AJAX Long Polling In PHP MySQL (Simple Example)

Welcome to a tutorial on how to do AJAX long polling. So you want to run a real-time application, but web sockets are too complicated? Or maybe you have heard of this long polling thing from somewhere and wondering what it does?

var poll = () => {
  fetch("SCRIPT.PHP")
  .then(res => res.text())
  .then(txt => { /* DO SOMETHING */ poll(); })
  .catch(err => poll());
};
poll();

This is extremely useful for processes that require “live updates” – Live chat, live game scores, live news, etc…  Let us walk through a simple example of long-polling a game score update in this guide, read on!

TABLE OF CONTENTS

Download & Notes

PHP MYSQL Long Polling

Extras

The End

EXAMPLE CODE DOWNLOAD

Source code on GitHub Gist

Just click on “download zip” or do a git clone. I have released it under the MIT license, so feel free to build on top of it or use it in your own project.

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TUTORIAL VIDEO

PART 1) GAME SCORE TABLE

CREATE TABLE `score` (
  `time` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP,
  `home` int(3) NOT NULL,
  `away` int(3) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

ALTER TABLE `score`
  ADD PRIMARY KEY (`time`);

For a start, this simple table will store the game scores.

• time Time at which the score is recorded. Primary key and defaults to the current time.
• home The score for the home team.
• away The score for the away team.

PART 2) SERVER-SIDE PHP

<?php
class Score {
  // (A) CONSTRUCTOR - CONNECT TO DATABASE
  protected $pdo = null;
  protected $stmt = null;
  function __construct () {
    $this->pdo = new PDO(
      "mysql:host=".DB_HOST.";dbname=".DB_NAME.";charset=".DB_CHARSET,
      DB_USER, DB_PASSWORD, [
      PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
      PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC
    ]);
  }

  // (B) DESTRUCTOR - CLOSE CONNECTION
  function __destruct () {
    if ($this->stmt !== null) { $this->stmt = null; }
    if ($this->pdo !== null) { $this->pdo = null; }
  }

  // (C) GET LATEST SCORE
  function getScore () {
    $this->stmt = $this->pdo->prepare(
      "SELECT *, UNIX_TIMESTAMP(`time`) AS `unix`
       FROM `score` ORDER BY `time` DESC LIMIT 1"
    );
    $this->stmt->execute();
    return $this->stmt->fetch();
  }
}

// (D) DATABASE SETTINGS - CHANGE THESE TO YOUR OWN!
define("DB_HOST", "localhost");
define("DB_NAME", "test");
define("DB_CHARSET", "utf8mb4");
define("DB_USER", "root");
define("DB_PASSWORD", "");

// (E) CHECK FOR SCORE UPDATES
if (isset($_POST["last"])) {
  // (E1) SET TIME LIMIT
  set_time_limit(30); // set an appropriate time limit
  ignore_user_abort(false); // stop when long polling breaks

  // (E2) LOOP UNTIL THERE ARE UPDATES OR TIMEOUT
  $_SCORE = new Score();
  while (true) {
    $score = $_SCORE->getScore();
    if (isset($score["unix"]) && $score["unix"] > $_POST["last"]) {
      echo json_encode($score);
      break;
    }
    sleep(1); // short pause to not break server
  }
}

The PHP script looks complicated, but keep calm and look carefully:

• (A, B, E2) When $_SCORE = new Score() is created, the constructor will connect to the database. The destructor closes the connection.
• (D) Remember to change the database settings to your own.
• (E) The HTML page will send $_POST["last"] over, a Unix timestamp of the last score update it got.
• (E1, E2, C) This part then fetches the latest score from the database in a loop. Respond only when there is an updated score.

PART 3) CLIENT-SIDE HTML JAVASCRIPT

<!-- (A) HTML SCOREBOARD -->
<div id="sTime"></div>
<div id="sBoard">
  <div id="sHome"></div>
  <div id="sAway"></div>
  <div id="sHomeT">Home</div>
  <div id="sAwayT">Away</div>
</div>
 
<script>
// (B) LAST UPDATED TIMESTAMP
var last = 0;
 
// (C) AJAX LONG POLL
function poll () {
  // (C1) FORM DATA
  let data = new FormData();
  data.append("last", last);
  console.log("Fetch run", last);
 
  // (C2) FETCH UPDATE ON SERVER RESPONSE
  fetch("2-score.php", { method:"POST", body:data })
  .then(res => res.json())
  .then(data => {
    // (C2-1) UPDATE HTML DISPLAY
    document.getElementById("sTime").innerHTML = data.time;
    document.getElementById("sHome").innerHTML = data.home;
    document.getElementById("sAway").innerHTML = data.away;
 
    // (C2-2) NEXT ROUND
    last = data.unix;
    poll();
  }) 
 
  // (C3) CATCH ERROR - LOOP ON TIMEOUT
  .catch(err => poll());
}
 
// (D) GO!
window.onload = poll;
</script>

As in the introduction, this does an AJAX request that is set to loop itself infinitely to check for updates from the server.

1. HTML “scoreboard” to display the scores.
   • <div id="sTime"> The time when the score update is received.
   • <div id="sHome"> Home team score.
   • <div id="sAway"> Away team score.
2. var last is used to hold the Unix timestamp of the current score update.
3. poll() Will send last to the server to check for score updates.
   • (C2 & C3) Notice how it loops itself endlessly to keep on getting “live updates”.

CHANGING THE TIMEOUT LIMIT FOR FETCH

// (A) NEW ABORT CONTROLLER - 5 SECONDS
const controller = new AbortController();
const timeout = setTimeout(() => controller.abort(), 5000);

// (B) FETCH WITH ABORT CONTROLLER
fetch(url, { signal: controller.signal })
.then(res => res.text())
.then(txt => { ... });

• Please take note that at the time of writing, the timeout limit for fetch() depends on the browser.
• Chrome seems to be 300 seconds and Firefox is 90 seconds.
• The “official way” to set the timeout for fetch() is seeming an AbortController. Credits to this post on Stack Overflow.

LIMITATIONS – LONG POLLING VS WEB SOCKET

Congratulations code ninja, you have mastered the arts of long polling. But there is one thing you need to understand – It is not a replacement for web sockets… Not even close. In a nutshell:

• An instance of the PHP server script will run for every client connection. If there are 1000 connections, the server will spawn 1000 instances.
• But when it comes to web sockets, we only create 1 instance of the server script to serve 1000 connections.

So yeah. Long polling may be a shortcut fix for servers that are not too busy, but it is a far cry from a true solution for a real-time system. Go invest some of your time in learning web sockets if you are interested.

PUSH NOTIFICATIONS

Also, times have changed. We no longer live in the Stone Age of the Internet, and there is something called “push notification”. Yep, this makes a lot more sense when pushing out updates and news. Check out the links below for more examples of push and WebSockets.

LINKS & REFERENCES

• PHP Push Notifications – Code Boxx
• Add push notifications to a web app – Google Developer
• PHP Live Chat – Code Boxx
• NodeJS Live Chat – Code Boxx