WP Super Cache (and I presume other caching plugins) will delete everything in the cache directory. It’s like putting important files in /tmp/ where files are routinely cleaned out on reboot.

My replies on the thread above might paint me as a cold heartless bastard but I am sorry those websites suffered data loss. However I’m shocked that they put links to uploaded images in a folder containing temporary files!

Edit (20 minutes later): the codex page has been updated, thanks Westi for your help. It now recommends using wp-content/ms-filemap/ rather than wp-content/cache/

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Mobile plugins need to tell WP Super Cache what user agents they support. I documented the filters you can use (“cached_mobile_browsers” and others) to do this but I don’t think they’ve been used by any plugin. It’s not hard to do and I added code that checks for WPTouch so when you visit the WP Super Cache settings page it updates the mobile user agents. So far it works for me but please feel free to view this site on your mobile phone and tell me if it looks ok! I also added support for the theme switcher in WPTouch based on code posted on the wporg forum.

It appears that the “random post on the front page” problem is a side effect of how PHP works. WordPress incorrectly reported that the current page wasn’t a search page, even though it was. I put an extra bit of code in checking if $_GET is non empty and that fixed it, so far.

Just in case anyone else is interested, this is why is_search() fails randomly when run during PHP shutdown. When a PHP process shuts down it starts by killing off objects. Unfortunately this happens before PHP stops executing your code, something that changed after PHP4.

Anything that runs when the output buffer finishes or that is registered by register_shutdown_function() better not depend on objects or classes. That means no using $wpdb, the object cache may disappear or to be completely paranoid don’t expect $wp_query to be around either! The functions is_search(), is_feed() and other related WordPress functions depend on $wp_query so you should cache the values of those functions earlier in the process. I’m thinking of hooking into wp_head but that depends on the theme unfortunately. Not every theme uses that action.

Many years ago I changed the format of the cache “meta file” from an object to an array because of the way the PHP desctruction process works. More recently, but still two years ago I had to remove all calls to get_option() and update_option() in the output buffer handler because occasionally people saw the error, “Call to a member function get() on a non-object in cache.php” in their error log. The object cache object had been destroyed by PHP! That was a right pain to figure out as the code looked perfect yet didn’t work right some of the time.

To help figure out problems I’ve added a lot more debugging to the plugin too. If $wp_query disappears it’ll appear in your debug log, and preloading will generate a lot more messages now.

Next up is caching is_search(), is_feed(), is_single() etc. Where should those be cached? The “init” action is too early for is_search and probably others but I don’t want to depend on actions that may not be in a theme.

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The first release of this plugin was in September 2009 so this has been a bit too long in the making. Back then the main caching plugin was WP Cache, which this plugin is based on, but now there are quite a few including W3 Total Cache, Hypercache, Quick Cache and many more.

Version 1.0 is an incremental upgrade from the previous release but it has a number of bugfixes and new features:

1. The all new scheduler and admin bar link should please many people, especially those who want to clear the cache on their site at particular times. The “Delete Cache” admin bar link is something people have been asking about for as long as WordPress has had that bar!
2. You can preload categories and taxonomies although this hasn’t really been tested as well as I’d like. It works for me but YMMV.
3. There’s better support for mobile and https users now. Mobile support should scale a lot more than previously.
4. The cache tester is fixed now and the plugin traps as many errors as possible. It’ll also spit out some helpful text if there is a problem. 99% of the time it’s because the server can’t request a page from itself.
5. At the risk of annoying users who comment on your site you can make them appear to be anonymous users which will drastically help your server if they like commenting a lot. Unfortunately it stops the comment form populating with their details so it might be worth using an external comment system like Intense Debate or Disqus!
6. The Advanced Settings page now lists a “do not cache page” secret key. Use this key to view any page of your site uncached.
7. The cache file listing and delete links should work again now.
8. And many many more bug fixes.

Once you upgrade go visit the settings page and check out what’s new there. The upgrade worked fine for me, but there’s no harm looking. Pay close attention to the new garbage collector. The scheduler is rather powerful and flexible so it’s worth setting up right.

If you have any problems please leave a comment here or use the support forum.

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Another major change was the addition of a “Delete Cache” link on the admin bar for logged in users. This may seem strange, but it will only show if you have disabled caching for known users. Unfortunately this is needed because of the security measures required. It only deletes the supercache file(s) for the current page, not any legacy files associated with it.

If you’re feeling adventurous go download the development version from this page and give it a whirl. I really need feedback that this works well so please leave a comment if you try it.

The version number won’t change and when the next release comes out you’ll still get the upgrade notification!

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Lupus causes the body’s immune system to go into overdrive and starts to attack itself. It is believed that over 50,000 people throughout the UK suffer with Lupus of whom 90% are female. The symptoms are many and varied, and the condition often seems to mimic other diseases. This gives rise to difficulty in diagnosis and the condition can be overlooked for years, unless the GP or consultant is alert to the possibility of lupus.

The Trust’s website must have experienced a torrent of traffic because Mr Fry is followed by gazillions of people and the tweet was retweeted by over 100 of his followers. The website stayed up and I’m glad that WP Super Cache played a part in helping them.

I wanted to mark the occasion of a website getting a Frying and surviving so I changed the “Digg proof” message in the plugin to “Stephen Fry Proof”. Download the development version from this page if you want a look. The link in the admin page goes to the Trust’s tweet so I hope I can play a small part in spreading the word.

Of course, just because you’re using this plugin is no guarantee that your site will survive a mention by Mr Fry or any other celebrity on Twitter (especially if your hosting is simply too limited) but With WP Super Cache at least it has a fighting chance!

BONUS: In case you didn’t see it, here’s Stephen Fry ag caint as Gaeilge (talking in Irish) as he appeared on an Irish language tv programme a while back:

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Some of the changes:

• The cache preloader will now preload tags and categories pages (and whatever other taxonomy you want by using the wp_cache_preload_taxonomies filter). It should also preload pages with custom post types, but I haven’t tested that yet as I don’t have any such pages here. I hope Adam does at least!
• You should be able to delete files from the cache file listing again.
• You can make “known users” anonymous so they’re served Supercache cache files. Logged in users will appear logged out and users who leave comments won’t have the comment form filled in however.
• And quite a few more bugfixes.

Only thing is, I need your help. The development version is running on this multisite install of WordPress and it’s working fine but I really need feedback, especially before 1.0 comes out.

So, if you’re comfortable with WordPress please go visit the download page and install the development version. Thanks!

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This version should be significantly faster if you have a lot of mobile visitors, plus it adds support for https secure pages. The speed came about by saving those files as static files in the supercache directories rather than the old “legacy caching” of WP Cache. You’ll see files like index-mobile.html(.gz) and index-https.html(.gz) in those directories. Be aware that if you use a flat permalink structure the plugin opens the root supercache directory for reading to find the index* files. This may or may not be an issue with directories containing a very large number of directories (10,000+ for example).

HTTPS support is built into the mod_rewrite rules so those files are served by the web server directly, but PHP serves the mobile clients. Only now you’ll be able to cache many times more mobile pages than before. There’s negligible speed difference between PHP and mod_rewrite modes anyway in normal operation.

There’s also a handy little “donotcachepage” GET parameter now. Add that, giving it the value of a secret key from the Advanced settings page and the current page will not be cached in any way.

There is lots of new code in the plugin. It’s running fine here, and a few other people have had no problems but I need more feedback. If you have a development server please give this a go, and if it works ok I believe it’s good enough to go live, under supervision. Don’t go install it on a Friday afternoon…

With the upgrade to WordPress 3.2 I moved this site and my photoblog to a Multisite install so I’ve been looking at making Supercache work better in that environment too.

I also installed Varnish as a front end cache and woah, I’m impressed. Setup was fairly easy though it’s still not 100% right. When I make posts the following page is a blank page except for the blog title, and when I (as a logged in user) leave a comment I get a 500 error and wp-comments-post.php doesn’t redirect. I’m still trying to debug those issues. I mentioned this on Google Plus a few days ago but expect a follow up post on Varnish with docs in the near future.

Now all I need is traffic. I should blog more!

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Version 0.9.9.8 has been released and has a number of bug fixes and changes including:

1. CDN updates: it can be switched off. Multiple CNAMEs can be used for serving static content to improve performance.
2. Uninstall process improved. Files that the plugin creates are removed, and the wp-config.php and .htaccess files are fixed. If the plugin can’t do any of these steps it will display a message explaining what to do.
3. Cached dynamic pages can now be stored in Supercache files and compressed. No need to use legacy caching any more.
4. 1and1 Webhosting fix. They use “/kunden/” in some of their paths but not others.
5. I removed the “log by email” functionality as it caused problems for users who were inundated by email.
6. Many more minor fixes and changes.

The old uninstall process has been completely revamped. Now all you need to do is deactivate it on the plugins page. It will then remove wp-cache-config.php and advanced-cache.php. It will also remove the extra code it added to wp-config.php and the .htaccess file. On the off chance it can’t remove those files it will print a helpful message asking you to fix the problem. Unfortunately this class of plugin (full page caching plugins) are by design more complicated than regular plugins and are more invasive.

Got a problem with the plugin? Use the support forum please. Support queries posted here may be ignored or deleted.

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I added an action on the deactivate hook for the plugin so when you deactivate it on the Plugins page it deletes the files the plugin created and removes the WP_CACHE definition from wp-config.php. It doesn’t yet remove the mod_rewrite rules from the .htaccess file though. Sometimes people put the WordPress rules in the same block of that file so removing the rules would stop the site serving requests! I will of course gladly accept code patches to do that job.

No more messing with uninstall.php now, just deactivate on your plugins page.

Besides that, the CDN page has an on/off switch and can use multiple hostnames now. The cache can be cleared when caching is disabled and dynamic pages can be served by using PHP mode rather than the slower “legacy mode”.

There are also numerous bug fixes so if you have a development site and a few minutes to spare why not give this a go? Please?

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I’ve just released a new version and the biggest change in this one is the addition of Content Delivery Network (CDN) support thanks to Mark Kubacki who allowed me to integrate his OSSDL CDN Off Linker plugin. (Please go visit his blog and say thank you if you use this feature!)

The CDN support simply rewrites images, CSS and Javascript files so they point at a different hostname. That hostname can be another virtual host on your own server (aka “Poor Man’s CDN”) pointing at your WordPress install or a fully fledged CDN. If your CDN supports “origin pull” then all the files on your server will be copied there as they are requested by visitors. Otherwise you’ll have to transfer the files over manually.

Apart from that, bugs have been fixed, a few features have been tweaked including the uninstall script which has been streamlined somewhat.

I also added links (on the “Easy” settngs page) to some plugins and tools you may find useful in making your site run faster.

Problems? Go to the forum and someone may already have had that problem and solved it. If not, post there and you’ll get a reply.

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