scrape with PrometheusMeterRegistry

Question

I have the following servlet:

@Servlet(\path : String -> path.matches("/metrics"))
class MetricsServlet_De extends HttpServlet {
  private var prometheusMeterRegistry : PrometheusMeterRegistry

  override function init() {
    configureMetrics()
  }

  protected override function doGet(request : HttpServletRequest, response : HttpServletResponse) {
    response.ContentType = TextFormat.CONTENT_TYPE_004
    response.setStatus(HttpServletResponse.SC_OK)

    prometheusMeterRegistry.scrape(response.Writer)
  }

  
  private function configureMetrics() : PrometheusMeterRegistry {
    prometheusMeterRegistry = new PrometheusMeterRegistry(PrometheusConfig.DEFAULT);
    new ClassLoaderMetrics().bindTo(prometheusMeterRegistry)
    new JvmMemoryMetrics().bindTo(prometheusMeterRegistry)
    new JvmGcMetrics().bindTo(prometheusMeterRegistry)
    new ProcessorMetrics().bindTo(prometheusMeterRegistry)
    new JvmThreadMetrics().bindTo(prometheusMeterRegistry)

    var toDoubleFunction : ToDoubleFunction<GameMetrics> = \status -> status.GameFinished
    Gauge.builder("game_status", new GameMetrics(), toDoubleFunction)
        .description("Return Game status")
        .register(prometheusMeterRegistry)

    return prometheusMeterRegistry
  }


}

Upon the first call to the Servlet, GameMetrics.GameFinished() is called and a value is properly displayed. For alls subsequent calls to the servlet GameMetrics.GameFinished() is no longer called and NaN is returned.

I can't figure out what I am doing wrong.

Best regards

Answer 1

Try to keep a reference (e.g.: a private final field) to the instances you use as state objects in Gauges (i.e.: GameMetrics).

Gauges are using a WeakReference to refer to their state object which does not prevent them to be collected by the GC. So I think in your case what happens is that the GC collects the GameMetrics instance so it will be null, see the docs: https://micrometer.io/docs/concepts#_why_is_my_gauge_reporting_nan_or_disappearing