ctx_with_value.go

func main() {

	mux := http.NewServeMux()

	finalHandler := http.HandlerFunc(func(writer http.ResponseWriter, request *http.Request) {
		// the context value is retrieved from the request
		user := request.Context().Value("user").(string)
		writer.Write([]byte(user))
	})

	mux.Handle("/", middleware(finalHandler))

	err := http.ListenAndServe(":3000", mux)
	log.Fatal(err)

}

// The middleware function will validate all the requests
// and propagate with context value, you can use the context with
// value to pass tracing id, log ref, user id, across the services.
func middleware(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		token := r.Header.Get("token") // this can be JWT, some other token
		user := "anonymous"
		if token != "" {
			// if token is not empty you can parse the token
			// parseTokenGetUser()
			user = "validated user"
		}

		// user value is passed along the request context
		ctx := context.WithValue(r.Context(), "user", user)
		// request.WithContext returns Request with context value included.
		next.ServeHTTP(w, r.WithContext(ctx))
	})
}