How Does Long Polling Efficiently Update Web Applications Without Constant Page Refreshing?

Understanding Long Polling: A Beginner's Guide

Long polling is a technique that allows web applications to efficiently wait for new data from the server without constantly refreshing the page. This is particularly useful for applications where data is updated infrequently, such as chat or social networking applications.

Implementing Basic Long Polling in PHP and JavaScript

While long polling can appear complex, its implementation can be surprisingly straightforward. Let's walk through a basic example using PHP and JavaScript:

PHP Script (msgsrv.php)

<?php if (rand(1, 3) == 1) {
    header("HTTP/1.0 404 Not Found");
    die();
}

sleep(rand(2, 10));
echo("Hi! Have a random number: " . rand(1, 10));
?>

This script simulates a simple message server. It generates a random number and sends it as a response, with a random delay between 2 and 10 seconds to mimic real-world conditions.

JavaScript Code (long_poller.htm)

    <script src="jquery.min.js"></script>
    <script>
        function addmsg(type, msg) {
            $("#messages").append("<div class='msg " + type + "'>" + msg + "");
        }

        function waitForMsg() {
            $.ajax({
                url: "msgsrv.php",
                success: function (data) {
                    addmsg("new", data);
                    setTimeout(waitForMsg, 1000);
                },
                error: function () {
                    addmsg("error", "Error");
                    setTimeout(waitForMsg, 15000);
                }
            });
        }

        $(document).ready(function () {
            waitForMsg();
        });
    </script>


    <div>
<p>This JavaScript code uses jQuery to request the PHP script asynchronously. If the request is successful, it adds the response to the #messages div. If the request fails, it displays an error and tries again after 15 seconds. The setTimeout function ensures that the browser requests the script at a regular interval, thus implementing long polling.</p>
<p><strong>Explanation</strong></p>
<p>This example demonstrates a simplified implementation of long polling. The PHP script simulates a server that sends data at irregular intervals. The JavaScript code repeatedly requests the script using AJAX, waiting for a response. When a response is received, the JavaScript code updates the UI. If there is no response, the script waits for a short period and tries again. This approach allows the web application to remain responsive while efficiently waiting for server-side updates.</p>
</div>

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