go 52 lines · 7 steps

Capturing response bodies in Gin middleware

A Gin middleware wraps the response writer to tee output into a buffer, then logs a structured audit record after the handler runs.

Explained by highlit
Gin Intermediate middleware response-capture struct-embedding structured-logging decorator
1package middleware
2 
3import (
4 "bytes"
5 "time"
6 
7 "github.com/gin-gonic/gin"
8 "github.com/rs/zerolog/log"
9)
10 
11type responseCapture struct {
12 gin.ResponseWriter
13 body *bytes.Buffer
14}
15 
16func (w *responseCapture) Write(b []byte) (int, error) {
17 w.body.Write(b)
18 return w.ResponseWriter.Write(b)
19}
20 
21func (w *responseCapture) WriteString(s string) (int, error) {
22 w.body.WriteString(s)
23 return w.ResponseWriter.WriteString(s)
24}
25 
26func AuditLogger(maxBody int) gin.HandlerFunc {
27 return func(c *gin.Context) {
28 capture := &responseCapture{
29 ResponseWriter: c.Writer,
30 body: bytes.NewBuffer(make([]byte, 0, 1024)),
31 }
32 c.Writer = capture
33 
34 start := time.Now()
35 c.Next()
36 
37 payload := capture.body.Bytes()
38 if len(payload) > maxBody {
39 payload = payload[:maxBody]
40 }
41 
42 log.Info().
43 Str("method", c.Request.Method).
44 Str("path", c.FullPath()).
45 Int("status", c.Writer.Status()).
46 Int("bytes", capture.body.Len()).
47 Dur("latency", time.Since(start)).
48 Str("client_ip", c.ClientIP()).
49 Bytes("response", payload).
50 Msg("request audited")
51 }
52}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Embedding an interface lets you override just the methods you care about while delegating the rest.
  2. 2Swapping c.Writer before calling c.Next lets middleware observe everything downstream handlers produce.
  3. 3Truncating captured payloads before logging bounds memory and log volume for large responses.

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