www.nico.schottelius.org/blog/puppet-sometimes-loads-a-class.mdwn

[[!meta title="Puppet: The quantum effect when loading classes"]]

After experiementing a bit with [puppet](http://reductivelabs.com/trac/puppet)
I found a very interesting phenomenon: Sometimes a host fails to load a class
with this error:

     err: Could not retrieve catalog: Could not find class common::nico_ethz at /etc/puppet/manifests/nodes/nico.pp:35 on node bach21.ethz.ch

The strange thing is that it *does* work sometimes. After describing
the situation in the [IRC channel #puppet](irc://irc.freenode.net/#puppet),
I got an "interesting" explanation for that behaviour:

First of all I did a mistake, because I placed the class
***common::nico_ethz*** into the file
**modules/common/manifests/nico.pp** instead of
**modules/common/manifests/nico_ethz.pp**.

But why does it work sometimes?
It works sometimes, because the ***puppetmaster*** compiles the catalog
for ***all*** nodes and reuses the compiled catalog for different
hosts. **If** a previous node loaded the class ***common::nico***,
the complete content of **modules/common/manifests/nico.pp**
is in the catalog, including ***common::nico_ethz***. This is the reason
why it sometimes works and that's also the reason why I am writing
this posting:

 * Dear other puppet users: Be aware that sometimes a class may be included
   indirectly and thus things work randomly (like
   [Schrödinger's cat](http://en.wikipedia.org/wiki/Schr%C3%B6dinger%27s_cat))!
 * Dear puppet developers: It would be way more helpful, if a wrong
   configuration **always** and not only **sometimes** fails!

If you've a comment to this blog article, please redirect it to
[the puppet users
mailinglist](http://reductivelabs.com/trac/puppet/wiki/GettingHelp#mailing-lists),
to which I sent a notice about this article.

[[!tag config unix]]