Puppet, extlookup, and yamlvar

When you dive in a complex project, even though you try to be as good as possible with documentation, sometime you just miss things.

I’ve had a good opportunity to realize this just recently while working on puppet. A common head scratcher in puppet is finding a way to keep system, technology and platform specifics separate. I will probably write at length on this particular subject a bit later, but to give a quick explanation, just consider this quick scenario:

Puppet allows you to get to the point where you can just write this:

class webserver { 
    include unix
    include nginx

    nginx::upstream { rails_app:
        servers => ["", ""]
    nginx::upstream_vhost { "www.example.com":
        upstream    => rails_app,
        listen      => 80
    nginx::static_vhost { "static.example.com":
        root    => "/srv/www/static.example.com",
        listen  => 80

This is great, generic and allows you to deploy configurations to many machines, so it’s a bit of a hassle to have to go through the trouble of going through case statements just for DNS.

module Puppet::Parser::Functions
    require 'yaml'

    newfunction(:yamlvar, :type => :rvalue) do |args|
        defval = args[1]
        yaml_path = args[2] || lookupvar('yamlvar_data')

        unless yaml_path or (yaml_path = lookupvar('yamlvar_data'))
            raise Puppet::ParseError, "No configuration for yamlvar"

            data = YAML.load_file(yaml_path)
            raise Puppet::ParseError, "Cannot read yaml data"

        unless data['order']
            data['order'] = %w(fqdn operatingsystem)

        # parse preference tab in this context
        prefs = data['order'].map{|x| lookupvar(x)}.select{|x| x}
        prefs << 'default'

        val = data['values'][args[0]] or args[1]
        unless val 
            raise Puppet::ParseError, "Cannot find #{args[0]}"

        # map strings to hashes
        if val.is_a? Hash
            # find the union of keys in our order tab and keys available
            key = (prefs & val.keys).first

            # this cannot happen
            raise Puppet::ParseError, "Key is nil ?" unless key 
            val = val[key]

        raise Puppet::ParseError, "Cannot find val for #{args[0]}" unless val 

This allows me do do clever local variables like these:

order:  [ fqdn, operatingsystem ]
        www01.remote.example.com: ""
        default: ""

And call them from classes like this:

class unix {
    $ns_server = yamlvar(ns_server)
    file { "/etc/resolv.conf":
        owner => root,
        group => $root_group,
        mode => 0644,
        content => template("unix/resolv.conf.erb")

Well this is all fine and dandy until I came upon this: [complex data and puppet](http://www.devco.net/archives/2009/08/31/complex_data_and_puppet.php][complex data and puppet). That’s right, @ripienaar already did the work and guess what? puppet 2.6.1 and extlookup it’s already in puppet 2.6.1!