Tag Archives: apache

Learning from my Apache/ CDN mistakes

Earlier this year I configured this here little blog to offload static resources to MaxCDN. I made some mistakes in the process, which I documented in a blogpost that has been in my drafts for too long. So here’s the gist;

  1. Etags misconfiguration:
    • Problem: By default Apache uses the file’s inode to calculate the Etag, but that inode is unknown to the CDN, so Etags can never match.
    • Solution: change the Apache config not to use inode, e.g. “FileETag MTime Size”
  2. Cookies & domains issues:
    • Problem: I created my CDN-domain as a sub-domain of my main one, which led to (small) performance and (potentially huge) security issues.
    • Solution: Make sure your CDN-domain is different from the domain on which your cookies are set. So for example for main domain blog.futtta.be don’t use static-cdn.blog.futtta.be, but rather blog-cdn.futtta.be (except if cookies would be set on futtta.be, in which case I would need cdn-futtta.be).
    • Todo: it might make sense to add some Apache magic to make sure that for requests from the CDN:

3 Apache mod_cache gotchas

If you want to avoid the learning curve of Squid and Varnish or the cost of a dedicated caching & proxying appliance, using Apache with mod_cache may seem like a good, simple and cheap solution. Rest assured, it can be -to some extent- but here are 3 gotchas I learned the hard way:

  1. mod_cache ignores Cache-control if Expires is in the past (which it shouldn’t according to RFC2616), so you might have to unset the Expires-header.
  2. mod_cache by default caches cookies! Let me repeat; cookies are cached! That might be a huge security-disaster waiting to happen; sessionid’s (that provide access for logged-on users) are generally stored in cookies. If a logged on user that request an uncached page, then that user’s cookie will get cached and sent to other users that request the same page. Do disable this by adding “CacheIgnoreHeaders Set-Cookie” to your config
  3. mod_cache by default treats all browsers like the one that triggered the caching of the object. In the field that approach can cause problems with e.g. CSS-files that are stored gzipped (because the first browser requested with header “Accept-Encoding: gzip, deflate”). If a browser that does not support gzipped content requests the same file, the CSS will be unreadable and thus not applied. The solution; make sure the “backend webserver” sends the “Vary: Accept-Encoding” header in the response (esp. for CSS-files). This will tell mod_cache to take different Accept-Encodings into account, storing and sending different versions of the same CSS-file.

Drupal, mod_cache & RFC2616 caching

Suppose you’re setting up a Drupal-based site for which you have to implement a caching reverse proxy and for reasons beyond your comprehension Varnish (or even Squid) are not an option. Oh no, you’re stuck with Apache’s mod_proxy and mod_cache! What should you do?

First of all, Drupal 6 doesn’t like reverse proxies. If you don’t want to wait for version 7, which should do better in this respect, you might want to look at Pressflow. This Drupal 6 “distro” has everything on board to work with reverse proxies. So install Pressflow (or try to apply this out of date diff to stock Drupal) and in the Performance-screen set “Caching Mode” to “External” and “Page Cache Maximum Age” to the number of minutes you consider a cached page valid. Voila, you’re done in Drupal (edit: almost, as you might also want to change the $base_url in sites/default/settings.php to reverse proxy URL after you configured Apache).

Next up: Apache! A simple configuration like this one should do the trick:

ProxyRequests Off
ProxyPass /rp_drupal http://localhost/pressflow
ProxyPassReverse /rp_drupal http://localhost/pressflow
CacheEnable disk /rp_drupal/
CacheRoot c:/TEMP/apacache
CacheDefaultExpire 3600

OK, this must surely work, no? Well it should, but it doesn’t! When setting your Apache-loglevel to debug you’ll see “not cached” entries in your error-log, with the following reason:

Expires header already expired, not cacheable

Expires in the past, what does Pressflow think it’s doing deep down in includes/bootstrap.inc?

// HTTP/1.0 proxies do not support the Vary header, so prevent any caching
// by sending an Expires date in the past. HTTP/1.1 clients ignores the
// Expires header if a Cache-Control: max-age= directive is specified (see RFC
// 2616, section 14.9.3).
drupal_set_header('Expires', 'Sun, 11 Mar 1984 12:00:00 GMT');
// [...]
$max_age = variable_get('cache', CACHE_DISABLED) == CACHE_AGGRESSIVE && (!isset($_COOKIE[session_name()]) || isset($hook_boot_headers['vary'])) ? variable_get('page_cache_max_age', 0) : 0;
$default_headers['Cache-Control'] = 'public, max-age=' . $max_age;

Darn, those Pressflow-guys seem to have read up on their RFC’s! And indeed, 2616 confirms that cache-control’s max-age overrules expires;

If a response includes both an Expires header and a max-age directive, the max-age directive overrides the Expires header, even if the Expires header is more restrictive. This rule allows an origin server to provide, for a given response, a longer expiration time to an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache.

Mod_cache’s code seems to take a much simpler approach; at line 503 it decides not to cache based on an Expires-header in the past, totally dismissing the potential presence of cache-control’s max-age.

else if (exp != APR_DATE_BAD && exp < r->request_time)
    {
        /* if a Expires header is in the past, don't cache it */
        reason = "Expires header already expired, not cacheable";
    }

But you’re not interested in code which does or does not adhere to whatever RFC some spec-buffs came up with, you just want to cache your frigging’ Drupal-site! Well, fear not little hacker-boy, here’s some Apache-magic to cure your ailments, to be copy/pasted in the config before ProxyPass and ProxyPassReverse:

<Location /rp_drupal>
     SetEnvIf Request_Protocol "HTTP/1.1" expires_overrule
     # homework: add a SetEnvIf to see if cache-control max-age is present
     Header unset Expires env=expires_overrule
</Location>

So there you have it, a rudimentary caching setup for Drupal (in the guise of Pressflow) using nothing but Apache’s mod_proxy and mod_cache. Now go do your homework and test and do some finetuning and test some more. Happy caching!