A contrived example of writing a transforming http proxy using oxy.

transforming-http-reverse-proxy.go

package main

import (
	"io"
	"net/http"
	"strconv"

	"github.com/mailgun/multibuf"
	"github.com/mailgun/oxy/forward"
	"github.com/mailgun/oxy/utils"
)

const (
	targetScheme = "http"
	targetHost   = "www.google.com"
	targetPath   = "/foo"
)

type counterWriter struct {
	Count  int64
	Writer io.Writer
}

func (c *counterWriter) Write(p []byte) (n int, err error) {
	written, err := c.Writer.Write(p)
	c.Count = c.Count + int64(written)
	return written, err
}

func transform(mediaType string, r io.Reader, w io.Writer) {
	// do something with r contents and write result to w
	io.Copy(w, r)
	w.Write([]byte("added content")) // just add some bytes
}

func proxy(w http.ResponseWriter, r *http.Request) {
	fwd, _ := forward.New()
	writer, _ := multibuf.NewWriterOnce(multibuf.MemBytes(64 * 1024))
	defer writer.Close()
	bw := utils.NewBufferWriter(writer)
	r.URL.Scheme = targetScheme
	r.URL.Host = targetHost
	r.URL.Path = targetPath + "/" + r.URL.Path
	fwd.ServeHTTP(bw, r)
	tw, _ := multibuf.NewWriterOnce(multibuf.MemBytes(64 * 1024))
	defer tw.Close()
	rdr, _ := writer.Reader()
	cnt := &counterWriter{0, tw}
	transform(bw.Header().Get("Content-Type"), rdr, cnt)
	tr, _ := tw.Reader()
	utils.CopyHeaders(w.Header(), bw.Header())
	w.Header().Set("Content-Length", strconv.FormatInt(cnt.Count, 10))
	w.WriteHeader(bw.Code)
	io.Copy(w, tr)
}

func main() {
	http.ListenAndServe(":8888", http.HandlerFunc(proxy))
}

Please note that for the sake of simplicity all error handling has been removed.