Error during file write in simple PHP caching

Question

I made a simple database driven php website. Now i am trying to implement some simple caching to the site. i tried this from somewhere

<?php
      $reqfilename="test";
      $cachetime = 60*5; // 5 minutes
      $cachefile = "cache/".$reqfilename.".html";
      $cf2="cache/".$reqfilename."2.html";
      if (file_exists($cachefile) && ((time() - $cachetime) < filemtime($cachefile))) 
      { 
         include($cachefile);
         exit;
      }
      ob_start(); 
?>
CONTENT OF THE PAGE GOES HERE
<?php   
        $fp = @fopen($cf2, 'w'); 
        if($fp){
            fwrite($fp, ob_get_contents());
            fclose($fp); 
            rename($cf2,$cachefile);
        }   
        ob_end_flush(); 
?>

But what if the cache file is being renamed and someone requests the page. Will there be an error displayed or just the user will experience a delay?

To reduce the time of the cache file being modified only, I am using rename instead of directly writing on the original cache file

Correct code to do this (based on answer by webbiedave below)

<?php
      $reqfilename="test";
      $cachetime = 60*5; // 5 minutes
      $cachefile = "cache/".$reqfilename.".html";

      if (file_exists($cachefile) && ((time() - $cachetime) < filemtime($cachefile))) 
      { 
         include($cachefile);
         exit;
      }
      ob_start(); 
?>
CONTENT OF THE PAGE GOES HERE
<?php   
        $fp = @fopen($cachefile, 'w'); 
    if (flock($fp, LOCK_EX | LOCK_NB)) {
    fwrite($fp, ob_get_contents());
    flock($fp, LOCK_UN);
    fclose($fp); 
    }

    ob_end_flush(); ?>

webbiedave · Accepted Answer · 2011-12-20 19:58:56Z

1

Renaming the file isn't the source of your problem. The main race condition with your script is multiple requests detecting an expired mtime and all writing to test2.html.

A better approach is to perform an exclusive, non-blocking flock (assuming non-windows) immediately after detecting an expired mtime, buffer the output, overwrite the file and release the lock. If flock returns false, then another process is writing to it and the current process should skip the write entirely.

If a request is made during the lock, the web server will wait until the file write is complete (and the lock released), then serve the file.

edited Dec 20, 2011 at 19:58

answered Dec 20, 2011 at 19:50

webbiedave

49k8 gold badges92 silver badges104 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

PRYM Over a year ago

Thanks.. thats exactly what I was looking for.. earlier didn't thought of LOCK_NB. Thanks again.

Simon Strasser · Accepted Answer · 2011-12-20 19:37:41Z

0

I don't think your source will work properly because you don't delete the cache-file

answered Dec 20, 2011 at 19:37

Simon Strasser

412 bronze badges

1 Comment

PRYM Over a year ago

Why should i delete the cache file. The file is being renamed as the file which will be used to display the page.

Collectives™ on Stack Overflow

Error during file write in simple PHP caching

2 Answers 2

1 Comment

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related