Metric Value from Database & avoid BlockingOperationNotAllowedException

[pepperbob]

My aim is to build a metric that essentially reads a number from a database table which then should be recorded in a Gauge.

So far I managed to register the metric via a BuildStep and a MetricsFactoryConsumerBuildItem which is working quite fine - the metric is registered and invoked.

However, when the metric is invoked, the registered Supplier<Number> tries execute a query using the EntityManager which produces a BlockingOperationNotAllowedException. Looking at the MetricFactory API I can't see way how to register something that helps with async. I wanted to avoid polling the database in the background - esp. as it feels quite inconvenient.

Any pointers/ideas how to accomplish this?

[jmartisk]

Are you using smallrye-metrics or micrometer? And what exporter? I believe that the Prometheus exporter for both these metric engines is configured to be on a blocking route, which should avoid your issue... Huh.

[pepperbob]

Just played around with this:

In my test, when I use the quarkus-smallrye-metrics artifact, exporting metrics via HTTP works without a problem - my metric is there, no warning.

When using the quarkus-micrometer-registry-prometheus artifact then I'll see the BlockingOperationNotAllowedException Warning in the log and my metric is not listed (HTTP Endpoint generally exports metrics, but mine is missing).

🤔

[pepperbob]

Okay, found some explanation: with quarkus-micrometer-registry-prometheus querying

/q/metrics

gives me my metric, no warning. Querying this as JSON (what I do in my test 🙈 as it fits RestAssured better) will give me the BlockingOperationNotAllowedException warning without my metric.

Opposing quarkus-smallrye-metrics where both is possible.

I believe that the Prometheus exporter for both these metric engines is configured to be on a blocking route

Ok, this explains why it works for Micrometer with Prometheus; so the difference is basically the different way that the "json exporter" (is this a thing?) is setup for micrometer and smallrye... unexpected though.

[jmartisk]

Ok so you've found a bug in the Micrometer JSON exporter. It's using a handler that is not blocking. Prometheus exporter is using a blocking handler, which is correct.
See #23661