broken_app.js

// In this example we'd like to have a factory that works per request and allows to keep track of created Models
// The singleton will however apply to all requests and thus keep growing
// For that reason we supply it with a reset function.
// This works fine until between receiving a request and sending a response there is no asynchronous code execution
// Once multiple requests come in in very short intervals, the asynchronous execution will lead to requests cross-polluting
// the singleton class

// factory.js
// 
// Should be per-request so we can keep track
// of all models that were instantiated during the request lifetime

var Model = require('backbone').Model;
var allModels = [];

module.exports = {
    createModel: function (data) {
        var model = new Model(data);
        allModels.push(model);
        return model;
    },
    serializeModels: function () {
        return allModels.map(function (model) {
            return model.toJSON();
        });
    },
    reset: function () {
        allModels = [];
    }
};

// view.js
var Factory = require('./factory');
var View = require('backbone').View;

return View.extend({
    initialize: function () {
        this.viewState = Factory.createModel({
            collapsed: true,
            hidden: false
        });
    }
});

// app.js
var express = require('express'),
    app = express(),
    factory = require('./factory'),
    View = require('./view');

app.all('*', function(req, res, next) {
    var view = new View();
    var responseTime = Math.random() * 2500; // random response time

    // Simulate async tasks
    setTimeout(function () {
        res.send(factory.serializeModels());
        factory.reset();
    }, responseTime);

});

app.listen(8000);