Hi everyone,
mod_cache behaves strangely, but I'm not sure if this is expected
behaviour or not. I have these cache-related lines in my virtual host
definition:
CacheRoot C:/temp
CacheEnable disk /
And this PHP file called test.php in /:
<?php
/* Generate our ETag. Assume that generating the ETag is
* a whole lot less expensive than generating the content
* (e.g. it could be based on revision counts for documents
* from a database).
*/
$etag="\"ComputedETag\"";
header("Etag: $etag");
//Expires ages away
header("Expires: " . gmdate("D, d M Y H:i:s", time()
+ 60 * 60 * 24 * 30) . " GMT");
//Let's work out what's going on, shall we?
$fp=fopen('log.txt','a');
if (isset($_SERVER['HTTP_IF_NONE_MATCH']) &&
$_SERVER['HTTP_IF_NONE_MATCH'] == $etag) {
/* At a users' request, the cache has been bypassed, but the
* document is still the same. Avoid costly response generation
* and waste of bandwidth by just sending not-modified.
*/
header('HTTP/1.0 304 Not Modified');
fwrite($fp, date('r')." - Response: 304 Not Modified\n");
fclose($fp);
exit(); //Don't generate or send the body
}
fwrite($fp, date('r')." - Response: 200. Generated document.\n");
fclose($fp);
echo "Document body goes here";
?>
My web browser requests the document the first time, the (trimmed)
response is:
Status=OK - 200
Date=Mon, 20 Jul 2009 07:16:05 GMT
Expires=Wed, 19 Aug 2009 07:16:05 GMT
Etag="ComputedETag"
The log performed by test.php indicates:
Mon, 20 Jul 2009 19:16:05 +1200 - Response: 200. Generated document.
So far so good. But now I press refresh in my web browser. This makes a
conditional request for the document:
If-None-Match="ComputedETag"
Cache-Control=max-age=0
With the max-age of 0, the cache must be bypassed. That's perfectly
fine. I understand that if I include the directive
"CacheIgnoreCacheControl yes" in my virtual host, it will force it to
use the cache and not pass it on to the backend. That isn't what I want,
though, I want the backend to really validate that it is still up to
date so it doesn't serve a stale response, so I'm not using
CacheIgnoreCacheControl.
Anyway, the cache passes this conditional request onto the backend, and
the backend logs it:
Mon, 20 Jul 2009 19:16:12 +1200 - Response: 304 Not Modified
So the backend is trying to tell the client that it already has an
up-to-date body. But the response sent to the browser by the caching
system is:
Status=OK - 200
Date=Mon, 20 Jul 2009 07:16:12 GMT
Etag="ComputedETag"
Expires=Wed, 19 Aug 2009 07:16:12 GMT
Argh, there goes my monthly bandwidth allowance! Why isn't mod_cache
passing the 304 response code onto the client? I have users pressing
refresh on dynamically-generated image pages all the time and I need to
reduce bandwidth in this usecase.
My Apache is:
Apache/2.2.11 (Win32) DAV/2 mod_ssl/2.2.11 OpenSSL/0.9.8i SVN/1.6.3
PHP/5.3.0
Cheers,
Nicholas Sherlock
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