ActivitySamplingResult.PropagationData overidden if not root span

[verdie-g]

Hi, I have an ASP.NET Core app with an endpoint that just writes a message to a Kafka topic. I'm using a ParentBasedSampler(TraceIdRatioBasedSampler(X)) sampler and I've noticed that whenever the trace is not sampled, the StartActivity will return null for the Kafka span when I would expect a non-recorded activity so I would still be able to propagate the tracing context.

When debugging the OTEL SDK, I've noticed this code

opentelemetry-dotnet/src/OpenTelemetry/Trace/TracerProviderSdk.cs

Lines 423 to 472 in 0f9f507

	var samplingResult = sampler.ShouldSample(samplingParameters);
	var activitySamplingResult = samplingResult.Decision switch
	{
	SamplingDecision.RecordAndSample => ActivitySamplingResult.AllDataAndRecorded,
	SamplingDecision.RecordOnly => ActivitySamplingResult.AllData,
	_ => ActivitySamplingResult.PropagationData,
	};
	if (activitySamplingResult != ActivitySamplingResult.PropagationData)
	{
	foreach (var att in samplingResult.Attributes)
	{
	options.SamplingTags.Add(att.Key, att.Value);
	}
	// https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/sdk.md#sampler
	// Spec requires clearing Tracestate if empty Tracestate is returned.
	// Since .NET did not have this capability, it'll break
	// existing samplers if we did that. So the following is
	// adopted to remain spec-compliant and backward compat.
	// The behavior is:
	// if sampler returns null, its treated as if it has no intend
	// to change Tracestate. Existing SamplingResult ctors will put null as default TraceStateString,
	// so all existing samplers will get this behavior.
	// if sampler returns non-null, then it'll be used as the
	// new value for Tracestate
	// A sampler can return string.Empty if it intends to clear the state.
	if (samplingResult.TraceStateString != null)
	{
	options = options with { TraceState = samplingResult.TraceStateString };
	}
	return activitySamplingResult;
	}
	return PropagateOrIgnoreData(options.Parent);
	}
	[MethodImpl(MethodImplOptions.AggressiveInlining)]
	private static ActivitySamplingResult PropagateOrIgnoreData(in ActivityContext parentContext)
	{
	var isRootSpan = parentContext.TraceId == default;
	// If it is the root span or the parent is remote select PropagationData so the trace ID is preserved
	// even if no activity of the trace is recorded (sampled per OpenTelemetry parlance).
	return (isRootSpan || parentContext.IsRemote)
	? ActivitySamplingResult.PropagationData
	: ActivitySamplingResult.None;
	}

If the ActivitySamplingResult is PropagationData but this is not the root span or the parent span is not a remote one then the ActivitySamplingResult is replaced by None.

I'm not sure how to propagate the tracing context 🤔

Potentially related issue #2887

[vishweshbankwar]

@verdie-g Would injecting Activity.Current work, instead of injecting the context from specific activity that you are creating? In your case, root activity's context will be injected when the sampling decision is to drop which should be ok if your intention is propagate the sampling decision to downstream service.

Related: #3290

[verdie-g]

I think using Activity.Current would propagate the ASP.NET Core span so the kafka consume span would have that span as parent which will be weird when viewing the trace.

[vishweshbankwar]

If the decision is to drop then that entire trace will not be exported so, that should not be an issue. If the decision is to record, your StartActivity will return non-null activity, which will be the current and will be propagated.

[verdie-g]

Makes sense. Thanks.