Paul's Journal

I started at Cloudkick in August, and today we announced our for-pay products & Freemium model.  (TechCrunch, GigaOm, ReadWriteWeb, VentureBeat, and more)

I’ve been working along with the entire Cloudkick Team on a few parts of our launch:

• Integration with Apache libcloud, so now Cloudkick supports EC2, Rackspace, Slicehost, RimuHosting, Linode, VPS.net and GoGrid.
• Our new monitoring Agent, Cloudkick Agent. Extremely light weight, written in C & Lua.  Hopefully I’l have some time to blog more about some of the cool technology we did here, but we were tired of seeing monitoring agents written in High Level languages using up tons of memory on a Cloud Server.
• Our Cloudkick Changelog Tool, aka “ckl”.  This tool lets you keep track of a large admin team and what everyone is doing.  The ASF Infrastructure team has already started using it outside of Cloudkick.  Of course, the Cloudkick UI is a little nicer than the demo one with the open source code.
• Our new Graphing and long term trending system, built on top of Reconnoiter and Apache Cassandra.
• Learned more about Django and JQuery than ever before.

Now that our big launch is out, hopefully I’ll find a little more time to post on this journal more.

mod_cache has a pretty inflexible configuration setup.  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.

Lets say you are running a website, and you don’t mind people hot linking images, like your Logo, or other resources, and at the same time, you want to enable a (short) Keep Alive timeout for your normal users.

Normal anti-hot linking recipes, like the one on the HTTPD Wiki are all about disabling access to the image completely.

If you have lots of people hot linking, these users can use up valuable Keep Alive sessions, so the easiest way to solve this problem is to disable Keep Alive for just those clients viewing a hot linked image.

This is possible by using mod_setenvif and the nokeepalive environment variable:

SetEnvIfNoCase Referer (.+) nokeepalive

SetEnvIfNoCase Referer (.*)example.com(.*) !nokeepalive

What this does is first disable KeepAlive for all users that have a Referer set, and then re-enable keepalive for those users who are coming from ‘exmaple.com’, which should be replaced with your site.