extract_backend.rb

# <modulepath>/extract_backend/lib/hiera/backend/extract_backend.rb
require 'yaml'

class Hiera
  module Backend
    class Extract_backend
      def lookup(key, scope, order_override, resolution_type)
        # Start your work here...
        Hiera.debug "We're in the lookup method!"

        # Use Backend.datasources to iterate through all data sources
        Backend.datasources(scope, order_override) do |source|
          # Get the path to the file we want to inspect by calling Backend.datafile
          # on the :extract_backend method and passing it the scope, source and file extension
          # This will query the hiera.yaml config file, look for the 'extract_backend' section,
          # and populate the path based on the values you pass.
          path = Backend.datafile(:extract, scope, source, 'yaml')
          next unless path

          Hiera.debug "Key: #{key} Path: #{path}"
          # Actually call the extract method below with the key and the path that
          # you retrieved in the previous step - it should return an answer
          value = extract(key, path)
          Hiera.debug "Your value is: #{value}"

          # Once you have a value from the etract method, we need to call Backend.parse_answer
          # with the value from the previous lookup and scope object - this will parse any
          # 'interpolation tokens' (e.g. %{environment}) out and replace it with actual values
          parsed_answer = Backend.parse_answer(value, scope)
          Hiera.debug "The parsed answer is: #{parsed_answer}"

          # FINALLY, check the resolution_type and return an answer.  If it's a priority
          # lookup, we'll just return the first answer we have. If it's :array or :hash,
          # then handle it accordingly...
          case resolution_type
          when :array
            Hiera.debug "Resolution type: array"
            @answer ||= []
            @answer << parsed_answer
          when :hash
            Hiera.debug "Resolution type: hash"
            @answer ||= {}
            @answer = parsed_answer.merge @answer
          else
            Hiera.debug "Resolution type: priority"
            # This means it's a priority lookup
            # We need the 'break' to jump out of the loop
            @answer = parsed_answer
            break
          end
          Hiera.debug "The @answer is: #{@answer}"

          @answer
        end

        @answer
      end

      def extract(key, path)
        # This is going to be a method VERY SIMILAR to the extract function we wrote
        # previously.

        datafile = YAML.load_file(path)

        # Handle any exception cases here
        #   (i.e. if the datafile is empty or doesn't have the key we want)
        return unless datafile
        return if datafile.empty?
        return unless datafile.include?(key)

        # Return the value
        return datafile[key]
      end
    end
  end
end