Answered: Help! WordPress keeps missing my scheduled posts!

I gave a new client a totally wrong answer the other day.  Well, it was right as far as I always knew.  Scheduled posts have been a bane to a bunch of blogs I’ve seen. I’d thought that there were bugs in wp-cron, that had been fought for years and years by dozens of people and no one had yet found the right fix.. I just stumbled across this post to the support forum made a year ago by Otto, well known WordPress Guru Supreme, that proved I was completely mistaken. With the additional source of completely swamped servers, I think this post explains every source of recent missed posts I’ve seen:

Short answer: Add this to the to defines in your wp-config.php file:

define(‘ALTERNATE_WP_CRON’, true);

Really long answer, for masochists: Scheduled posts are not now, and have never been, “broken”. The developers of WordPress cannot fix it because there is nothing to fix. The problem lies in the fact that your server, for some reason, cannot properly execute the wp-cron process. This process is WordPress’ timing mechanism, it handles everything from scheduled posts to sending pingbacks to XMLRPC pings, etc. The way it works is pretty simple. Whenever a WordPress page loads, internally WordPress checks to see if it needs to fire off wp-cron (by comparing the current time with the last time wp-cron ran). If it does need to run wp-cron, then it tries to make an HTTP connection back to itself, calling the wp-cron.php file. This connection back to itself is there for a reason. wp-cron has a lot of work to do, and that work takes time. Delaying the user seeing his webpage while it does a bunch of stuff is a bad idea, so by making that connection back to itself, it can run the wp-cron program in a separate process. Since WordPress itself doesn’t care about the result of the wp-cron, it only waits a second, then goes back to rendering the webpage for the user. Meanwhile, wp-cron, having been launched, does its work until it’s finished or it runs out of execution time. That HTTP connection is where some badly configured systems fail. Basically, WordPress is acting like a web browser. If your site was http://example.com/blog, then WP will call http://example.com/blog/wp-cron.php to start the process. However, some servers simply can’t do that for some reason. Among the possible reasons:

  • Server doesn’t have DNS, and so it can’t figure out who “example.com” is, even though it is *itself*.
  • Server administrators, in a misguided attempt at security, have blocked “loopback” requests, so it can’t actually make a call back to itself.
  • Server is running something called “mod_security” or similar, which actively blocks the call due to brain-dead configuration.
  • Something else.

The point is that for whatever reason, your web server is configured in some non-standard way that is preventing WordPress from doing its job. WordPress simply can’t fix that. However, if you have this condition, there is a workaround. Add this to the to defines in your wp-config.php file:

define(‘ALTERNATE_WP_CRON’, true);

This alternate method uses a redirection approach, which makes the users browser get a redirect when the cron needs to run, so that they come back to the site immediately while cron continues to run in the connection they just dropped. This method is a bit iffy sometimes, which is why it’s not the default.

I would add three things to this explanation:

  • mod_security can be configured to block the ip address of the server, the domain name itself or even wp-cron specifically.  You can do some testing of this by doing a wget for your domain from your server. (wget http://example.com/wp-cron.php) and look at the results.  If there’s a problem and you can’t change the mod_security config itself, the fix may be configuring .htaccess to specifically allow from your server’s external IP address access to that file.
  • The loopback address failure can be caused by the Linux networking subsystem failing to load completely. Diagnosing that is off topic here, but you could look for it and see if you can “ping 127.0.0.1” and your domain from your server. If that works, loopback config isn’t the problem. If you it doesn’t, try running “ifconfig lo up” and doing it again. If it works, you’ve got some networking issues to resolve. If once device fails due to a config error, the rest don’t come up in systems I’ve seen.
  • It has been reported and disputed that the line in cron.php that calls the script has  a timeout that is too short or a value that is incompatible with some configurations. I also suspect that this can be a problem on extremely overloaded systems.  However, the fix might be worse than the original symptom.

The line in question is:

wp_remote_post($cron_url, array(‘timeout’ => 0.01, ‘blocking’ => false));

Some people have changed 0.01 to 1 and it works however, this adds a full second to every page load that calls wp-cron – and that could be very costly Google wise.  So be conservative  using this “fix”. On your overloaded system, adding milliseconds or seconds or more might not be noticeable, but then again – you probably have more pressing things you should be worrying about than a few missed posts.