go / gzip

updated February 01, 2026

I use a simple middleware to gzip HTTP responses, with a sync.Pool to reuse gzip writers.

The pattern

package gzip

import (
	"compress/gzip"
	"io"
	"net/http"
	"strings"
	"sync"
)

var pool = sync.Pool{
	New: func() any {
		w, _ := gzip.NewWriterLevel(nil, gzip.BestSpeed)
		return w
	},
}

func Middleware(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		if !strings.Contains(r.Header.Get("Accept-Encoding"), "gzip") {
			next.ServeHTTP(w, r)
			return
		}

		w.Header().Set("Content-Encoding", "gzip")
		w.Header().Add("Vary", "Accept-Encoding")

		gz := pool.Get().(*gzip.Writer)
		gz.Reset(w)
		defer func() {
			gz.Close()
			pool.Put(gz)
		}()

		next.ServeHTTP(&gzipResponseWriter{gz: gz, ResponseWriter: w}, r)
	})
}

type gzipResponseWriter struct {
	gz *gzip.Writer
	http.ResponseWriter
}

func (w *gzipResponseWriter) Write(b []byte) (int, error) {
	return w.gz.Write(b)
}

Key points:

• Pooled writers: gzip.NewWriter allocates ~256KB for compression tables. Reusing writers via sync.Pool reduces allocations significantly. See pool for when you need handle-based removal instead.
• BestSpeed: Level 1 compression is usually the right trade-off for HTTP. CPU cost is low, and most compressible content (JSON, HTML) compresses well even at low levels.
• Vary header: Tells caches the response varies by Accept-Encoding.

Usage

mux := http.NewServeMux()
mux.HandleFunc("/api/data", handleData)

http.ListenAndServe(":8080", gzip.Middleware(mux))

When to use

• JSON APIs with large responses
• HTML pages
• Any text-based content (CSS, JavaScript, XML)

When not to use

• Already-compressed content (images, video, zip files)
• Very small responses (gzip overhead exceeds savings under ~150 bytes)
• WebSocket connections
• Server-sent events (streaming)

Streaming caveat

This middleware buffers the entire response before sending. For streaming responses, you'd need to call gz.Flush() periodically, but that reduces compression ratio.

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