How to push a gauge observation (JS)?

[triblondon]

I am using the JavaScript oTel libraries to configure telemetry on my NodeJS app. One of my metrics is a gauge whose value doesn't often change. How do I push an updated value into it?

const meterProvider = new MeterProvider();
const metricExporter = new OTLPMetricExporter({ url: process.env.OTEL_HTTP_COLLECTOR_BASE_URL + "/v1/metrics" });
const metricReader = new PeriodicExportingMetricReader({ exporter: metricExporter });
meterProvider.addMetricReader(metricReader);
const oTelMeter = meterProvider.getMeter('Foo app');
const activeExecSessionsCount = oTelMeter.createObservableGauge('active_exec_sessions_count');

It looks like an observation() function used to exist because when I first started writing this code I wrote this:

export const setActiveExecSessions = (n: number): void => {
  activeExecSessionsCount.observation(n);   // <-- this no longer works (maybe never did, unsure)
};

But now it looks like Observables can only update their value by giving them a callback to invoke. There is a Counter and an ObservableCounter, so why isn't there a Gauge that is not observable? What is the best practice for monitoring a value that fluctuates, is measured absolutely rather than in increments/decrements, doesn't change often, and where on each change an event is triggered in which I'd like to update the value of the gauge?

[legendecas]

Synchronous instruments are designed for accumulating metric events and asynchronous instruments are observing snapshots of metric values.

For Gauge, there is no essential difference between the proactive synchronous recording and passive asynchronous observation: only the final observation is exported (with default LastValueAggregation). If you are interested in a summary of "gauge" metric events, I'd recommend the Histogram instrument for the case.

[triblondon]

Thanks @legendecas. The problem is, without making changes to my application, it seems the only way to migrate to OpenTelemetry from my current metrics library (which supports myGauge.updateValue(newValue)) would be to do something like this:

const activeExecSessionsCount = oTelMeter.createObservableGauge('active_exec_sessions_count');

let gaugeValue;
activeExecSessionsCount.addCallback((result) => result.observe(gaugeValue));

export const setActiveExecSessions = (n: number): void => {
  gaugeValue = n;
};

This seems pretty unpleasant and you might observe an undefined value during startup. I could refactor my app so that the telemetry module can directly observe the current value of the thing this gauge is monitoring, but it's a really large app with a lot of gauges and it seems like that would create a significant tight coupling between the telemetry module and lots of other modules all over the app.

[triblondon]

Please do, I'm not quite sure exactly how to frame what I'm asking for.

[dyladan]

Can you tell us which existing metrics system you're currently using so we can be sure we understand the feature and argue for it effectively in the specification?

[triblondon]

Sure! I was using node-statsd, initially, which has this:

client.gauge('my_gauge', 123.45);

We migrated to prom-client, which does this:

const gauge = new client.Gauge({ name: 'my_gauge' });
gauge.set(10); // Set to 10
gauge.inc(); // Increment 1
gauge.inc(10); // Increment 10

These clients are designed to work with the StatsD and Prometheus metric servers, which seem like two of the most popular, so I feel like this is a pretty common use case - at least I don't think I'm trying to do anything weird!

[manvydasu]

👍 on this.
Let's assume scenario exists, where scheduled job is being ran and we want to update some metrics (i.e. amount of unprocessed items) at the end of that job.
With the current API (callback for gauge) - job's metric can not be manually updated at the end of the processing.

[manvydasu]

Seems like there is an issue in specificaton repo created for this already.
open-telemetry/opentelemetry-specification#2318

[naorpeled]

Hey @legendecas,
sorry for bumping this.

Any updates on this topic?
We can't find a way to perform this in the latest version of the lib.
Is it something I can work on? I'd love to contribute to this lib.

[JCMais]

update is that the spec has been merged and that there is an ticket to implement it in the js repo here: #4296