go / gzip

updated February 20, 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)

See readwrap for patterns to chain gzip readers with proper resource cleanup.

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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