Prefer w3c traceparent header over legacy elastic-apm-traceparent

CHANGELOG.asciidoc

@@ -23,6 +23,7 @@ endif::[]
 
 https://github.com/elastic/apm-agent-go/compare/v1.12.0...master[View commits]
 
+- Prefer w3c traceparent header over legacy elastic-apm-traceparent {pull}963[#(963)]
 
 [[release-notes-1.x]]
 === Go Agent version 1.x

module/apmgrpc/server.go

@@ -160,9 +160,9 @@ func startTransaction(ctx context.Context, tracer *apm.Tracer, name string) (*ap
 	var opts apm.TransactionOptions
 	md, ok := metadata.FromIncomingContext(ctx)
 	if ok {
-		traceContext, ok := getIncomingMetadataTraceContext(md, elasticTraceparentHeader)
+		traceContext, ok := getIncomingMetadataTraceContext(md, w3cTraceparentHeader)
 		if !ok {
-			traceContext, _ = getIncomingMetadataTraceContext(md, w3cTraceparentHeader)
+			traceContext, _ = getIncomingMetadataTraceContext(md, elasticTraceparentHeader)
 		}
 		opts.TraceContext = traceContext
 	}

module/apmhttp/handler.go

@@ -110,9 +110,9 @@ func (h *handler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
 //
 // DEPRECATED. Use StartTransactionWithBody instead.
 func StartTransaction(tracer *apm.Tracer, name string, req *http.Request) (*apm.Transaction, *http.Request) {
-	traceContext, ok := getRequestTraceparent(req, ElasticTraceparentHeader)
+	traceContext, ok := getRequestTraceparent(req, W3CTraceparentHeader)
 	if !ok {
-		traceContext, ok = getRequestTraceparent(req, W3CTraceparentHeader)
+		traceContext, ok = getRequestTraceparent(req, ElasticTraceparentHeader)
 	}
 	if ok {
 		traceContext.State, _ = ParseTracestateHeader(req.Header[TracestateHeader]...)

module/apmot/tracer.go

@@ -141,20 +141,19 @@ func (t *otTracer) Extract(format interface{}, carrier interface{}) (opentracing
 		var tracestateHeaderValues []string
 		switch carrier := carrier.(type) {
 		case opentracing.HTTPHeadersCarrier:
-			traceparentHeaderValue = http.Header(carrier).Get(apmhttp.ElasticTraceparentHeader)
+			traceparentHeaderValue = http.Header(carrier).Get(apmhttp.W3CTraceparentHeader)
 			if traceparentHeaderValue == "" {
-				traceparentHeaderValue = http.Header(carrier).Get(apmhttp.W3CTraceparentHeader)
+				traceparentHeaderValue = http.Header(carrier).Get(apmhttp.ElasticTraceparentHeader)
 			}
 			tracestateHeaderValues = http.Header(carrier)[apmhttp.TracestateHeader]
 		case opentracing.TextMapReader:
 			carrier.ForeachKey(func(key, val string) error {
 				switch textproto.CanonicalMIMEHeaderKey(key) {
-				case apmhttp.ElasticTraceparentHeader:
-					traceparentHeaderValue = val
 				case apmhttp.W3CTraceparentHeader:
-					// The Elastic header value always trumps the W3C one,
-					// to ensure backwards compatibility, hence we only set
-					// the value if not set already.
+					traceparentHeaderValue = val
+				case apmhttp.ElasticTraceparentHeader:
+					// The W3C header value always trumps the Elastic one,
+					// hence we only set the value if not set already.
 					if traceparentHeaderValue == "" {
 						traceparentHeaderValue = val
 					}