Exported vars through PuppetDB I find it useful sometimes to be able to share bits of information between nodes. For example, I can have all of the nodes in a cluster register their IP addresses.

example.pp

  ::magic::register { 'hadoop_datanode_fqdn':
   value => $fqdn,
  }

  ::magic::register { 'hadoop_datanode_ip':
    value => $ip,
  }

  $reg = query_registrations('hadoop_datanode_fqdn')
  
  # Now $reg has an array of all of the FQDN's of the hadoop datanodes
  

query_registrations.rb

Puppet::Parser::Functions::newfunction(:query_registrations, :type => :rvalue) do |args|
  require 'puppet'
  require 'puppet/util/puppetdb'

  host = Puppet::Util::Puppetdb.server || 'localhost'
  port = Puppet::Util::Puppetdb.port || 8080

  title = args[0]
  tag   = args[1]

  raise(Puppet::Error, 'Title is a required parameter') unless title

  if tag.to_s.empty?
    tag = title
  end

  # https://github.com/ripienaar/ruby-puppetdb
  # https://github.com/ripienaar/ruby-puppetdb/blob/master/lib/puppetdb.rb#L196
  def query_puppetdb(host, port, query)
    require 'puppet/network/http_pool'
    http = Puppet::Network::HttpPool.http_instance(host, port)

    headers = { "Accept" => "application/json" }

    if query == :empty
      resp, data = http.get("/nodes", headers)
      return PSON.parse(data)
    else
      type = query.keys.first

      case type
        when "resources"
          query = "/resources?query=%s" % URI.escape(query[type].to_pson)         
          resp, data = http.get(query, headers)
          return PSON.parse(data)
        when "nodes"
          query = "/nodes?query=%s" % URI.escape(query[type].to_pson)
          resp, data = http.get(query, headers)
          return PSON.parse(data)
        when "facts"
          query = "/facts/#{query[type]}"
          resp, data = http.get(query, headers)
          return PSON.parse(data)
      end
    end
  end

  def extract_registrations(nodes)
    registrations = Array.new
    nodes.each do |resources|
      simple_resources = simplify_resource(resources)
      registrations.push(simple_resources['parameters']['message'])
    end
    registrations
  end

  # converts a resource_hash returned by puppetdb into something
  # much simpler
  def simplify_resource(resource_hash)
    {
      'parameters'    => resource_hash['parameters'],
      'hash'          => resource_hash['resource'],
      'certname'      => resource_hash['certname']
    }
  end

  nodes = query_puppetdb(host, port, {"resources" => ["and", ["=", "type", "Notify"], ["=", "title", title], ["=", "tag", tag]]})
  extract_registrations(nodes)
end

register.pp

define magic::register($value, $customtag=$title) {
  @@notify { "${customtag}_${value}":
    message => $value,
    tag     => $customtag,
  }
}

This is based on some magic I learned from Andrew Gaffney many years ago, I had to write a new function to collect the registrations from PuppetDB though, his implementation used storeconfigs.