metrics-capture-proxy.go

package main

import (
	"bytes"
	"encoding/json"
	"flag"
	"io/ioutil"
	"log"
	"net/http"
	"net/http/httputil"
	"net/url"
	"strconv"
)

func parseResponse(res *http.Response, unmarshalStruct interface{}) error {
	body, err := ioutil.ReadAll(res.Body)
	if err != nil {
		return err
	}
	res.Body.Close()

	res.Body = ioutil.NopCloser(bytes.NewBuffer(body))
	return json.Unmarshal(body, unmarshalStruct)
}

func captureMetrics(m map[string]interface{}) error {
	// Add your metrics capture code here
	log.Printf("captureMetrics = %+v\n", m)
	return nil
}

func main() {
	port := flag.Int("port", 8080, "port to listen on")
	targetURL := flag.String("target-url", "", "downstream service url to proxy to")
	flag.Parse()

	u, err := url.Parse(*targetURL)
	if err != nil {
		log.Fatalf("Could not parse downstream url: %s", *targetURL)
	}

	proxy := httputil.NewSingleHostReverseProxy(u)

	director := proxy.Director
	proxy.Director = func(req *http.Request) {
		director(req)
		req.Header.Set("X-Forwarded-Host", req.Header.Get("Host"))
		req.Host = req.URL.Host
	}

	proxy.ModifyResponse = func(res *http.Response) error {
		responseContent := map[string]interface{}{}
		err := parseResponse(res, &responseContent)
		if err != nil {
			return err
		}

		return captureMetrics(responseContent)
	}

	http.HandleFunc("/", proxy.ServeHTTP)
	log.Printf("Listening on port %d", *port)
	log.Fatal(http.ListenAndServe(":"+strconv.Itoa(*port), nil))
}