node.js http long polling server. using a messaging as an example of longpoll usage.

messagequeue.js

//
// Crude node.js longpoll example via a simple message queue
//

//--------------------
// app.js
//--------------------
var queue = require('./queue/messagequeue');
app.get('/messages/:queueName/:lastMsgId', queue.getMessages);
app.post('/messages/:queueName', queue.postMessages);


//--------------------------
// queue/messagequeue.js
//--------------------------

// message lists keyed by queuenames (dictionary of arrays)
var messages = {};

// counter for each queuename - represents next id to insert @ for a given queue (dictionary of numbers)
var counters = {};

// dictionary by queueName with a list of requests for that queueName
var queuedRequests = {}


exports.postMessages = function(req, res) {
	var msgs = req.body;
	var queueName = req.params.queueName;

	if ((!msgs || !queueName) || (!(msgs instanceof Array))){
		res.status(400).send('Bad request. Body must be an array of messages.');
		return;
	}

	if (!messages.hasOwnProperty(queueName)) {
		console.log('creating queue ' + queueName);
		counters = {};
		counters[queueName] = 1;  // start @ 1 so client starts with lastMsgId of 0
		messages[queueName] = [{"data":"0"}];
	}

	messages[queueName] = messages[queueName].concat(msgs);
	counters[queueName] += msgs.length;

	// send queued messages
	if (queuedRequests.hasOwnProperty(queueName)) {
		queuedRequests[queueName].forEach(function(queuedMessage){
			sendMessages(queueName, queuedMessage.lastMsgId, queuedMessage.response);
		});
	}

	res.status(200).send(msgs);	
}

exports.getMessages = function(req, res) {
	var queueName = req.params.queueName;
	var lastMsgId = req.params.lastMsgId;

	console.log('getMessages ' + queueName);

	if ((!messages.hasOwnProperty(queueName)) || 
		(lastMsgId == counters[queueName] - 1)) {
		console.log('queuing response ...');
		queueResponse(queueName, lastMsgId, res);
		return;
	}

	if  (lastMsgId >= counters[queueName]) {
		res.status(400).send('Bad request.  Invalid lastMsgId.');
	}

	sendMessages(queueName, lastMsgId, res);
}

var queueResponse = function(queueName, lastMsgId, res) {
	if (!queuedRequests.hasOwnProperty(queueName)) {
		queuedRequests[queueName] = [];
	}

	queuedRequests[queueName].push({"lastMsgId": lastMsgId, "response": res});
}

var sendMessages = function(queueName, lastMsgId, res) {
    var msgs = messages[queueName].slice(parseInt(lastMsgId) + 1, counters[queueName]);
	res.status(200).send({"lastMsgId": counters[queueName] - 1, "data": msgs});	
}

//---------------------------------------------------------------------------------------
// client messagelistener (rest.js is just a class to do get requests and return JSON)
//---------------------------------------------------------------------------------------
var rest = require("./rest");

var _queueName;
var _host;
var _port;
var _basePath = "/messages";
var _lastMsgId = 0;
var _callback;

exports.start = function(queueName, host, port, callback) {
	_queueName = queueName;
	_host = host;
	_port = port;
	_callback = callback;

	getMessages();
}

var getMessages = function() {
	var path = _basePath + "/" + _queueName + "/" + _lastMsgId;
	var options = {
	    host: _host,
	    port: _port,
	    path: path,
	    method: 'GET',
	    headers: {
	        'Content-Type': 'application/json'
	    }
	};

	rest.getJSON(options, 
		function(statusCode, payload) {
			_lastMsgId = payload.lastMsgId;
			console.log('lastMsgId: ' + _lastMsgId);
			_callback(payload.data);

			getMessages();
		});
}

//-----------------------------------------------
// client.js
//-----------------------------------------------
var listener = require('./messagelistener');
var queueName = "myqueue";
var host = "localhost";
var port = 3000;

console.log('listening to ' + queueName);
listener.start(queueName, host, port, function(messages) {
	console.log('callback ...')
	console.log(JSON.stringify(messages, null, 2));	
});