Common usage approach. Note that app.js has it's own interface :
module.exports = {
startServer : startServer // return httpServer (or another one) instance
}
var app = require(__dirname + '/server/app');
var clusterHelper = require('./node_user_modules/cluster_helper');
var clusterInstance = new clusterHelper();
clusterInstance.init(app);
clusterInstance.setProcessing();