ddtrace/tracer: mark top level spans

.gitignore

@@ -10,6 +10,8 @@ bin/
 *.cov
 *.lock
 *.swp
+.idea
+dd-trace-go.iml
 
 /contrib/google.golang.org/grpc.v12/vendor/
 

ddtrace/tracer/span.go

@@ -381,4 +381,7 @@ const (
 	keyRulesSamplerAppliedRate = "_dd.rule_psr"
 	keyRulesSamplerLimiterRate = "_dd.limit_psr"
 	keyMeasured                = "_dd.measured"
+	// keyTopLevel is the key of top level metric indicating if a span is top level.
+	// A top level span is a local root (parent span of the local trace) or the first span of each service.
+	keyTopLevel = "_top_level"
 )

ddtrace/tracer/span_test.go

@@ -244,7 +244,7 @@ const (
 func TestSpanSetMetric(t *testing.T) {
 	for name, tt := range map[string]func(assert *assert.Assertions, span *span){
 		"init": func(assert *assert.Assertions, span *span) {
-			assert.Equal(2, len(span.Metrics))
+			assert.Equal(3, len(span.Metrics))
 			_, ok := span.Metrics[keySamplingPriority]
 			assert.True(ok)
 			_, ok = span.Metrics[keySamplingPriorityRate]
@@ -279,7 +279,7 @@ func TestSpanSetMetric(t *testing.T) {
 		"finished": func(assert *assert.Assertions, span *span) {
 			span.Finish()
 			span.SetTag("finished.test", 1337)
-			assert.Equal(2, len(span.Metrics))
+			assert.Equal(3, len(span.Metrics))
 			_, ok := span.Metrics["finished.test"]
 			assert.False(ok)
 		},

ddtrace/tracer/tracer.go

@@ -307,6 +307,11 @@ func (t *tracer) StartSpan(operationName string, options ...ddtrace.StartSpanOpt
 	for k, v := range t.config.globalTags {
 		span.SetTag(k, v)
 	}
+	if context == nil || context.span == nil || context.span.Service != span.Service {
+		span.setMetric(keyTopLevel, 1)
+		// all top level spans are measured. So the measured tag is redundant.
+		delete(span.Metrics, keyMeasured)
+	}
 	if t.config.version != "" && span.Service == t.config.serviceName {
 		span.SetTag(ext.Version, t.config.version)
 	}

ddtrace/tracer/tracer_test.go

@@ -212,10 +212,19 @@ func TestTracerStartSpan(t *testing.T) {
 		assert.Equal(t, "/home/user", span.Resource)
 	})
 
-	t.Run("measured", func(t *testing.T) {
+	t.Run("measured_top_level", func(t *testing.T) {
 		tracer := newTracer()
 		span := tracer.StartSpan("/home/user", Measured()).(*span)
-		assert.Equal(t, 1.0, span.Metrics[keyMeasured])
+		_, ok := span.Metrics[keyMeasured]
+		assert.False(t, ok)
+		assert.Equal(t, 1.0, span.Metrics[keyTopLevel])
+	})
+
+	t.Run("measured_non_top_level", func(t *testing.T) {
+		tracer := newTracer()
+		parent := tracer.StartSpan("/home/user").(*span)
+		child := tracer.StartSpan("home/user", Measured(), ChildOf(parent.context)).(*span)
+		assert.Equal(t, 1.0, child.Metrics[keyMeasured])
 	})
 }
 
@@ -277,7 +286,6 @@ func TestTracerStartSpanOptions(t *testing.T) {
 		ResourceName("test.resource"),
 		StartTime(now),
 		WithSpanID(420),
-		Measured(),
 	}
 	span := tracer.StartSpan("web.request", opts...).(*span)
 	assert := assert.New(t)
@@ -287,7 +295,7 @@ func TestTracerStartSpanOptions(t *testing.T) {
 	assert.Equal(now.UnixNano(), span.Start)
 	assert.Equal(uint64(420), span.SpanID)
 	assert.Equal(uint64(420), span.TraceID)
-	assert.Equal(1.0, span.Metrics[keyMeasured])
+	assert.Equal(1.0, span.Metrics[keyTopLevel])
 }
 
 func TestTracerStartChildSpan(t *testing.T) {

ddtrace/tracer/transport.go

@@ -20,6 +20,12 @@ import (
 	"gopkg.in/DataDog/dd-trace-go.v1/internal/version"
 )
 
+const (
+	// headerComputedTopLevel specifies that the client has marked top-level spans, when set.
+	// Any non-empty value will mean 'yes'.
+	headerComputedTopLevel = "Datadog-Client-Computed-Top-Level"
+)
+
 var defaultClient = &http.Client{
 	// We copy the transport to avoid using the default one, as it might be
 	// augmented with tracing and we don't want these calls to be recorded.
@@ -114,6 +120,7 @@ func (t *httpTransport) send(p *payload) (body io.ReadCloser, err error) {
 	}
 	req.Header.Set(traceCountHeader, strconv.Itoa(p.itemCount()))
 	req.Header.Set("Content-Length", strconv.Itoa(p.size()))
+	req.Header.Set(headerComputedTopLevel, "yes")
 	response, err := t.client.Do(req)
 	if err != nil {
 		return nil, err