librato_metric.rb

require 'lazy'
require 'system_timer'

class LibratoMetric
  COUNT      = 10
  RESOLUTION = 1

  def initialize(metric_name, timeout = 4)
    @metric_name = metric_name
    @timeout     = timeout

    # Start loading data
    data
  end

  def data
    @data ||= Lazy.future do
      begin
        with_timeout do
          fetch(@metric_name, :count => COUNT, :resolution => RESOLUTION, :summarize_time => true)
        end
      rescue Timeout::Error, Librato::Metrics::CredentialsMissing
        nil
      end
    end
  end

  def value
    @value ||= begin
      if d = Lazy.demand(data)
        d.sum { |source, data| data[0]['value'] }
      end
    end
  end

  def time
    if measure_time
      measure_time + COUNT.minutes
    end
  end

  def measure_time
    @measure_time ||= begin
      if d = Lazy.demand(data)
        Time.at(d.collect { |source, data| data[0]['measure_time'] }.min)
      end
    end
  end

  def with_timeout(&block)
    SystemTimer.timeout_after(@timeout, Timeout::Error, &block)
  end

  def fetch(*args)
    client = Librato::Metrics::Client.new
    client.authenticate(Librato::Metrics.client.email, Librato::Metrics.client.api_key)
    client.fetch(*args)
  end
end