I can find lots of information on how Long Polling works (For example, this, and this), but no simple examples of how to implement this in code.
All I can find is cometd, which relies on the Dojo JS framework, and a fairly complex server system..
Basically, how would I use Apache to serve the requests, and how would I write a simple script (say, in PHP) which would "long-poll" the server for new messages?
The example doesn't have to be scaleable, secure or complete, it just needs to work!
Tornado is designed for long-polling, and includes a very minimal (few hundred lines of Python) chat app in /examples/chatdemo , including server code and JS client code. It works like this:
Clients use JS to ask for an updates since (number of last message), server URLHandler receives these and adds a callback to respond to the client to a queue.
When the server gets a new message, the onmessage event fires, loops through the callbacks, and sends the messages.
The client-side JS receives the message, adds it to the page, then asks for updates since this new message ID.
I used this to get to grips with Comet, I have also set up Comet using the Java Glassfish server and found lots of other examples by subscribing to cometdaily.com
Below is a long polling solution I have developed for Inform8 Web. Basically you override the class and implement the loadData method. When the loadData returns a value or the operation times out it will print the result and return.
If the processing of your script may take longer than 30 seconds you may need to alter the set_time_limit() call to something longer.
Apache 2.0 license. Latest version on github https://github.com/ryanhend/Inform8/blob/master/Inform8-web/src/config/lib/Inform8/longpoll/LongPoller.php
Ryan
abstract class LongPoller {
protected $sleepTime = 5;
protected $timeoutTime = 30;
function __construct() {
}
function setTimeout($timeout) {
$this->timeoutTime = $timeout;
}
function setSleep($sleep) {
$this->sleepTime = $sleepTime;
}
public function run() {
$data = NULL;
$timeout = 0;
set_time_limit($this->timeoutTime + $this->sleepTime + 15);
//Query database for data
while($data == NULL && $timeout < $this->timeoutTime) {
$data = $this->loadData();
if($data == NULL){
//No new orders, flush to notify php still alive
flush();
//Wait for new Messages
sleep($this->sleepTime);
$timeout += $this->sleepTime;
}else{
echo $data;
flush();
}
}
}
protected abstract function loadData();
}
Here is a simple long-polling example in PHP by Erik Dubbelboer using the Content-type: multipart/x-mixed-replace header:
<?
header('Content-type: multipart/x-mixed-replace; boundary=endofsection');
// Keep in mind that the empty line is important to separate the headers
// from the content.
echo 'Content-type: text/plain
After 5 seconds this will go away and a cat will appear...
--endofsection
';
flush(); // Don't forget to flush the content to the browser.
sleep(5);
echo 'Content-type: image/jpg
';
$stream = fopen('cat.jpg', 'rb');
fpassthru($stream);
fclose($stream);
echo '
--endofsection
';
And here is a demo:
You can try icomet(https://github.com/ideawu/icomet), a C1000K C++ comet server built with libevent. icomet also provides a JavaScript library, it is easy to use as simple as
var comet = new iComet({
sign_url: 'http://' + app_host + '/sign?obj=' + obj,
sub_url: 'http://' + icomet_host + '/sub',
callback: function(msg){
// on server push
alert(msg.content);
}
});
icomet supports a wide range of Browsers and OSes, including Safari(iOS, Mac), IEs(Windows), Firefox, Chrome, etc.
Take a look at this blog post which has code for a simple chat app in Python/Django/gevent.
The WS-I group published something called "Reliable Secure Profile" that has a Glass Fish and .NET implementation that apparently inter-operate well.
With any luck there is a Javascript implementation out there as well.
There is also a Silverlight implementation that uses HTTP Duplex. You can connect javascript to the Silverlight object to get callbacks when a push occurs.
There are also commercial paid versions as well.
I used this to get to grips with Comet, I have also set up Comet using the Java Glassfish server and found lots of other examples by subscribing to cometdaily.com
Take a look at this blog post which has code for a simple chat app in Python/Django/gevent.
This is one of the scenarios that PHP is a very bad choice for. As previously mentioned, you can tie up all of your Apache workers very quickly doing something like this. PHP is built for start, execute, stop. It's not built for start, wait...execute, stop. You'll bog down your server very quickly and find that you have incredible scaling problems.
That said, you can still do this with PHP and have it not kill your server using the nginx HttpPushStreamModule: http://wiki.nginx.org/HttpPushStreamModule
You setup nginx in front of Apache (or whatever else) and it will take care of holding open the concurrent connections. You just respond with payload by sending data to an internal address which you could do with a background job or just have the messages fired off to people that were waiting whenever the new requests come in. This keeps PHP processes from sitting open during long polling.
This is not exclusive to PHP and can be done using nginx with any backend language. The concurrent open connections load is equal to Node.js so the biggest perk is that it gets you out of NEEDING Node for something like this.
You see a lot of other people mentioning other language libraries for accomplishing long polling and that's with good reason. PHP is just not well built for this type of behavior naturally.
Why not consider the web sockets instead of long polling? They are much efficient and easy to setup. However they are supported only in modern browsers. Here is a quick reference.
Tornado is designed for long-polling, and includes a very minimal (few hundred lines of Python) chat app in /examples/chatdemo , including server code and JS client code. It works like this:
Clients use JS to ask for an updates since (number of last message), server URLHandler receives these and adds a callback to respond to the client to a queue.
When the server gets a new message, the onmessage event fires, loops through the callbacks, and sends the messages.
The client-side JS receives the message, adds it to the page, then asks for updates since this new message ID.
You can try icomet(https://github.com/ideawu/icomet), a C1000K C++ comet server built with libevent. icomet also provides a JavaScript library, it is easy to use as simple as
var comet = new iComet({
sign_url: 'http://' + app_host + '/sign?obj=' + obj,
sub_url: 'http://' + icomet_host + '/sub',
callback: function(msg){
// on server push
alert(msg.content);
}
});
icomet supports a wide range of Browsers and OSes, including Safari(iOS, Mac), IEs(Windows), Firefox, Chrome, etc.
This is one of the scenarios that PHP is a very bad choice for. As previously mentioned, you can tie up all of your Apache workers very quickly doing something like this. PHP is built for start, execute, stop. It's not built for start, wait...execute, stop. You'll bog down your server very quickly and find that you have incredible scaling problems.
That said, you can still do this with PHP and have it not kill your server using the nginx HttpPushStreamModule: http://wiki.nginx.org/HttpPushStreamModule
You setup nginx in front of Apache (or whatever else) and it will take care of holding open the concurrent connections. You just respond with payload by sending data to an internal address which you could do with a background job or just have the messages fired off to people that were waiting whenever the new requests come in. This keeps PHP processes from sitting open during long polling.
This is not exclusive to PHP and can be done using nginx with any backend language. The concurrent open connections load is equal to Node.js so the biggest perk is that it gets you out of NEEDING Node for something like this.
You see a lot of other people mentioning other language libraries for accomplishing long polling and that's with good reason. PHP is just not well built for this type of behavior naturally.
Thanks for the code, dbr. Just a small typo in long_poller.htm around the line
1000 /* ..after 1 seconds */
I think it should be
"1000"); /* ..after 1 seconds */
for it to work.
For those interested, I tried a Django equivalent. Start a new Django project, say lp for long polling:
django-admin.py startproject lp
Call the app msgsrv for message server:
python manage.py startapp msgsrv
Add the following lines to settings.py to have a templates directory:
import os.path
PROJECT_DIR = os.path.dirname(__file__)
TEMPLATE_DIRS = (
os.path.join(PROJECT_DIR, 'templates'),
)
Define your URL patterns in urls.py as such:
from django.views.generic.simple import direct_to_template
from lp.msgsrv.views import retmsg
urlpatterns = patterns('',
(r'^msgsrv\.php$', retmsg),
(r'^long_poller\.htm$', direct_to_template, {'template': 'long_poller.htm'}),
)
And msgsrv/views.py should look like:
from random import randint
from time import sleep
from django.http import HttpResponse, HttpResponseNotFound
def retmsg(request):
if randint(1,3) == 1:
return HttpResponseNotFound('<h1>Page not found</h1>')
else:
sleep(randint(2,10))
return HttpResponse('Hi! Have a random number: %s' % str(randint(1,10)))
Lastly, templates/long_poller.htm should be the same as above with typo corrected. Hope this helps.
For a ASP.NET MVC implementation, look at SignalR which is available on NuGet.. note that the NuGet is often out of date from the Git source which gets very frequent commits.
Read more about SignalR on a blog on by Scott Hanselman
The WS-I group published something called "Reliable Secure Profile" that has a Glass Fish and .NET implementation that apparently inter-operate well.
With any luck there is a Javascript implementation out there as well.
There is also a Silverlight implementation that uses HTTP Duplex. You can connect javascript to the Silverlight object to get callbacks when a push occurs.
There are also commercial paid versions as well.
This is a nice 5-minute screencast on how to do long polling using PHP & jQuery: http://screenr.com/SNH
Code is quite similar to dbr's example above.
Here is a simple long-polling example in PHP by Erik Dubbelboer using the Content-type: multipart/x-mixed-replace header:
<?
header('Content-type: multipart/x-mixed-replace; boundary=endofsection');
// Keep in mind that the empty line is important to separate the headers
// from the content.
echo 'Content-type: text/plain
After 5 seconds this will go away and a cat will appear...
--endofsection
';
flush(); // Don't forget to flush the content to the browser.
sleep(5);
echo 'Content-type: image/jpg
';
$stream = fopen('cat.jpg', 'rb');
fpassthru($stream);
fclose($stream);
echo '
--endofsection
';
And here is a demo:
I used this to get to grips with Comet, I have also set up Comet using the Java Glassfish server and found lots of other examples by subscribing to cometdaily.com
Here are some classes I use for long-polling in C#. There are basically 6 classes (see below).
I've got a really simple chat example as part of slosh.
Edit: (since everyone's pasting their code in here)
This is the complete JSON-based multi-user chat using long-polling and slosh. This is a demo of how to do the calls, so please ignore the XSS problems. Nobody should deploy this without sanitizing it first.
Notice that the client always has a connection to the server, and as soon as anyone sends a message, everyone should see it roughly instantly.
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<!-- Copyright (c) 2008 Dustin Sallings <[email protected]> -->
<html lang="en">
<head>
<title>slosh chat</title>
<script type="text/javascript"
src="http://code.jquery.com/jquery-latest.js"></script>
<link title="Default" rel="stylesheet" media="screen" href="style.css" />
</head>
<body>
<h1>Welcome to Slosh Chat</h1>
<div id="messages">
<div>
<span class="from">First!:</span>
<span class="msg">Welcome to chat. Please don't hurt each other.</span>
</div>
</div>
<form method="post" action="#">
<div>Nick: <input id='from' type="text" name="from"/></div>
<div>Message:</div>
<div><textarea id='msg' name="msg"></textarea></div>
<div><input type="submit" value="Say it" id="submit"/></div>
</form>
<script type="text/javascript">
function gotData(json, st) {
var msgs=$('#messages');
$.each(json.res, function(idx, p) {
var from = p.from[0]
var msg = p.msg[0]
msgs.append("<div><span class='from'>" + from + ":</span>" +
" <span class='msg'>" + msg + "</span></div>");
});
// The jQuery wrapped msgs above does not work here.
var msgs=document.getElementById("messages");
msgs.scrollTop = msgs.scrollHeight;
}
function getNewComments() {
$.getJSON('/topics/chat.json', gotData);
}
$(document).ready(function() {
$(document).ajaxStop(getNewComments);
$("form").submit(function() {
$.post('/topics/chat', $('form').serialize());
return false;
});
getNewComments();
});
</script>
</body>
</html>
Simplest NodeJS
const http = require('http');
const server = http.createServer((req, res) => {
SomeVeryLongAction(res);
});
server.on('clientError', (err, socket) => {
socket.end('HTTP/1.1 400 Bad Request\r\n\r\n');
});
server.listen(8000);
// the long running task - simplified to setTimeout here
// but can be async, wait from websocket service - whatever really
function SomeVeryLongAction(response) {
setTimeout(response.end, 10000);
}
Production wise scenario in Express for exmaple you would get response in the middleware. Do you what you need to do, can scope out all of the long polled methods to Map or something (that is visible to other flows), and invoke <Response> response.end() whenever you are ready. There is nothing special about long polled connections. Rest is just how you normally structure your application.
If you dont know what i mean by scoping out, this should give you idea
const http = require('http');
var responsesArray = [];
const server = http.createServer((req, res) => {
// not dealing with connection
// put it on stack (array in this case)
responsesArray.push(res);
// end this is where normal api flow ends
});
server.on('clientError', (err, socket) => {
socket.end('HTTP/1.1 400 Bad Request\r\n\r\n');
});
// and eventually when we are ready to resolve
// that if is there just to ensure you actually
// called endpoint before the timeout kicks in
function SomeVeryLongAction() {
if ( responsesArray.length ) {
let localResponse = responsesArray.shift();
localResponse.end();
}
}
// simulate some action out of endpoint flow
setTimeout(SomeVeryLongAction, 10000);
server.listen(8000);
As you see, you could really respond to all connections, one, do whatever you want. There is id for every request so you should be able to use map and access specific out of api call.
For a ASP.NET MVC implementation, look at SignalR which is available on NuGet.. note that the NuGet is often out of date from the Git source which gets very frequent commits.
Read more about SignalR on a blog on by Scott Hanselman
Here are some classes I use for long-polling in C#. There are basically 6 classes (see below).
I think the client looks like a normal asynchronous AJAX request, but you expect it to take a "long time" to come back.
The server then looks like this.
while (!hasNewData())
usleep(50);
outputNewData();
So, the AJAX request goes to the server, probably including a timestamp of when it was last update so that your hasNewData() knows what data you have already got.
The server then sits in a loop sleeping until new data is available. All the while, your AJAX request is still connected, just hanging there waiting for data.
Finally, when new data is available, the server gives it to your AJAX request and closes the connection.
This is a nice 5-minute screencast on how to do long polling using PHP & jQuery: http://screenr.com/SNH
Code is quite similar to dbr's example above.
I used this to get to grips with Comet, I have also set up Comet using the Java Glassfish server and found lots of other examples by subscribing to cometdaily.com
Simplest NodeJS
const http = require('http');
const server = http.createServer((req, res) => {
SomeVeryLongAction(res);
});
server.on('clientError', (err, socket) => {
socket.end('HTTP/1.1 400 Bad Request\r\n\r\n');
});
server.listen(8000);
// the long running task - simplified to setTimeout here
// but can be async, wait from websocket service - whatever really
function SomeVeryLongAction(response) {
setTimeout(response.end, 10000);
}
Production wise scenario in Express for exmaple you would get response in the middleware. Do you what you need to do, can scope out all of the long polled methods to Map or something (that is visible to other flows), and invoke <Response> response.end() whenever you are ready. There is nothing special about long polled connections. Rest is just how you normally structure your application.
If you dont know what i mean by scoping out, this should give you idea
const http = require('http');
var responsesArray = [];
const server = http.createServer((req, res) => {
// not dealing with connection
// put it on stack (array in this case)
responsesArray.push(res);
// end this is where normal api flow ends
});
server.on('clientError', (err, socket) => {
socket.end('HTTP/1.1 400 Bad Request\r\n\r\n');
});
// and eventually when we are ready to resolve
// that if is there just to ensure you actually
// called endpoint before the timeout kicks in
function SomeVeryLongAction() {
if ( responsesArray.length ) {
let localResponse = responsesArray.shift();
localResponse.end();
}
}
// simulate some action out of endpoint flow
setTimeout(SomeVeryLongAction, 10000);
server.listen(8000);
As you see, you could really respond to all connections, one, do whatever you want. There is id for every request so you should be able to use map and access specific out of api call.
Below is a long polling solution I have developed for Inform8 Web. Basically you override the class and implement the loadData method. When the loadData returns a value or the operation times out it will print the result and return.
If the processing of your script may take longer than 30 seconds you may need to alter the set_time_limit() call to something longer.
Apache 2.0 license. Latest version on github https://github.com/ryanhend/Inform8/blob/master/Inform8-web/src/config/lib/Inform8/longpoll/LongPoller.php
Ryan
abstract class LongPoller {
protected $sleepTime = 5;
protected $timeoutTime = 30;
function __construct() {
}
function setTimeout($timeout) {
$this->timeoutTime = $timeout;
}
function setSleep($sleep) {
$this->sleepTime = $sleepTime;
}
public function run() {
$data = NULL;
$timeout = 0;
set_time_limit($this->timeoutTime + $this->sleepTime + 15);
//Query database for data
while($data == NULL && $timeout < $this->timeoutTime) {
$data = $this->loadData();
if($data == NULL){
//No new orders, flush to notify php still alive
flush();
//Wait for new Messages
sleep($this->sleepTime);
$timeout += $this->sleepTime;
}else{
echo $data;
flush();
}
}
}
protected abstract function loadData();
}
I've got a really simple chat example as part of slosh.
Edit: (since everyone's pasting their code in here)
This is the complete JSON-based multi-user chat using long-polling and slosh. This is a demo of how to do the calls, so please ignore the XSS problems. Nobody should deploy this without sanitizing it first.
Notice that the client always has a connection to the server, and as soon as anyone sends a message, everyone should see it roughly instantly.
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<!-- Copyright (c) 2008 Dustin Sallings <[email protected]> -->
<html lang="en">
<head>
<title>slosh chat</title>
<script type="text/javascript"
src="http://code.jquery.com/jquery-latest.js"></script>
<link title="Default" rel="stylesheet" media="screen" href="style.css" />
</head>
<body>
<h1>Welcome to Slosh Chat</h1>
<div id="messages">
<div>
<span class="from">First!:</span>
<span class="msg">Welcome to chat. Please don't hurt each other.</span>
</div>
</div>
<form method="post" action="#">
<div>Nick: <input id='from' type="text" name="from"/></div>
<div>Message:</div>
<div><textarea id='msg' name="msg"></textarea></div>
<div><input type="submit" value="Say it" id="submit"/></div>
</form>
<script type="text/javascript">
function gotData(json, st) {
var msgs=$('#messages');
$.each(json.res, function(idx, p) {
var from = p.from[0]
var msg = p.msg[0]
msgs.append("<div><span class='from'>" + from + ":</span>" +
" <span class='msg'>" + msg + "</span></div>");
});
// The jQuery wrapped msgs above does not work here.
var msgs=document.getElementById("messages");
msgs.scrollTop = msgs.scrollHeight;
}
function getNewComments() {
$.getJSON('/topics/chat.json', gotData);
}
$(document).ready(function() {
$(document).ajaxStop(getNewComments);
$("form").submit(function() {
$.post('/topics/chat', $('form').serialize());
return false;
});
getNewComments();
});
</script>
</body>
</html>
Thanks for the code, dbr. Just a small typo in long_poller.htm around the line
1000 /* ..after 1 seconds */
I think it should be
"1000"); /* ..after 1 seconds */
for it to work.
For those interested, I tried a Django equivalent. Start a new Django project, say lp for long polling:
django-admin.py startproject lp
Call the app msgsrv for message server:
python manage.py startapp msgsrv
Add the following lines to settings.py to have a templates directory:
import os.path
PROJECT_DIR = os.path.dirname(__file__)
TEMPLATE_DIRS = (
os.path.join(PROJECT_DIR, 'templates'),
)
Define your URL patterns in urls.py as such:
from django.views.generic.simple import direct_to_template
from lp.msgsrv.views import retmsg
urlpatterns = patterns('',
(r'^msgsrv\.php$', retmsg),
(r'^long_poller\.htm$', direct_to_template, {'template': 'long_poller.htm'}),
)
And msgsrv/views.py should look like:
from random import randint
from time import sleep
from django.http import HttpResponse, HttpResponseNotFound
def retmsg(request):
if randint(1,3) == 1:
return HttpResponseNotFound('<h1>Page not found</h1>')
else:
sleep(randint(2,10))
return HttpResponse('Hi! Have a random number: %s' % str(randint(1,10)))
Lastly, templates/long_poller.htm should be the same as above with typo corrected. Hope this helps.
I've got a really simple chat example as part of slosh.
Edit: (since everyone's pasting their code in here)
This is the complete JSON-based multi-user chat using long-polling and slosh. This is a demo of how to do the calls, so please ignore the XSS problems. Nobody should deploy this without sanitizing it first.
Notice that the client always has a connection to the server, and as soon as anyone sends a message, everyone should see it roughly instantly.
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<!-- Copyright (c) 2008 Dustin Sallings <[email protected]> -->
<html lang="en">
<head>
<title>slosh chat</title>
<script type="text/javascript"
src="http://code.jquery.com/jquery-latest.js"></script>
<link title="Default" rel="stylesheet" media="screen" href="style.css" />
</head>
<body>
<h1>Welcome to Slosh Chat</h1>
<div id="messages">
<div>
<span class="from">First!:</span>
<span class="msg">Welcome to chat. Please don't hurt each other.</span>
</div>
</div>
<form method="post" action="#">
<div>Nick: <input id='from' type="text" name="from"/></div>
<div>Message:</div>
<div><textarea id='msg' name="msg"></textarea></div>
<div><input type="submit" value="Say it" id="submit"/></div>
</form>
<script type="text/javascript">
function gotData(json, st) {
var msgs=$('#messages');
$.each(json.res, function(idx, p) {
var from = p.from[0]
var msg = p.msg[0]
msgs.append("<div><span class='from'>" + from + ":</span>" +
" <span class='msg'>" + msg + "</span></div>");
});
// The jQuery wrapped msgs above does not work here.
var msgs=document.getElementById("messages");
msgs.scrollTop = msgs.scrollHeight;
}
function getNewComments() {
$.getJSON('/topics/chat.json', gotData);
}
$(document).ready(function() {
$(document).ajaxStop(getNewComments);
$("form").submit(function() {
$.post('/topics/chat', $('form').serialize());
return false;
});
getNewComments();
});
</script>
</body>
</html>
I've got a really simple chat example as part of slosh.
Edit: (since everyone's pasting their code in here)
This is the complete JSON-based multi-user chat using long-polling and slosh. This is a demo of how to do the calls, so please ignore the XSS problems. Nobody should deploy this without sanitizing it first.
Notice that the client always has a connection to the server, and as soon as anyone sends a message, everyone should see it roughly instantly.
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<!-- Copyright (c) 2008 Dustin Sallings <[email protected]> -->
<html lang="en">
<head>
<title>slosh chat</title>
<script type="text/javascript"
src="http://code.jquery.com/jquery-latest.js"></script>
<link title="Default" rel="stylesheet" media="screen" href="style.css" />
</head>
<body>
<h1>Welcome to Slosh Chat</h1>
<div id="messages">
<div>
<span class="from">First!:</span>
<span class="msg">Welcome to chat. Please don't hurt each other.</span>
</div>
</div>
<form method="post" action="#">
<div>Nick: <input id='from' type="text" name="from"/></div>
<div>Message:</div>
<div><textarea id='msg' name="msg"></textarea></div>
<div><input type="submit" value="Say it" id="submit"/></div>
</form>
<script type="text/javascript">
function gotData(json, st) {
var msgs=$('#messages');
$.each(json.res, function(idx, p) {
var from = p.from[0]
var msg = p.msg[0]
msgs.append("<div><span class='from'>" + from + ":</span>" +
" <span class='msg'>" + msg + "</span></div>");
});
// The jQuery wrapped msgs above does not work here.
var msgs=document.getElementById("messages");
msgs.scrollTop = msgs.scrollHeight;
}
function getNewComments() {
$.getJSON('/topics/chat.json', gotData);
}
$(document).ready(function() {
$(document).ajaxStop(getNewComments);
$("form").submit(function() {
$.post('/topics/chat', $('form').serialize());
return false;
});
getNewComments();
});
</script>
</body>
</html>
I think the client looks like a normal asynchronous AJAX request, but you expect it to take a "long time" to come back.
The server then looks like this.
while (!hasNewData())
usleep(50);
outputNewData();
So, the AJAX request goes to the server, probably including a timestamp of when it was last update so that your hasNewData() knows what data you have already got.
The server then sits in a loop sleeping until new data is available. All the while, your AJAX request is still connected, just hanging there waiting for data.
Finally, when new data is available, the server gives it to your AJAX request and closes the connection.
Why not consider the web sockets instead of long polling? They are much efficient and easy to setup. However they are supported only in modern browsers. Here is a quick reference.
Source: Stackoverflow.com