Traces Sampler ignoring false boolean

[magnusburton]

Is there an existing issue for this?

• I have checked for existing issues https://github.com/getsentry/sentry-javascript/issues
• I have reviewed the documentation https://docs.sentry.io/
• I am using the latest SDK release https://github.com/getsentry/sentry-javascript/releases

How do you use Sentry?

Sentry Saas (sentry.io)

Which SDK are you using?

@sentry/node - fastify

SDK Version

9.42.0

Framework Version

No response

Link to Sentry event

https://lisa-tk.sentry.io/explore/traces/trace/6dd111760bb8edb07b37883a9d35b9db/?node=span-68cf894ebe343333

Reproduction Example/SDK Setup

Sentry.init({
	tracesSampler: ({ attributes, parentSampled }) => {
		const { removeSample } = attributes ?? {};

		if (removeSample !== undefined && removeSample === true) {
			return false;
		}
	},
});

function cancelSpan() {
	Sentry.getActiveSpan()?.setAttribute('removeSample', true);
}

const data = [];
Sentry.startSpan({
	name: 'Ingester',
	op: 'queue.task',
}, () => {
	if (!data.length) {
		cancelSpan();
	}
});

Steps to Reproduce

Returning false in tracesSampler sends the trace/span to Sentry anyways. In the URL you can see the attribute removeSample set to true. I can confirm that the tracesSampler is being invoked but I'm curious as to why spans are being sent when I explicitly return false.

Expected Result

Should filter out trace

Actual Result

Trace shows up in Sentry

[linear]

JS-777 Traces Sampler ignoring false boolean

[andreiborza]

Hi @magnusburton, thanks for filing this.

The tracesSampler runs on span creation, by the time you modify the attribute the span has already run through the function. To make this specific example work, you'd have to pass in the removeSample attribute to the startSpan options.

Sentry.startSpan({
	name: 'Ingester',
	op: 'queue.task',
        attributes: {
           removeSample: !data.length
        }
}, () => {
	...
});

[magnusburton]

Thanks for your swift reply @andreiborza !

I understand and we've applied this where appropriate. In some cases, we figure out after some time within the span that it should be deleted (eg. after select from db to realise it's a duplicate and thus would affect our avg span durations in Sentry). Do you have insights on how we could think in these cases?

[andreiborza]

@magnusburton you could filter these in beforeSendTransaction.