Caching With Apache's mod_cache On Debian Etch

Version 1.0
Author: Falko Timme <ft [at] falkotimme [dot] com>
Last edited 12/23/2008

This article explains how you can cache your web site contents with Apache's mod_cache on Debian Etch. If you have a high-traffic dynamic web site that generates lots of database queries on each request, you can decrease the server load dramatically by caching your content for a few minutes or more (that depends on how often you update your content).

I do not issue any guarantee that this will work for you!

1 Preliminary Note

I'm assuming that you have a working Apache2 setup (Apache 2.2.x - prior to that version, mod_cache is considered experimental) from the Debian repositories - the Apache version in the Debian Etch repositories is 2.2.3 so you should be good to go.

I'm using the document root /var/www here for my test vhost - you must adjust this if your document root differs.

2 Enabling mod_cache

mod_cache has two submodules that manage the cache storage, mod_disk_cache (for storing contents on the hard drive) and mod_mem_cache (for storing contents in memory which is faster than disk caching). Decide which one you want to use and continue either with chapter 2.1 (mod_disk_cache) or 2.2 (mod_mem_cache).

2.1 mod_disk_cache

The mod_disk_cache configuration is stored in /etc/apache2/mods-available/disk_cache.conf, so let's edit that one:

vi /etc/apache2/mods-available/disk_cache.conf

Make sure you uncomment the CacheEnable disk / line, so that the minimal configuration looks as follows:

# a2enmod-note: needs-configuration

<IfModule mod_disk_cache.c>
        CacheRoot /var/cache/apache2/mod_disk_cache

# If you enable disk caching, you need to use htcacheclean from the
# apache2-utils package to ensure that the cache does not grow indefinitely.
# See the htcacheclean man page for details.

# There is currently no mechanism in the Debian package to start htcacheclean
# automatically, but it is planned to add such a mechanism in the future.

        CacheEnable disk /

        CacheDirLevels 5
        CacheDirLength 3
</IfModule>

You can find explanations for these configuration options and further configuration options on http://httpd.apache.org/docs/2.2/mod/mod_disk_cache.html.

Now we can enable mod_cache and mod_disk_cache:

a2enmod cache
a2enmod disk_cache

/etc/init.d/apache2 force-reload

To make sure that our cache directory /var/cache/apache2/mod_disk_cache doesn't fill up over time, we have to clean it with the htcacheclean command. That command is part of the apache2-utils package which we install as follows:

apt-get install apache2-utils

Afterwards, we can start htcacheclean as a daemon like this:

htcacheclean -d30 -n -t -p /var/cache/apache2/mod_disk_cache -l 100M -i

This will clean our cache directory every 30 minutes and make sure that it will not get bigger than 100MB. To learn more about htcacheclean, take a look at

man htcacheclean

Of course, you don't want to start htcacheclean manually each time you reboot the server - therefore we edit /etc/rc.local...

vi /etc/rc.local

... and add the following line to it, right before the exit 0 line:

[...]
/usr/sbin/htcacheclean -d30 -n -t -p /var/cache/apache2/mod_disk_cache -l 100M -i
[...]

This will start htcacheclean automatically each time you start the server.