+1, this idea is doused in awesome sauce.
On Mon, Mar 30, 2009 at 7:38 PM, Luke Kanies <l...@madstop.com> wrote:
> Hi all,
>
> We at Reductive Labs keep running into clients who need something like
> an attribute class - that is, for a given module, they want a single
> class that handles all of the variable setting and overriding, and
> then they want that attribute class to be merged into all or some of
> the classes in the module. (Teyo has especially been pushing me to
> solve this problem.)
>
> This looks a lot like composition, which Puppet doesn't currently
> support - you include one class's behaviour in another, rather than
> inheriting. This lack of composition is, I think, partially why
> people keep trying to do variable inheritance and getting frustrated
> that it doesn't work well.
>
> While doing a code audit for a client this week, though, I found their
> solution to this problem to be close enough that I was able to take it
> almost the rest of the of way. They had what amounts to a 'data'
> module with a class for each class that needed external data, and then
> they defined all of the attributes in that data class. That is, if
> you have an 'apache' class, you'd also have a 'data::apache' class
> with a bunch of attributes. Then all of your usage of those
> attributes would say '$data::apache::variable'.
>
> The big benefit of this model for them is that it allows clean change
> control of all of the important configuration data (as opposed to
> manifest structure and resources), and, again, it looks a lot like
> composition. In their case, they branch this module for all of their
> environments, but none of the other modules.
>
> As soon as I saw it, though, I thought of a way that might make it
> better, so I wrote a function that enables that way (and a converter
> for their existing data).
>
> Basically, I created (and have attached) a simple function that knows
> how to find and load a yaml file from a data directory, and it loads
> that file as a hash of parameters that should be set as local
> variables in the class.
>
> For instance, say you have this class apache; you'd create this file:
>
> data/apache.yaml
>
> And put all of the attributes you care about in that file.
>
> Then, in your apache class, you'd say:
>
> class apache {
> load_data()
> ...
> }
>
> It would pick the right file based on the class name (although the
> current function allows you to specify the class, also), load it, and
> set all of the contained attributes as local variables in the class.
> So, if your apache.yaml file looks like this:
>
> ---
> host: myhost.com
> port: 80
>
> Then this 'load_data' call is equivalent to this Puppet code:
>
> class apache {
> $host = "myhost.com"
> $port = 80
> }
>
> The function is currently set up to support one big 'data' directory
> for all of your modules. One could argue that it should instead
> support a data file per module, but the benefit of this one big data
> directory is that it makes it *much* easier to write sharable modules
> - you extract all of your site-specific data into this data dir, and
> you share the module with essentially no site data. It's probably
> most reasonable to support an in-module data file and a site-wide data
> directory to make it easy to provide defaults.
>
> I'm beginning to think that this, or a function a lot like this,
> should be included directly into Puppet, and data should get loaded
> automatically, rather than requiring the call to the 'load_data'
> function.
>
> What do you think?
>
> --
> Fallacies do not cease to be fallacies because they become fashions.
> --G. K. Chesterton
> ---------------------------------------------------------------------
> Luke Kanies | http://reductivelabs.com | http://madstop.com
>
> >
>
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