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// Copyright The OpenTelemetry Authors
// 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.
package otel
import "context"
// TextMapCarrier is the storage medium used by a TextMapPropagator.
type TextMapCarrier interface {
// Get returns the value associated with the passed key.
Get(key string) string
// Set stores the key-value pair.
Set(key string, value string)
// TextMapPropagator propagates cross-cutting concerns as key-value text
// pairs within a carrier that travels in-band across process boundaries.
type TextMapPropagator interface {
// Inject set cross-cutting concerns from the Context into the carrier.
Inject(ctx context.Context, carrier TextMapCarrier)
// Extract reads cross-cutting concerns from the carrier into a Context.
Extract(ctx context.Context, carrier TextMapCarrier) context.Context
// Fields returns the keys who's values are set with Inject.
Fields() []string
type compositeTextMapPropagator []TextMapPropagator
func (p compositeTextMapPropagator) Inject(ctx context.Context, carrier TextMapCarrier) {
for _, i := range p {
i.Inject(ctx, carrier)
func (p compositeTextMapPropagator) Extract(ctx context.Context, carrier TextMapCarrier) context.Context {
for _, i := range p {
ctx = i.Extract(ctx, carrier)
return ctx
func (p compositeTextMapPropagator) Fields() []string {
unique := make(map[string]struct{})
for _, i := range p {
for _, k := range i.Fields() {
unique[k] = struct{}{}
fields := make([]string, 0, len(unique))
for k := range unique {
fields = append(fields, k)
return fields
// NewCompositeTextMapPropagator returns a unified TextMapPropagator from the
// group of passed TextMapPropagator. This allows different cross-cutting
// concerns to be propagates in a unified manner.
// The returned TextMapPropagator will inject and extract cross-cutting
// concerns in the order the TextMapPropagators were provided. Additionally,
// the Fields method will return a de-duplicated slice of the keys that are
// set with the Inject method.
func NewCompositeTextMapPropagator(p ...TextMapPropagator) TextMapPropagator {
return compositeTextMapPropagator(p)