Handling gem dependencies in custom puppet providers

If you are writing or using custom puppet providers that rely on external libraries or gems, you might run into problems where puppet would either simply refuse to retrieve the catalog for you due to unmet dependencies or will need multiple runs to reach the desired state. I ran into these problems while trying out the sensu-puppet module which relies on the json library for its providers to work. More details documented in #17747 and other related tickets. Note : this applies for puppet 2.7.20, please refer to 3.x changelogs to see which 3.x version is this available in.

What we want:
1. Distribute the dependencies through our puppet modules, and
2. Make them available during the puppet run to be used by the custom providers without needing successive runs or failing catalogs.

To do this, we will add a custom feature and confine our provider on systems with this feature.

Features can be distributed like normal plugins, and they should be under module/lib/puppet/feature/foo.rb.
Example: sensu/lib/puppet/feature/json.rb

Puppet.features.add(:json, :libs => ["json"])

There are a bunch of standard features shipped with puppet and can be checked here.

Now, to confine the provider, for example, in the sensu_client_config provider, we can do:

require 'rubygems' if RUBY_VERSION < '1.9.0' && Puppet.features.rubygems?
require 'json' if Puppet.features.json?

Puppet::Type.type(:sensu_client_config).provide(:json) do
  confine :feature => :json
  ...
end 

So, if you do not have json lib available, you will simply see “Could not find a suitable provider for sensu_client_config” but the catalog will compile, and other modules applied without problem. This isnt possible if not using the above approach, in which case puppet will just fail to retrieve the catalog.

Now with a relationship like

Class["ruby::json"]->Sensu::Client[$::fqdn]

where ruby::json simply installs the json library and sensu::client uses sensu_client_config provider under the hood, you can be assured that puppet will do the right thing by checking again for the existence of the json library when it comes across it in sensu_client_config, and reach the desired state. Many thanks to Jeff McCune, Josh Cooper from PL & Dominic for their suggestions & consideration !