go 56 lines · 7 steps

How gzip HTTP middleware works in Go

A net/http middleware that compresses responses on the fly while pooling gzip writers to avoid per-request allocations.

Explained by highlit
Intermediate middleware compression object-pooling http decorator
1package middleware
2 
3import (
4 "compress/gzip"
5 "io"
6 "net/http"
7 "strings"
8 "sync"
9)
10 
11var gzipPool = sync.Pool{
12 New: func() any {
13 return gzip.NewWriter(io.Discard)
14 },
15}
16 
17type gzipResponseWriter struct {
18 http.ResponseWriter
19 gw *gzip.Writer
20}
21 
22func (w *gzipResponseWriter) Write(b []byte) (int, error) {
23 if w.Header().Get("Content-Type") == "" {
24 w.Header().Set("Content-Type", http.DetectContentType(b))
25 }
26 return w.gw.Write(b)
27}
28 
29func (w *gzipResponseWriter) Flush() {
30 _ = w.gw.Flush()
31 if f, ok := w.ResponseWriter.(http.Flusher); ok {
32 f.Flush()
33 }
34}
35 
36func Gzip(next http.Handler) http.Handler {
37 return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
38 if !strings.Contains(r.Header.Get("Accept-Encoding"), "gzip") {
39 next.ServeHTTP(w, r)
40 return
41 }
42 
43 gw := gzipPool.Get().(*gzip.Writer)
44 gw.Reset(w)
45 defer func() {
46 _ = gw.Close()
47 gzipPool.Put(gw)
48 }()
49 
50 w.Header().Set("Content-Encoding", "gzip")
51 w.Header().Add("Vary", "Accept-Encoding")
52 w.Header().Del("Content-Length")
53 
54 next.ServeHTTP(&gzipResponseWriter{ResponseWriter: w, gw: gw}, r)
55 })
56}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Embedding http.ResponseWriter lets you override just Write while inheriting the rest of the interface for free.
  2. 2sync.Pool recycles expensive objects like gzip writers so high-traffic handlers avoid constant allocation.
  3. 3Compression middleware must negotiate via Accept-Encoding and fix up headers like Content-Encoding and Content-Length.

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