def track_external_change(app_id, action, document)
      return unless Conf.redis
      Conf.redis.rpush("#{KEYS[:changes]}-#{app_id}", MultiJson.encode([action, document]))
    end
def track_external_change(app_id, action, document)
      return unless Conf.redis
      Conf.redis.rpush("#{KEYS[:changes]}-#{app_id}", MultiJson.encode([action, document]))
    end
use this method when you are tracking the change of another app
 you must pass the app_id of the app being affected by the change