PHP Programming

While trying to speed up my wordpress installation I noticed that there seemed to be a lot of plugins that generate a static home page – they all used complex methods to store the files, checking the last modified times and implementing complex checking methods to see if the content has changed.

It struck me as strange that no one had suggested using APC cache – I don’t know if this is because its not available on all commercial hosting packages (such as dreamhost) or if its just due to lack of knowledge about the apc_store and apc_fetch commands. APC doesn’t just cache PHP opcode, it can cache user defined PHP variables as well.

I’ve come up with a small script that is capable of caching my PHP pages generated by wordpress. It uses the APC cache to store and manage the page data and it guarantees that the page will never be more than a specified number of seconds out of date.

In order to use this plugin you only have to make 2 changes go index.php in the wordpress root. Firstly you place this code at the top of your index.php

<? 
   if (empty($_POST)) {

        $cache_key = md5($_SERVER['REQUEST_URI'] .serialize($_GET));
        $cache_data = apc_fetch($cache_key, $cache_result);

        if ($cache_result == true) {
            echo($cache_data);
            die;
        }

        ob_start();

    } 
?>

And then place this code at the base of index.php

<?
   if (empty($_POST)) {
    $cache_data = ob_get_clean();
    apc_store($cache_key, $cache_data, 300);
    echo($cache_data);
   } 
?>

The code works by creating a MD5 hash of the URL and the GET request – this is important as it means that pages that take variables as arguments, such as search, will create their own cached pages. We don’t use the cache if there are POST variables present as we can probably assume that the output of the page is very dynamic. If you don’t want to cache pages that use a GET request as well you could use something like this

if ((empty($_POST)) && (empty($_GET)) {

It would be unwise to cache separate pages for each set of POST data as POST data is generally much larger than data sent via GET.

We use the MD5 of the URI and GET parameters to act as a key for our storage, if we find the key in memory and it hasn’t expired, we can use the data retrieved from the key to display the page – this bypasses all the MySQL and other PHP that was needed to create the original page.

Wordpress Static vs Dynamic benchmarks

If we now look at some benchmarks of before and after we can see the difference. I’ve use ab (that comes with apache) to perform the tests

You can see that without the caching that the server can serve 10 requests per second and after the caching it can serve 136 requests per second. That’s a 13 times increase in speed. Not bad for a server with only 256mb of RAM and a single core CPU. I’m pretty sure the server could handle even more load but at this point the upstream connection became saturated.

There are however some pitfalls with caching. Currently when users are logged in we cache their page, we need to add a condition to prevent this caching. Its not a problem on my server however as I am the only personal allowed to blog. Other pitfalls are the freshness of data, we can set this to whatever we want (from 1 second to infinity) but its never going to be as “fresh” as a page that isn’t cached.

Caching isn’t for everyone but if you do want your site to run faster, can afford to have some slight inconsistencies in your data and don’t mind waiting a few minutes for a comment to appear then you can achieve some really good results.