Question

Here’s my code:

      $cachefile = "cache/ttcache.php";
      if(file_exists($cachefile) && ((time() - filemtime($cachefile)) < 900))
      {
            include($cachefile);
      }
      else
      {
            ob_start();

            /*resource-intensive loop that outputs 
            a listing of the top tags used on the website*/

            $fp = fopen($cachefile, 'w');
            fwrite($fp, ob_get_contents());
            fflush($fp);
            fclose($fp);
            ob_end_flush();
      }

This code seemed like it worked fine at first sight, but I found a bug, and I can’t figure out how to solve it. Basically, it seems that after I leave the page alone for a period of time, the cache file empties (either that, or when I refresh the page, it clears the cache file, rendering it blank). Then the conditional sees the now-blank cache file, sees its age as less than 900 seconds, and pulls the blank cache file’s contents in place of re-running the loop and refilling the cache.

I catted the cache file in the command line and saw that it is indeed blank when this problem exists.

I tried setting it to 60 seconds to replicate this problem more often and hopefully get to the bottom of it, but it doesn’t seem to replicate if I am looking for it, only when I leave the page and come back after a while.

Any help?

Answer 1

In the caching routines that I write, I almost always check the filesize, as I want to make sure I’m not spewing blank data, because I rely on a bash script to clear out the cache.

if(file_exists($cachefile) && (filesize($cachefile) > 1024) && ((time() - filemtime($cachefile)) < 900))

This assumes that your outputted cache file is > 1024 bytes, which, usually it will be if it’s anything relatively large. Adding a lock file would be useful as well, as noted in the comments above to avoid multiple processes trying to write to the same lock file.