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fix(ddtrace/opentelemetry): propagate sampling decision to child spans #3673

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darccio wants to merge 1 commit into from
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fix(ddtrace/opentelemetry): propagate sampling decision to child spans #3673

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3 changes: 3 additions & 0 deletions ddtrace/opentelemetry/tracer.go
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Original file line number Diff line number Diff line change
Expand Up @@ -44,6 +44,9 @@ func (t *oteltracer) Start(ctx context.Context, spanName string, opts ...oteltra
// if the span doesn't originate from the Datadog tracer,
// use SpanContextW3C implementation struct to pass span context information
ddopts = append(ddopts, tracer.ChildOf(tracer.FromGenericCtx(&otelCtxToDDCtx{sctx})))
if sctx.IsSampled() {
ddopts = append(ddopts, tracer.Tag(ext.ManualKeep, true))
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@sdwa0 sdwa0 Jun 22, 2025

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This doesn't look right.

If the parent span is sampled, the new span shouldn't need to get force-sampled to get sampled. This will incorrectly set the ingestion_reason span metadata to manual instead of rule (relevant doc).

Also, this doesn't cover the case when the parent span is not sampled. In that case, the child span shouldn't be sampled either. However, with this code, the child span will independently decide its sampling decision, so it might be sampled.

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@darccio darccio Jul 3, 2025

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You are right. Closing this in favour of #3718 (in progress).

}
}
}
if t := ssConfig.Timestamp(); !t.IsZero() {
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25 changes: 25 additions & 0 deletions ddtrace/opentelemetry/tracer_test.go
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Expand Up @@ -121,6 +121,31 @@ func TestSpanContext(t *testing.T) {
assert.Equal(true, sctx.IsRemote())
}

func TestSamplingDecision(t *testing.T) {
assert := assert.New(t)
tp := NewTracerProvider(
tracer.WithSamplingRules([]tracer.SamplingRule{
{Rate: 0}, // This should be applied only when a brand new root span is started and should be ignored for a non-root span
}),
)
defer tp.Shutdown()
otel.SetTracerProvider(tp)
tr := otel.Tracer("")

parentSpanContext := oteltrace.NewSpanContext(oteltrace.SpanContextConfig{
TraceID: oteltrace.TraceID{0xAB},
SpanID: oteltrace.SpanID{0x01},
TraceFlags: oteltrace.FlagsSampled, // the parent span is sampled, so its child spans should be sampled too
})
ctx := oteltrace.ContextWithSpanContext(context.Background(), parentSpanContext)
_, span := tr.Start(ctx, "test")
span.End()

childSpanContext := span.SpanContext()
assert.Equal(parentSpanContext.TraceID(), childSpanContext.TraceID())
assert.True(childSpanContext.IsSampled(), "parent span is sampled, but child span is not sampled")
}

func TestForceFlush(t *testing.T) {
const (
UNSET = iota
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