      //
      req.session = {};
  
      //
      // If we don't have a session id we are done.
      //
      if (!sid) return next();
  
      //
      // Grab the session from the store.
      //
      store.get(sid, function (err, session) {
        //
        // We don't want to kill the connection when we get an error from the
        // session store so we just log the error.
        //
        if (err) {
          primus.emit('log', 'error', err);
          return next();
        }
  
        if (session) req.session = session;
  
        next();
      });
    }
docs: The actual session middleware. This middleware is async so we need 3 arguments.