Debugging: Random Redirect and WP Super Cache

I think the quip, which I most often see attributed to Thomas Fuller, that “All things are difficult before they are easy” is so clearly borne out by debugging that the truth of it cannot be doubted. You can easily spend minutes, hours, even days bashing your head against a metaphorical wall only to notice that a misplaced colon — which you of course, didn’t realize was misplaced — was the cause of reams of unnecessary pain.

The Problem: Incompatible Rules

During my work on Kaleidoscope, that’s the theme this site is running, I decided that a random post link in the footer would be nice. A quick search yeilded Matt’s Random Redirect Plugin, which was more than up to the job. Without a need to reinvent the wheel, I just borrowed the core functionality of Matt’s plugin function and dropped it into my theme’s functions.php file.

And on my personal test server, it worked well. And when I put it on the Ikiru Demo Blog, there too it worked fine. But I found problems on Ikiru Design. More frustratingly, those problems would sometimes seem to suddenly disappear. (It was only later that I realized that it was whether or not I’d logged in that was the cause of that disparity.)

As you may suspect from what I’ve said so far, Ikiru Design has the WP Super Cache plugin running (though it has rarely needed it). Looking desperately to figure out why my redirect link in the footer worked on the demo blog but not on the main one, I decided to look through their directories.

Solution One: Changing .htaccess

Mercifully, I noticed that the .htaccess files were drastically different sizes. And I remembered that SuperCache depends on your making changes to that file. And indeed, comparing the two showed these lines added to Ikiru’s .htaccess file:

# BEGIN WPSuperCache

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{QUERY_STRING} !.*s=.*
RewriteCond %{HTTP_COOKIE} !^.*(comment_author_|wordpress|wp-postpass_).*$
RewriteCond %{HTTP:Accept-Encoding} gzip
RewriteCond %{DOCUMENT_ROOT}/wp-content/cache/supercache/%{HTTP_HOST}/$1/index.html.gz -f
RewriteRule ^(.*) /wp-content/cache/supercache/%{HTTP_HOST}/$1/index.html.gz [L]

RewriteCond %{QUERY_STRING} !.*s=.*
RewriteCond %{HTTP_COOKIE} !^.*(comment_author_|wordpress|wp-postpass_).*$
RewriteCond %{DOCUMENT_ROOT}/wp-content/cache/supercache/%{HTTP_HOST}/$1/index.html -f
RewriteRule ^(.*) /wp-content/cache/supercache/%{HTTP_HOST}/$1/index.html [L]

# END WPSuperCache

I now understand what this does, but at first I didn’t. It looked like a mash of random characters. But I know enough about programming conventions to guess that an “!” means not. And I also know enough about this modern world to know to Google things I don’t understand. “RewriteRule” seemed a reasonable phrase to start with.

I’ll spare you the full narrative of the search, but I’ll explain what I learned. RewriteCond provides the conditions that determine whether or not a RewriteRule is used by the server. Essentially, if a user trying to access a given file on the server gets through all the conditions in the RewriteCond stack, they’ll be made to follow the RewriteRule.

In this case, that means that if their query string — that’s the ?s or ?p=187 or ?random that tells the server what dynamic features are wanted — doesn’t start with ?s, and if the user doesn’t have a cookie specifying that they’ve either logged in or commented, and if the proper static file has been generated, send them that static file. (You see essentially the same rules there twice, the top one is for if you’re using compression, the other for if you’re not.) This is exactly how Super Cache is advertised to work.

But it means that the server will ignore the query string ?random that Matt’s Random Redirect function relies on. So the user will get, as I was, the cached version of the index page. And they’ll get it before Matt’s plugin has had a chance to well, redirect them.

Having figured that this was what was probably happening, I felt reasonably hopeful that I could finally fix this week-old problem. After all, the random link was working when I was logged in — thus not making me follow the RewriteRule — but it was failing when I wasn’t.

So I added was this line to the RewriteCond stacks:

RewriteCond %{QUERY_STRING} !.*random*

This adds an exclusion, which says that if the url looks something like “”, exclude the user from the RewriteRule. Uploading the modified file and opening the link from every conceivable page I could find showed that it was finally working.

Solution Two: Changing the Function

But this can’t be packaged into a theme. Fortunately, a usable but imperfect solution can be. The basic problem is that redirects break with WP Super Cache. The solution is to do this same thing without a redirect.

Essentially, all this take is simplifying Matt’s Random Redirect Plugin even further. The result was this function:

function sc_safe_random() {
	global $wpdb;
	$query = "SELECT ID FROM $wpdb->posts WHERE post_type = 'post' AND post_password = '' AND post_status = 'publish' ORDER BY RAND() LIMIT 1";
	$random_id = $wpdb->get_var( $query );
	return get_permalink($random_id);

This is essentially the first three lines of Matt’s function — which picks a random post from among those eligible on your blog — and adds a line that just tells it to pass the permalink for that post to the place where the function is called. Then you simply call the function for your random link, like:

<a href="<?php echo sc_safe_random() ?>">a random post</a>

This solution works, but it’s got a problem. If I were to click the redirect version into five new tabs from a single page, I’d receive five tabs each with a different random post in it. If I do the same with this version, I get the same post each time.

The likelihood of someone doing this is obviously questionable, but it’s a feature that I’d rather not lose. For that reason, I’m planning on making sure that I allow a user who doesn’t have WP Super Cache enabled, or who have implemented Solution One, to use the redirect version. Those who have Super Cache enabled, or who don’t mind the limitations of this second solution, would be able to continue to use it.

5 Responses to “Debugging: Random Redirect and WP Super Cache”

    Perhaps I should add that line to the plugin. I already use it on my own blog as I use the random post plugin there. If you remove the rules, and make sure you’re using the latest version of wp-super-cache it will add rules for the very handy “Subscribe to comments” plugin too!

    Thanks so much for the post! I just added that code right above the QUERY_STRING in the .htaccess and it worked like a charm.

    Worked beautifully, you’re a life saver (method 1). Will this have any effect on my posts if, for example, one of my posts has the word “Random” in the title?

    Again, thanks for the fix!

    Thank you for this post! It really helped me out today :)

    Diogenes said:  02/01/12


    Just commenting to say that your post helped me a lot! Thank You for your investigation on the issue and resolution. Now my Random Redirect works with WP Super Cache plugin seamlessly

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