scout_metric.rb

require 'lazy'
require 'system_timer'

class ScoutMetric
  M60_ALPHA = (1 - Math.exp(-5.minutes / 1.hour.to_f))
  NS_IN_SECONDS = 1000000000

  def initialize(metric_id, timeout = 5)
    @metric_id = metric_id
    @timeout   = timeout

    @start = 6.hours.ago
    @cache = {}
  end

  def metric
    @metric ||= with_timeout { Scout::Metric.first(@metric_id) }
  end

  def timeseries
    @timeseries ||= Lazy.future do
      begin
        m = Lazy.demand(metric)
        with_timeout { m.average(:start => @start).to_array }
      rescue Exception
        []
      end
    end
  end

  def trend_value
    @trend_value ||= Lazy.future do
      ts = Lazy.demand(timeseries)
      values = ts.collect { |(time, value)| value }
      rate = change_per_hour(values)
    end
  end

  def change_per_hour(values)
    rate, rates = nil, []

    values.each_cons(2) do |first, second|
      current_rate = (second - first).to_f / 5.minutes / NS_IN_SECONDS

      if rate
        rate += (M60_ALPHA * (current_rate - rate))
      else
        rate = current_rate
      end
      rates << rate
    end

    rate *= NS_IN_SECONDS * 1.hour if rate
    rate
  end

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