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// Copyright 2017 Google Inc. All Rights Reserved.
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
// +build go1.7
package trace
import (
// Transport is an http.RoundTripper that traces the outgoing requests.
// Transport is safe for concurrent usage.
type Transport struct {
// Base is the base http.RoundTripper to be used to do the actual request.
// Optional. If nil, http.DefaultTransport is used.
Base http.RoundTripper
// RoundTrip creates a trace.Span and inserts it into the outgoing request's headers.
// The created span can follow a parent span, if a parent is presented in
// the request's context.
func (t Transport) RoundTrip(req *http.Request) (*http.Response, error) {
span := FromContext(req.Context()).NewRemoteChild(req)
resp, err := t.base().RoundTrip(req)
// TODO(jbd): Is it possible to defer the span.Finish?
// In cases where RoundTrip panics, we still can finish the span.
return resp, err
// CancelRequest cancels an in-flight request by closing its connection.
func (t Transport) CancelRequest(req *http.Request) {
type canceler interface {
if cr, ok := t.base().(canceler); ok {
func (t Transport) base() http.RoundTripper {
if t.Base != nil {
return t.Base
return http.DefaultTransport
// HTTPHandler returns a http.Handler from the given handler
// that is aware of the incoming request's span.
// The span can be extracted from the incoming request in handler
// functions from incoming request's context:
// span := trace.FromContext(r.Context())
// The span will be auto finished by the handler.
func (c *Client) HTTPHandler(h http.Handler) http.Handler {
return &handler{traceClient: c, handler: h}
type handler struct {
traceClient *Client
handler http.Handler
func (h *handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
traceID, parentSpanID, options, optionsOk, ok := traceInfoFromHeader(r.Header.Get(httpHeader))
if !ok {
traceID = nextTraceID()
t := &trace{
traceID: traceID,
client: h.traceClient,
globalOptions: options,
localOptions: options,
span := startNewChildWithRequest(r, t, parentSpanID)
span.span.Kind = spanKindServer
span.rootSpan = true
configureSpanFromPolicy(span, h.traceClient.policy, ok)
defer span.Finish()
r = r.WithContext(NewContext(r.Context(), span))
if ok && !optionsOk {
// Inject the trace context back to the response with the sampling options.
// TODO(jbd): Remove when there is a better way to report the client's sampling.
w.Header().Set(httpHeader, spanHeader(traceID, parentSpanID, span.trace.localOptions))
h.handler.ServeHTTP(w, r)