Real time notifications with PHP

March 14, 2011March 15, 2011 ~ Gonzalo Ayuso

Real time communications are cool, isn’t it? Something impossible to do five years ago now (or almost impossible) is already available. Nowadays we have two possible solutions. WebSockets and Comet. WebSockets are probably the best solution but they’ve got two mayor problems:

Not all browsers support them.
Not all proxy servers allows the communications with websokets.

Because of that I prefer to use comet (at least now). It’s not as good as websockets but pretty straightforward ant it works (even on IE). Now I’m going to explain a little script that I’ve got to perform a comet communications, made with PHP. Probably it’s not a good idea to use it in a high traffic site, but it works like a charm in a small intranet. If you want to use comet in a high traffic site maybe you need have a look to Tornado, twisted, node.js or other comet dedicated servers.

Normally when we are speaking about real-time communications, all the people are thinking about a chat application. I want to build a simpler application. A want to detect when someone clicks on a link. Because of that I will need a combination of HTML, PHP and JavaScript. Let’s start:

For the example I’ll use jquery library, so we need to include the library in our HTML file. It will be a blend of JavaScrip and PHP:

<html>
    <head>
        <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
        <title>Comet Test</title>
    </head>
    <body>
        <p><a class='customAlert' href="#">publish customAlert</a></p>
        <p><a class='customAlert2' href="#">publish customAlert2</a></p>
        <script src="http://ajax.googleapis.com/ajax/libs/jquery/1.5/jquery.min.js" type="text/javascript"></script>
        <script src="NovComet.js" type="text/javascript"></script>
        <script type="text/javascript">
NovComet.subscribe('customAlert', function(data){
    console.log('customAlert');
    //console.log(data);
}).subscribe('customAlert2', function(data){
    console.log('customAlert2');
    //console.log(data);
});

$(document).ready(function() {
    $("a.customAlert").click(function(event) {
        NovComet.publish('customAlert');
    });
    
    $("a.customAlert2").click(function(event) {
        NovComet.publish('customAlert2');
    });
    NovComet.run();
});
        </script>
    </body>
</html>

The client code:

//NovComet.js
NovComet = {
    sleepTime: 1000,
    _subscribed: {},
    _timeout: undefined,
    _baseurl: "comet.php",
    _args: '',
    _urlParam: 'subscribed',

    subscribe: function(id, callback) {
        NovComet._subscribed[id] = {
            cbk: callback,
            timestamp: NovComet._getCurrentTimestamp()
        };
        return NovComet;
    },

    _refresh: function() {
        NovComet._timeout = setTimeout(function() {
            NovComet.run()
        }, NovComet.sleepTime);
    },

    init: function(baseurl) {
        if (baseurl!=undefined) {
            NovComet._baseurl = baseurl;
        }
    },

    _getCurrentTimestamp: function() {
        return Math.round(new Date().getTime() / 1000);
    },

    run: function() {
        var cometCheckUrl = NovComet._baseurl + '?' + NovComet._args;
        for (var id in NovComet._subscribed) {
            var currentTimestamp = NovComet._subscribed[id]['timestamp'];

            cometCheckUrl += '&' + NovComet._urlParam+ '[' + id + ']=' +
               currentTimestamp;
        }
        cometCheckUrl += '&' + NovComet._getCurrentTimestamp();
        $.getJSON(cometCheckUrl, function(data){
            switch(data.s) {
                case 0: // sin cambios
                    NovComet._refresh();
                    break;
                case 1: // trigger
                    for (var id in data['k']) {
                        NovComet._subscribed[id]['timestamp'] = data['k'][id];
                        NovComet._subscribed[id].cbk(data.k);
                    }
                    NovComet._refresh();
                    break;
            }
        });

    },

    publish: function(id) {
        var cometPublishUrl = NovComet._baseurl + '?' + NovComet._args;
        cometPublishUrl += '&publish=' + id;
        $.getJSON(cometPublishUrl);
    }
};

The server-side PHP

// comet.php
include('NovComet.php');

$comet = new NovComet();
$publish = filter_input(INPUT_GET, 'publish', FILTER_SANITIZE_STRING);
if ($publish != '') {
    echo $comet->publish($publish);
} else {
    foreach (filter_var_array($_GET['subscribed'], FILTER_SANITIZE_NUMBER_INT) as $key => $value) {
        $comet->setVar($key, $value);
    }
    echo $comet->run();
}

and my comet library implementation:

// NovComet.php
class NovComet {
    const COMET_OK = 0;
    const COMET_CHANGED = 1;

    private $_tries;
    private $_var;
    private $_sleep;
    private $_ids = array();
    private $_callback = null;

    public function  __construct($tries = 20, $sleep = 2)
    {
        $this->_tries = $tries;
        $this->_sleep = $sleep;
    }

    public function setVar($key, $value)
    {
        $this->_vars[$key] = $value;
    }

    public function setTries($tries)
    {
        $this->_tries = $tries;
    }

    public function setSleepTime($sleep)
    {
        $this->_sleep = $sleep;
    }

    public function setCallbackCheck($callback)
    {
        $this->_callback = $callback;
    }

    const DEFAULT_COMET_PATH = "/dev/shm/%s.comet";

    public function run() {
        if (is_null($this->_callback)) {
            $defaultCometPAth = self::DEFAULT_COMET_PATH;
            $callback = function($id) use ($defaultCometPAth) {
                $cometFile = sprintf($defaultCometPAth, $id);
                return (is_file($cometFile)) ? filemtime($cometFile) : 0;
            };
        } else {
            $callback = $this->_callback;
        }

        for ($i = 0; $i < $this->_tries; $i++) {
            foreach ($this->_vars as $id => $timestamp) {
                if ((integer) $timestamp == 0) {
                    $timestamp = time();
                }
                $fileTimestamp = $callback($id);
                if ($fileTimestamp > $timestamp) {
                    $out[$id] = $fileTimestamp;
                }
                clearstatcache();
            }
            if (count($out) > 0) {
                return json_encode(array('s' => self::COMET_CHANGED, 'k' => $out));
            }
            sleep($this->_sleep);
        }
        return json_encode(array('s' => self::COMET_OK));
    }

    public function publish($id)
    {
        return json_encode(touch(sprintf(self::DEFAULT_COMET_PATH, $id)));
    }
}

As you can see in my example I’ve created a personal protocol for the communications between the client (js at browser), and the server (PHP). It’s a simple one. If you’re looking for a “standard” protocol maybe you need have a look to bayeux protocol from Dojo people.

Let me explain a little bit the usage of the script:

In the HTML page we start the listener (NovComet.subscribe).
We can subscribe to as many events we want (OK it depends on our resources)
When we subscribe to one event we pass a callback function to be triggered.
When we subscribe to the event, we pass the current timestamp to the server.
Client side script (js with jquery) will call to server-side script (PHP) with the timestamp and will wait until server finish.
Server side script will answer when even timestamp changes (someone has published the event)
Server side will no keep waiting forever. If nobody publish the event, server will answer after a pre-selected timeout
client side script will repeat the process again and again.

There’s something really important with this technique. Our server-side event check need to be as simpler as we can. We cannot execute a SQL query for example (our sysadmin will kill us if we do it). We need to bear in mind that this check will be performed again and again per user, because of that it must be as light as we can. In this example we are checking the last modification date of a file (filemtime). Another good solution is to use a memcached database and check a value.

For the test I’ve also created a publishing script (NovComet.publish). This is the simple part. We only call a server-side script that touch the event file (changing the last modification date), triggering the event.

Now I’m going to explain what we can see on the firebug console:

The first iteration nothing happens. 200 OK Http code after the time-out set in the PHP script
As we can see here the script returns a JSON with s=0 (nothing happens)
Now we publish an event. Script returns a 200 OK but now the JSON is different. s=1 and the time-stamp of the event
Our callback has been triggered
And next iteration waiting

And that’s all. Simple and useful. But remember, you must take care if you are using this solution within a high traffic site. What do you think? Do you use lazy comet with PHP in production servers or would you rather another solution?

You can get the code at github here.

Published by Gonzalo Ayuso

Web Architect. PHP, Python, Node, Angular, ionic, PostgreSQL, Linux, ... Always learning. View all posts by Gonzalo Ayuso

42 thoughts on “Real time notifications with PHP”

Kristopher says:

March 15, 2011 at 3:03 pm

Nice write up. You’re absolutely correct: we deployed this technique on a high traffic website and found the server frequently bottlenecking. We ended up using writing our own custom little server to handle the requests, as well as some back end caching to prevent unnecessary database calls.

Reply
1. Gonzalo Ayuso says:
  
  March 15, 2011 at 9:47 pm
  
  I must admit I didn’t use it in a hight traffic site but it works like a charm in a medium intranet. I avoid any database connection in the comet script and it works without significant server load. BTW I’ve got in my mind a similar script using node.js instead of PHP.
  
  Reply
Martin says:

March 15, 2011 at 3:32 pm

Very nice indeed…

But line 41 in NowComet.php should be
if (is_null($this->_callback)) {

Reply
1. Gonzalo Ayuso says:
  
  March 15, 2011 at 4:06 pm
  
  Ups. You’re right. Many txs. Updating …
  
  Reply
Scott Mattocks says:

March 16, 2011 at 3:10 pm

With this example you are still just doing short polling. The difference is that you have moved the polling from the client to the server. If you use a more event driven model, you can greatly improve performance. You won’t have blackout periods between polling memcached or while sending the data out to the client. I’ve written a small PHP HTTP/WebSocket server that allows you to take incoming messages from your main server and distribute them to many client connections at once. The smaller server (which is based off of Tornado and called WaterSpout) listens for events from the main server and then distributes them immediately to the proper client connections. It uses WebSockets to ensure a constant connection but will fail over to long polling if WebSockets aren’t available.

If you are going to be at Tek11 this year, stop by my talk on Realtime Communications. I’d love to chat about the challenges you’ve faced and the way you went about solving them.

Reply
1. Gonzalo Ayuso says:
  
  March 16, 2011 at 5:49 pm
  
  I know. I need to use websockets to improve the performance. The main problem I’ve got with websockets is the browser (only a few of them) and proxy servers (they normally kill websockes). But this solution works without any problem within a small/medium intranet (the number of users is limited). My sysadmin did not kill me (yet), and the server load isn’t altered. I’ve taken care with databases (no db connection at serverside), and I check the modification date of files located in RAM (/dev/shm). The apache connections are limited by a timeout (they’re not alive forever). OK, you’re right. That’s still short polling and it’s not a “real” real-time, but it looks like real-time. Good part: works with all browsers, and I don’t need to take care about internet proxies. Bad part: if I scale it, my sysadmin will definitely kill me. Now I’m playing with node.js + socket.io library (in order to use websockets)
  
  I’ve seen your talk about it at phpconference2010 (BCN). Definitely your talk inspired me a lot. In fact I’m the one who spoke with you (at the end of your great talk) at about the problems with proxy servers and websokets ;).
  
  Reply
Pingback: Anonymous
Pingback: Real time notifications (part II). Now with node.js and socket.io « Gonzalo Ayuso | Web Architect
Angelo Calabrò says:

July 2, 2011 at 8:06 am

How can I implement it on my site?

Reply
1. Gonzalo Ayuso says:
  
  July 4, 2011 at 12:37 pm
  
  It depends on your site. I’ve tried to develop an aseptic library without external dependencies to make it easy to adapt it. Anyway I recomend to check the second part of this mini-serie:
  
  https://gonzalo123.wordpress.com/2011/05/23/real-time-notifications-part-ii-now-with-node-js-and-socket-io/
  
  Nodejs solution maybe is an smart and probably easy solution to the problem.
  
  Reply
Pingback: mymoe » 基于push技术的即时聊天室
InfoMan says:

February 8, 2012 at 8:28 pm

Great work, I found it very helpful.

Reply
Pingback: Notifications in Node.js | t1u
jimmy says:

September 15, 2012 at 5:55 am

Really interesting. Do you think Facebook/Gmail using the same way like this? I know they are using long-polling/comet. Is this the same? Why these two website still works like a charm? They have billions of users… Because of their amazing servers?

Reply
1. Gonzalo Ayuso says:
  
  September 15, 2012 at 10:43 am
  
  No. This technique is not suitable for big traffic. Your server load will raise to hell, and that’s too expensive. I’ve wrote a second part of the post (https://gonzalo123.wordpress.com/2011/05/23/real-time-notifications-part-ii-now-with-node-js-and-socket-io/) doing the same with node.js and websockets/socket.io. That’s a better solution if you need to scale. FB/Gmail/Twitter’s servers aren’t amazing. The only amazing thing is the number of servers :). The solution of this post “works” but the solution of the second post works too and scales too. The “problem” with the second solution is that you need one extra server for Real Time communications.
  
  The problem with the PHP’s solution (The first post) is the IDLE mechanism used (the sleep function). Node uses functional programming and its way to send process to IDLE is brilliant (with callbacks). So different ways to solve the same problem, wiht its pro and cons.
  
  Reply
  1. jimmy says:
    
    September 15, 2012 at 3:16 pm
    
    Thanks for your explanation. I’m still curious tho, is Facebook using node.js to get this work?
  2. Gonzalo Ayuso says:
    
    September 15, 2012 at 3:35 pm
    
    I don’t think so. They published couple of years ago one similar open source project called Tornado (python based). It’s similar than Twisted (python too). As far as I know FB uses python in the back-end and probably they have one customized version of Tornado. The idea is the same than node.js.
  3. jimmy says:
    
    September 15, 2012 at 5:02 pm
    
    You are right. They are using tornado. How do I implement it on tornado? or I can simply use node.js? since i tried facebook on firebugs and found that they were using long-polling php as well. Should I set up another server to get node.js/tornado work?
    
    Thanks!
Payal Gupta says:

December 7, 2012 at 5:24 pm

hi i am using your script but not getting the correct manner to use it.While i used it i have got some error like / comet.php

Warning: touch() [function.touch]: Unable to create file /dev/customAlert2.comet because No such file or directory in C:\xampp\htdocs\realtime\NovComet.php on line 73
false

in firebug response box.
Actually i want to make real time notification in my php website but not geeting any idea.as go through the google i got your script but not working it too for me please help me.
thanks

Reply
1. Gonzalo Ayuso says:
  
  December 10, 2012 at 1:51 pm
  
  In the example I’m using /dev/shm to store files.That’s because it’s virtual file system mounted in memory (Linux systems). As I see within your comment you are using Windows, so you need to change the path of the files (hardcoded in DEFAULT_COMET_PATH constant). Anyway I recommend to use another alternatives with node.js or react (with PHP). The approach of this post works, but it will throw problems if you need to scale. (you can see examples in newer posts here for example)
  
  Reply
Chris says:

August 29, 2013 at 3:10 am

I’ve been programming notifications and all kinds of real-time updates (eg. live-chats) for the last 10 years using PHP and plain Javascript! “impossible 5 years ago”. What are you talking about???

Reply
1. Gonzalo Ayuso says:
  
  August 30, 2013 at 8:00 pm
  
  I sad that it was “almost” impossible. In fact this implementation is also PHP and plain js and it was available 5 years ago, but nowadays websockets plays in another league. Without websockets we could do it using exotic implementations (like the example of this post) or “emulate” real time with timers. The problems. With a limited number of users those implementations was viable. I have something similar than this post in production for for years in a intranet and it works like a charm indeed, but I now that that if I need to scale it I will have serious problems. Nothing compared to the simplicity of a node.js server with socket.io or even PHP with rachet.
  
  Reply
Claudio says:

August 29, 2013 at 11:30 am

Can you reliably compare timestamps issued by different machines?

Reply
1. Gonzalo Ayuso says:
  
  August 30, 2013 at 9:45 am
  
  If they have the same time (synchronized with a NTP server), it should be. Maybe it’s not perfect for ultra critical situations, but good enough for the rest of them
  
  Reply
Amar says:

September 2, 2013 at 6:29 am

I am getting syntax error

Parse error: syntax error, unexpected T_FUNCTION in /home/user/web/test/notification/NovComet.php on line 44

This is line no. 44
$callback = function($id) use ($defaultCometPAth) {

Reply
1. Gonzalo Ayuso says:
  
  September 2, 2013 at 1:00 pm
  
  it looks like you’re using a old version of PHP (at least 5.3)
  
  Reply
jlcurty says:

March 1, 2014 at 6:22 pm

hi, your example is working great “out of the box”,
I was wondering if it can support a publish action publish( ” my message”,”subscribed channel”) and subscribe (“subscribed channel” alert (“message” )?
thanks
jean-louis

Reply
jlcurty says:

March 1, 2014 at 6:24 pm

I was wondering if it was possible to modify it to support a publish function publish(“message”,”channel”) and a function subscribe ( “channel” ) ?
thanks,
jean-louis

Reply
ksprogrammer says:

April 9, 2014 at 3:14 pm

Great script, thank you

Reply
vivek says:

August 1, 2014 at 7:02 am

Hie i need a help in php. when i insert a data into sql at that time after trigger is fired i need to send the data to client atomatically

after trigger fired i need to run the php script automatically plz help for this problem

Thank you

Reply
1. Gonzalo Ayuso says:
  
  August 3, 2014 at 3:27 pm
  
  I don’t understand your problem. Maybe you need to have look to https://gonzalo123.com/2012/11/26/sending-sockets-from-postgresql-triggers-with-python/ or provably to https://gonzalo123.com/2013/12/24/integrating-websockets-with-php-applications-silex-and-socket-io-playing-together/
  
  Reply
Pingback: [html] Real time notifications | PipisCrew Official Homepage
Max says:

March 4, 2015 at 11:24 pm

I was expecting some comment inside the code but u did not provide the code with comet therefore it is almost impossible for a beginner to understand ur code

Reply
maulana says:

May 17, 2015 at 1:19 pm

How to use master ?

Reply
Jitesh says:

August 22, 2016 at 12:04 pm

I was wondering if it was possible to modify it to support a publish function publish(“message”,”channel”) and a function subscribe ( “channel” ) ?
thanks,

Reply
1. Ben says:
  
  September 22, 2016 at 3:29 pm
  
  @Jitesh have a look at ZeroMQ it works well, especially if you have nodejs background
  
  Reply
Ben says:

September 22, 2016 at 3:16 pm

considering this article is from 2011 and now its 2016 (going 2017), this was a good write-up…

I also came across ZeroMQ which is obviously using sockets to send/receive.. any comments on that?

Reply
Deb says:

November 29, 2016 at 9:19 am

I am getting error on log console. Notice: Undefined variable: out in D:\Xampp\htdocs\RTN\NovComet.php on line 63. what is $out on this file. and how to solve this. Thanks.

Reply
1. ben nxumaloBen says:
  
  November 30, 2016 at 2:33 pm
  
  I believe $out needs to be initialized maybe as an array inside that function…
  $out = array();
  
  Reply
Banago says:

April 14, 2017 at 5:49 pm

Considering this post was written 6 years ago, I find it very impressive. I’m wandering if you have replaced this solution already on your intranet. 🙂

Reply
1. Gonzalo Ayuso says:
  
  April 16, 2017 at 4:01 pm
  
  Of course not :). I use to use this tricky technique before websockets become mainstream. Nowadays I use socket.io for real time notifications. Anyway If I’d need to build a websocket system from scratch I don’t know if I’ll use socket.io or crossbar.io
  
  Reply
Nilay says:

September 4, 2018 at 5:48 am

i just want to add extra parameter can you guide me?

Reply