Multiple caching: splitting the Web page content - PHP

Dynamic Web pages take longer to load than static ones, which forces visitors to your site to wait -- and we all know they won't wait for very long. Output caching is a powerful technique you can use to shorten that wait and keep them from leaving.

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By: Alejandro Gervasio

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January 11, 2005

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As stated above, since most websites are composed of a header section, body and footer section, we are going to implement server side caching for each part. Also, we need to set up some kind of criteria for checking whether cache files are valid. The most common caching policy approaches are the following:

Time triggered caching (expiry timestamp).
Content change triggered caching (sensitive content has changed, so cache must be updated).
Manually triggered caching (manually inform the application that information is outdated, and force a new cache creation).

For the scope of this article, we will use a time triggered caching policy for each part of the web page.

Let’s see how it works:

First, we define that the page header will have an expiry time of one day or 86400 seconds (only for purposes of example). Our body section will have an expiry time of ten seconds, and our page footer will be valid for one day.

Having defined expiry times for each section, let’s build two very intuitive and useful functions. The first one will take care of creating the cache files for the proper section. The second will check to see whether there is a valid cached version for the section. If there is, it will return the cached content extracted from the cache file. Otherwise, it will return false.

The functions are as follow:

function createCache ( $content , $cacheFile ) {
$fp = fopen( $cachefile , ‘w’ );
fwrite( $fp , $content );
fclose( $fp);
}
function getCache ( $cacheFile , $expireTime ) {
if ( file_exists ( $cacheFile ) && filemtime ( $cacheFile ) >( time() - $expireTime ) ) {
return file_get_contents( $cacheFile );
}
return false;
}

The function createCache takes two parameters: content from the output buffer and the cache filename where the output will be stored. Once the buffer content is obtained, it is stored as a file. Similarly, the function getCache accepts two arguments: the cache filename and expiry time (expressed in seconds). The function checks to see whether there is a cache file and whether it is valid. Cache validity is tested to see whether the cache file modification timestamp is greater than the current time less the expiry time. If the cache file is found to be valid, then cache content is returned. Otherwise it returns false.

Having defined this two core functions, we are able to build the new script:

<?php
//   start output buffering
ob_start();
// check if a valid header cache exists
if ( !$header = getCache( ‘headerCache.txt’ , 86400 ) {
// display header section
?>
<html>
<head>
<title>Cached Page</title>
</head>
<body>
The header section is updated on a daily basis.
<?php
$header = ob_get_contents();
ob_clean();
createCache( $header , ‘headerCache.txt’ );
}
// check if a valid body cache exists
if ( !$body = getCache( ‘bodyCache.txt’ , 10 ) {
// display body section
?>
<h1>This section is updated every 10 seconds.</h1>
<?php
$body = ob_get_contents();
ob_clean();
createCache( $body , ‘bodyCache.txt’ );
}
// check if a valid footer cache exists
if ( !$footer = getCache( ‘footerCache.txt’ , 86400 ) {
// display footer section
?>
The footer section is updated on a daily basis.
</body>
</html>
<?php
$footer = ob_get_contents();
ob_clean();
createCache( $footer , ‘footerCache.txt’ );
}
//   stops output buffering
ob_end_clean();
//   display the complete page
echo $header . $body . $footer;
?>

The script is pretty straightforward in its functionality. Let’s see what it does in detail.

Once output buffering is started, the script checks in turn for the corresponding cache files, which store header, body and footer sections. If the cache files are not valid based on expiry times given for each section, new content is generated, stored in the buffer and finally written to the proper cache files. When the whole page content is obtained either from the buffer or from cache files, it will be displayed in the user’s browser.

The script’s output is the following: