does OpenTelemetry.NET support sampling by request status?

[fkpwolf]

I want to record all failed requests (status code=500). For other requests, sampling rate is 10%.

In Application Insight, we implement by this code:

    public class ExcludeTelemetryFromSamplingTelemetryInitializer : ITelemetryInitializer
    {
        public void Initialize(ITelemetry telemetry)
        {
            // Sample 5xx requests and exception
            if (telemetry is ExceptionTelemetry || (telemetry is RequestTelemetry rt && rt.ResponseCode.StartsWith('5')))
            {
                ((ISupportSampling)telemetry).SamplingPercentage = 100;
            }
        }
    }

[utpilla]

I'm assuming you're using AspNetCore instrumentation package. If yes, you could use the EnrichWithHttpResponse option and mark an activity to not be recorded based on a custom condition. For your requirements, you could write something like this:

.AddAspNetCoreInstrumentation(options =>
{
    options.EnrichWithHttpResponse = (activity, response) =>
    {
        if (response.StatusCode < 500)
        {
            if (Random.Shared.Next(0, 100) >= 10)
            {
                activity.IsAllDataRequested = false;
                activity.ActivityTraceFlags &= ~ActivityTraceFlags.Recorded;
            }
        }
    };
});

Note: This is different from sampling in that it does not prevent an activity from being created. You would just be marking an activity (that's already created) to not be recorded.

[cijothomas]

does OpenTelemetry.NET support sampling by request status?

No. OpenTelemetry sampling is head-based, so sampling decision is taken at the beginning of the Span, and can only leverage information available at that time. Things like "Status", "Success", etc. are not available when a Span starts, so they cannot be leveraged by OTel sampler.

Apart from the suggestion to leverage Instrumentation Library's capabilities, also consider : https://github.com/open-telemetry/opentelemetry-dotnet/tree/main/docs/trace/tail-based-sampling-span-level. There is also the option to use tail-sampler in OTel Collector.