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Created July 12, 2016 05:52
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An example of turning into usable HTTP middleware for Go.
package handler
import (
type logger struct {
handler http.Handler
output log.Logger
// opts options
// NewLogger returns a handler for logging the HTTP request, including status
// code, HTTP method, URI (path), duration, bytes written and client address.
func NewLogger(output log.Logger) func(http.Handler) http.Handler {
return func(h http.Handler) http.Handler {
lgr := &logger{
handler: h,
output: output,
return lgr
func (lg *logger) ServeHTTP(w http.ResponseWriter, r *http.Request) {
ww := wrapWriter(w)
start := time.Now()
lg.handler.ServeHTTP(ww, r)
end := time.Now()
duration := end.Sub(start)
"ts", time.Now().Format(time.RFC3339),
"status", ww.Status(),
"method", r.Method,
"uri", r.URL.Path,
"duration", duration,
"bytes", ww.BytesWritten(),
"from", r.RemoteAddr,
// The below writer-wrapping code has been lifted from
// - because
// it does exactly what is needed, and it's unlikely to change in any
// significant way that makes copying worse-off than importing. MIT licensed
// and (c) Carl Jackson.
// writerProxy is a proxy around an http.ResponseWriter that allows you to hook
// into various parts of the response process.
type writerProxy interface {
// Status returns the HTTP status of the request, or 0 if one has not
// yet been sent.
Status() int
// BytesWritten returns the total number of bytes sent to the client.
BytesWritten() int
// Tee causes the response body to be written to the given io.Writer in
// addition to proxying the writes through. Only one io.Writer can be
// tee'd to at once: setting a second one will overwrite the first.
// Writes will be sent to the proxy before being written to this
// io.Writer. It is illegal for the tee'd writer to be modified
// concurrently with writes.
// Unwrap returns the original proxied target.
Unwrap() http.ResponseWriter
// wrapWriter wraps an http.ResponseWriter, returning a proxy that allows you to
// hook into various parts of the response process.
func wrapWriter(w http.ResponseWriter) writerProxy {
_, cn := w.(http.CloseNotifier)
_, fl := w.(http.Flusher)
_, hj := w.(http.Hijacker)
_, rf := w.(io.ReaderFrom)
bw := basicWriter{ResponseWriter: w}
if cn && fl && hj && rf {
return &fullWriter{bw}
if fl {
return &flushWriter{bw}
return &bw
// basicWriter wraps a http.ResponseWriter that implements the minimal
// http.ResponseWriter interface.
type basicWriter struct {
wroteHeader bool
code int
bytes int
tee io.Writer
func (b *basicWriter) WriteHeader(code int) {
if !b.wroteHeader {
b.code = code
b.wroteHeader = true
func (b *basicWriter) Write(buf []byte) (int, error) {
n, err := b.ResponseWriter.Write(buf)
if b.tee != nil {
_, err2 := b.tee.Write(buf[:n])
// Prefer errors generated by the proxied writer.
if err == nil {
err = err2
b.bytes += n
return n, err
func (b *basicWriter) maybeWriteHeader() {
if !b.wroteHeader {
func (b *basicWriter) Status() int {
return b.code
func (b *basicWriter) BytesWritten() int {
return b.bytes
func (b *basicWriter) Tee(w io.Writer) {
b.tee = w
func (b *basicWriter) Unwrap() http.ResponseWriter {
return b.ResponseWriter
// fullWriter is a writer that additionally satisfies http.CloseNotifier,
// http.Flusher, http.Hijacker, and io.ReaderFrom. It exists for the common case
// of wrapping the http.ResponseWriter that package http gives you, in order to
// make the proxied object support the full method set of the proxied object.
type fullWriter struct {
func (f *fullWriter) CloseNotify() <-chan bool {
cn := f.basicWriter.ResponseWriter.(http.CloseNotifier)
return cn.CloseNotify()
func (f *fullWriter) Flush() {
fl := f.basicWriter.ResponseWriter.(http.Flusher)
func (f *fullWriter) Hijack() (net.Conn, *bufio.ReadWriter, error) {
hj := f.basicWriter.ResponseWriter.(http.Hijacker)
return hj.Hijack()
func (f *fullWriter) ReadFrom(r io.Reader) (int64, error) {
if f.basicWriter.tee != nil {
return io.Copy(&f.basicWriter, r)
rf := f.basicWriter.ResponseWriter.(io.ReaderFrom)
return rf.ReadFrom(r)
var _ http.CloseNotifier = &fullWriter{}
var _ http.Flusher = &fullWriter{}
var _ http.Hijacker = &fullWriter{}
var _ io.ReaderFrom = &fullWriter{}
type flushWriter struct {
func (f *flushWriter) Flush() {
fl := f.basicWriter.ResponseWriter.(http.Flusher)
var _ http.Flusher = &flushWriter{}
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