httpd: mod_cache only caching your homepage

modcache has a pretty inflexible configuration setup.  [CacheEnable](http://httpd.apache.org/docs/2.2/mod/modcache.html#cacheenable) can only take a prefix of a path to be cached, and to disable a sub-path with CacheDisable, you need to list all of the possible prefixes (ie, no regular expressions).

Lets say you want to cache just your root page, aka ’/’, for your website, just in case you get hit by a Slashdot Effect.

For Apache httpd 2.2.12 or newer, you can do this by first enabling Caching on All pages, then setting the no-cache enviroment variable globally, and then unsetting it for a specific path:

CacheDirLevels 2
CacheDirLength 1
CacheEnable disk /
CacheRoot /var/cache/apache2/mod_disk_cache
CacheIgnoreHeaders Set-Cookie
CacheIgnoreNoLastMod On
CacheMaxExpire 600
SetEnv no-cache
<LocationMatch "^/$">
UnsetEnv no-cache
</LocationMatch>

For Apache httpd before 2.2.12, you need a different method of disabling caching globally, and then re-enabling it.  The easiest way is using mod_headers, to muck with Vary header

Header set Vary *
<LocationMatch "^/$">
Header unset Vary
</LocationMatch>

Strictly speaking, doing this to the Vary header is an RFC violation, and you best bet is to upgrade to a newer httpd version.  :-)

This works because mod_cache will refuse to cache any HTTP resource with a Vary value of ”*”, because this is saying that every response form the origin will be different.