Custom TracesSampler ignores sampling decision from sentry-trace header

Package

Sentry.AspNetCore

.NET Flavor

.NET Core

.NET Version

6.0.0

OS

Any

SDK Version

3.17.1

Self-Hosted Sentry Version

No response

Steps to Reproduce

Set up an ASP.NET Core application, include .UseSentryTracing(), and add a custom TracesSampler:

webBuilder.UseSentry(o =>
{
    o.TracesSampler = ctx =>
    {
        return 1;
    }
})

Run it, make a request using Postman (or similar tool), and include the sentry-trace HTTP request header:

sentry-trace: 00000000000000000000000000000000-0000000000000000-0

The -0 at the end indicates the transaction should not be sampled.

Expected Result

The transaction should not be sent to Sentry.

Actual Result

The transaction is sent to Sentry anyway, because the TracesSampler return value overrides the value in the sentry-trace header.

[mattjohnsonpint]

OK. Then nothing to do here. Closing as works as designed. Thanks for the clarity.

[bruno-garcia]

The background story on why the sampler callback always runs (even if the sentry-trace has a sampled transaction) is that otherwise there’s no way for a backend to control the volume if it gets overwhelmed by incoming transactions initiated by a front end. That said, I agree it’s a pitfall, and the only thing done, to my knowledge, to hedge the risk of users falling into this issue, was to add to our docs in all sample code snippets that users should peek into the parent sampling decision if they care about that, examples:

• .NET: https://docs.sentry.io/platforms/dotnet/configuration/sampling/#setting-a-sampling-function
• Java: https://docs.sentry.io/platforms/java/configuration/sampling/#setting-a-sampling-function

More context:

• https://github.com/getsentry/sentry-docs/issues/3230
• https://github.com/getsentry/sentry-docs/pull/3231
• https://github.com/getsentry/sentry-java/issues/1315