main.go

package main

import (
	"net/http"
	"fmt"
	"io/ioutil"
	"math/rand"
)

var letters = []byte("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ")

const HEADER_CORRELATION_ID = "correlationId"

type Frontend struct {
	client *http.Client
}

func NewFrontend() *Frontend {
	return &Frontend{client: &http.Client{}}
}

func (f *Frontend) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	var correlationId = r.Header.Get(HEADER_CORRELATION_ID)
	if correlationId == "" {
		correlationId = newCorrelationId()
	}

	request, err := http.NewRequest("GET", "http://localhost:8082/url", nil)
	request.Header.Set(HEADER_CORRELATION_ID, correlationId)

	//resp, err := http.Get("http://localhost:8082/url")
	resp, err := f.client.Do(request)
	if err != nil {
		panic(err)
	}

	defer resp.Body.Close()
	body, err := ioutil.ReadAll(resp.Body)
	if err != nil {
		panic(err)
	}

	fmt.Fprint(w, "Hello (frontend)\n")
	w.Write(body)
}

func randSeq(n int) string {
	b := make([]byte, n)
	for i := range b {
		b[i] = letters[rand.Intn(len(letters))]
	}
	return string(b)
}

func newCorrelationId() string {
	return randSeq(16)
}

func backendHandler(w http.ResponseWriter, r *http.Request) {
	var correlationId = r.Header.Get(HEADER_CORRELATION_ID)
	fmt.Fprint(w, "Hello (backend)\n")
	fmt.Fprintf(w, "Correlation ID: %s\n", correlationId)
}

func main() {
	// backend service
	var backendService *http.ServeMux = http.NewServeMux()
	backendService .HandleFunc("/", backendHandler)
	go http.ListenAndServe(":8082", backendService)

	// frontend service
	var frontendService = http.NewServeMux()
	frontendService.Handle("/", NewFrontend())
	http.ListenAndServe(":8081", frontendService)
}