How to remove no longer valid Gauges

Question

I use the Prometheus Java Client to export session information of my application. We want to show how long sessions have been idle.

The problem is that we have a maximum of 1000 sessions and sessions are removed after a certain period. Unfortunately they do not disappear from Prometheus:

My code looks like this:

static final Gauge sessionInactivity = Gauge.build()
    .name("sessions_inactivity_duration")
    .labelNames("internal_key", "external_key", "browser")
    .help("Number of milliseconds a certain session has been inactive")
    .register();

sessionInactivity.labels(internalKey, externalKey, browser).set(inactivityTime);

I tried to do sessionInactivity.clear() during scrapes but obviously this does not empty the content of the Gauge.

Answer 1

The Gauge class has a remove method which has the same signature as the labels method. For your specific example, removing the metrics associated with that gauge would look like this

sessionInactivity.remove(internalKey, externalKey, browser);

The client library documentation states:

Metrics with labels SHOULD support a remove() method with the same signature as labels() that will remove a Child from the metric no longer exporting it, and a clear() method that removes all Children from the metric. These invalidate caching of Children.

Answer 2

Since you are registrating the gauge, you need to remove the gauges which are expired from the registry.

protected MeterRegistry registry;

  ...
  static final Gauge sessionInactivity = Gauge.build()
    .name("sessions_inactivity_duration")
    .labelNames("internal_key", "external_key", "browser")
    .help("Number of milliseconds a certain session has been inactive")
    .register(registry); // use registry

  ...
  // remove expired gauges from registry
  registry.remove(sessionInactivity);