@fastify/otel: Ignoring or disabling a path does not affect sub-spans

[hogklint]

💬 Question here

Hi,

I have a health endpoint that is often called and I want to disable all tracing for the endpoint and any sub-spans that may occor. I've tried using the ignorePaths and route config (config { otel: false }), and they do disable the endpoint in question, but any sub-spans are still registered, e.g. @opentelemetry/instrumentation-http and DB calls are still traced and are reported as root-spans. This creates more spam than just enabling the endpoint. I can't imagine this is the inteded way it should work, what am I doing wrong?

I tried using a parent based sampler but that has no affect.

export const fastifyOtelInstrumentation = new FastifyOtelInstrumentation({
  registerOnInitialization: true,
  ignorePaths: (opts) => {
    return opts.url.startsWith('/healthz');
  },
});

sdk = new NodeSDK({
  sampler: new ParentBasedSampler({
    root: new AlwaysOnSampler(),
  }),
  instrumentations: [
    getNodeAutoInstrumentations({
      '@opentelemetry/instrumentation-fastify': {
        enabled: false,
      },
    }),
  ],
})
sdk.start()

Your Environment

• node version: 22.16
• fastify version: 5.3.3
• fastify/otel version: 0.9.1
• os: Linux

[metcoder95]

As you are using the auto instrumentation from opentelemetry, you need to also disable the HTTP one (and your database driver as well) directly at the getNodeAutoInstrumentations

[hogklint]

Thank you for the reply. Yes, I'm using the auto instrumentation from @opentelemetry/auto-instrumentations-node.

I'm not following your suggestion, though. Let's say I have two endpoints

• /user - I want all the tracing I can get here. Any incoming request, outgoing requests, db calls etc
• /healthz - I don't want any tracing here.

If I disable HTTP and DB instrumentation in getNodeAutoInstrumentations then /user won't have traces either. Unless you mean they need to be disabled in a similar manner to how @fastify/otel has ignorePaths, on a per-route basis? I don't think all of them support it, and ...well, that seems to require a big effort that would be quite finicky.

Is the conclusion that disabling a route in @fastify/otel only affects spans generated by @fastify/otel? I hate to be negative, but it doesn't seem very useful. Maybe it's a limitiation of how OpenTelemetry works?

[metcoder95]

If I disable HTTP and DB instrumentation in getNodeAutoInstrumentations then /user won't have traces either. Unless you mean they need to be disabled in a similar manner to how @fastify/otel has ignorePaths, on a per-route basis? I don't think all of them support it, and ...well, that seems to require a big effort that would be quite finicky.

opentelemetry/http provides some configuration to allow you ignore paths while endorsing requests.
If you want to disable these spans, you'll need to do it in combination with fastify/otel.

I'm unaware of what database driver you are using, tho I don't think you'll be able to disable it per-query.

Usually is also common to use two different endpoints when it comes to health checks, having a readinz one that executes dependency check, and an alive one that just returns plain responses hinting is alive. Maybe is a path you can follow (unless you are already doing this)

Is the conclusion that disabling a route in @fastify/otel only affects spans generated by @fastify/otel? I hate to be negative, but it doesn't seem very useful. Maybe it's a limitiation of how OpenTelemetry works?

Yes, exactly; that's how the instrumentations (not only fastify/otel) works.

[hogklint]

I'm using the pg db driver, but as you say the instrumentation could possibly do it per-query, but I want the deciding factor to be the endpoint.

It sound like I would have to write a custom sampler together with a parent based sampler to discard whole traces, not just disabled spans from certain intstrumentation. Was hoping to avoid that.

Thank you for the help

[hogklint]

Disabling a route in @opentelemetry/instrumentation-http suppresses tracing (http.ts#L607) which achieve what I want. Sounds like something that would be useful for @fastify/otel too.

A lot of the opentelemetry-js instrumentation packages also has an option to require a parent span (E.g. mongoose) which is handy when @opentelemetry/instrumentation-http cannot be used. Not all of them have this though.

[metcoder95]

TIL of the supressTracing, I believe that can be easily added if the route is set to otel: false or within the ignore routes.

would you lie to send a PR to address that?

[hogklint]

I'm open to submitting a PR, but I'm not sure when I can get to it. I have no ETA for it at this point.

[metcoder95]

No pressure, if no one has submitted a PR by the time you are free, feel free to open one