metric instance per class instance or shared across instances?

[Ben McCann]

Hi,

I've setup Guice dependency inject to provide a MetricRegistry as a Singleton. For the classes where I want to have metrics, do those metrics provider need to be singletons or will they share counters if registered with the same name? E.g. can I do the following:

    public class MailSender {

      private final Counter attemptCount;

      @Inject

      public MailSender(MetricRegistry metrics) {

        this.attemptCount = metrics.counter(MetricRegistry.name(MailSender .class, "attempts"));

      }

    }

This would create a new attemptCount Counter for everytime I create a MailSender to send an email, but those counters would all share the same MetricRegistry and the same name. Is that okay or do I need to make it static or a singleton so that all the MailSender instances share the same Counter?

Thanks,

Ben

[Ryan Rupp]

The methods on the MetricsRegistry such as counter, timer, meter etc. will return the existing metric if one has already been registered under that name so what you're doing is fine. For reference see - https://github.com/codahale/metrics/blob/master/metrics-core/src/main/java/com/codahale/metrics/MetricRegistry.java#L307 - which is use by all those methods.

[Ben McCann]

Thanks. Any advice on registering metrics computed from other metrics? While you can recreate simple metrics in constructors, it seems like I'll have to make all my metrics singletons in order to allow them to be used in calculating ratio gauges and other calculated metrics. E.g. I tried to do this and it didn't work:

    public class MailSender {

      private final Counter attemptCount;

      @Inject

      public MailSender(MetricRegistry metrics) {

        this.cacheHitCount = metrics.counter(MetricRegistry.name(MailSender .class, "cache-hits"));

        this.cacheAttemptCount = metrics.counter(MetricRegistry.name(MailSender .class, "cache-attempts"));

        metrics.register(MetricRegistry.name(MailSender.class, "cache-hit-ratio"),

            new HitRateGauge(cacheHitCount, cacheAttemptCount));

      }

    }

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