The "Expires" header for caching to work

The page consists of dozens of files, and 95% of the files can be shared by all pages of the site. Caching was invented to avoid downloading this information every time.

There are 3 caching mechanisms:

The “cache-control” or “expires” headers indicate how long the content will be relevant. During this time, the user’s browser uses the saved copy of the file and does not make any requests to the server.
Caching via “etag”. The server generates a unique string based on the content and the user’s browser asks each time if the file has changed and provides this string.
The title is “Last-Modified”. The server reports the date of the last file change, and the browser asks each time if the file has changed since that time.

If the server does not use the “cache-control” or “expires” headers when transferring files, the browser will send a request to download files or check the cache by “etag” or “Last-Modified” every time the page is visited. This will slow down the page loading speed.

Instruction [How to set up caching] (/blog/11).

Demonstration of page loading speed with caching enabled and disabled:

Demonstration

PHP code

HTML code

Open a demo Download the example code

<?php
if(isset($_GET['img'])) {
    header('Content-Type: image/jpeg');
    echo file_get_contents('photo.jpg');
    die;
}
?><!DOCTYPE html>
<html>
    <head>
        <meta name="viewport" content="width=device-width, initial-scale=1.0">
        <style>
            .example {
                display: flex;
            }
            .img {
                position: relative;
            }
            img {
                width: 250px;
            }
            .time {
                position: absolute;
                left: 5px;
                top: 5px;
                background-color: #fff;
                padding: 1px 4px;
            }
            .btn {
                color: #000;
                display: inline-block;
                margin-bottom: 5px;
                text-decoration: none;
                padding: 5px 10px;
                border: 1px solid #000;
                border-radius: 5px;
            }
            .btn:hover {
                background-color: #ddd;
            }
            .btn.active {
                background-color: #000;
                color: #fff;
            }
        </style>
        <script>
            // Функция, которая добавляет на изображение время окончания загрузки
            function endLoading(el) {
                var time = performance.now()/1000;
                el.parentNode.insertAdjacentHTML("beforeend", '<div class="time">'+time+' с</div>');
            }
        </script>
    </head>
    <body style="font-family: sans-serif;">
        <a class="btn" href="./cache_time_header.php">Обновить страницу</a><br /><br />
        <div class="example">
            <div class="element">
                Кеширование отключено<br />
                <div class="img">
                    <img src="./cache_time_header.php?img=1" onload="endLoading(this)" />
                </div>
            </div>
            <div class="element">
                Кеширование работает<br />
                <div class="img">
                    <img src="./photo.jpg" onload="endLoading(this)" />
                </div>
            </div>
        </div>
    </body>
</html>

<!DOCTYPE html>
<html>
    <head>
        <meta name="viewport" content="width=device-width, initial-scale=1.0">
        <style>
            .example {
                display: flex;
            }
            .img {
                position: relative;
            }
            img {
                width: 250px;
            }
            .time {
                position: absolute;
                left: 5px;
                top: 5px;
                background-color: #fff;
                padding: 1px 4px;
            }
            .btn {
                color: #000;
                display: inline-block;
                margin-bottom: 5px;
                text-decoration: none;
                padding: 5px 10px;
                border: 1px solid #000;
                border-radius: 5px;
            }
            .btn:hover {
                background-color: #ddd;
            }
            .btn.active {
                background-color: #000;
                color: #fff;
            }
        </style>
        <script>
            // Функция, которая добавляет на изображение время окончания загрузки
            function endLoading(el) {
                var time = performance.now()/1000;
                el.parentNode.insertAdjacentHTML("beforeend", '<div class="time">'+time+' с</div>');
            }
        </script>
    </head>
    <body style="font-family: sans-serif;">
        <a class="btn" href="./cache_time_header.php">Обновить страницу</a><br /><br />
        <div class="example">
            <div class="element">
                Кеширование отключено<br />
                <div class="img">
                    <img src="./cache_time_header.php?img=1" onload="endLoading(this)" />
                </div>
            </div>
            <div class="element">
                Кеширование работает<br />
                <div class="img">
                    <img src="./photo.jpg" onload="endLoading(this)" />
                </div>
            </div>
        </div>
    </body>
</html>

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