Logging middleware

Learn to write a middleware that logs requests.

Logging middleware

The request method, path and duration are easy to log; but the status code requires wrapping the ResponseWriter, because the standard interface doesn't say which code the handler wrote:

type kayitci struct {
    http.ResponseWriter
    kod int
}

func (k *kayitci) WriteHeader(c int) {
    k.kod = c
    k.ResponseWriter.WriteHeader(c)
}

Structured logging with slog

With Go 1.21's log/slog package fields are logged separately, and log search tools can query them:

k := &kayitci{ResponseWriter: w, kod: 200}
bas := time.Now()
next.ServeHTTP(k, r)
slog.Info("istek", "yol", r.URL.Path, "kod", k.kod, "sure", time.Since(bas))

Exercise Record the status code

Write the kayitci type (embedded http.ResponseWriter and kod int) and its WriteHeader method. The handler calls w.WriteHeader(404); run it with k := &kayitci{ResponseWriter: rec, kod: 200} and print k.kod.

Show example solution

package main

import (
    "fmt"
    "net/http"
    "net/http/httptest"
)

type kayitci struct {
    http.ResponseWriter
    kod int
}

func (k *kayitci) WriteHeader(c int) {
    k.kod = c
    k.ResponseWriter.WriteHeader(c)
}

func main() {
    rec := httptest.NewRecorder()
    k := &kayitci{ResponseWriter: rec, kod: 200}
    h := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) { w.WriteHeader(404) })
    h.ServeHTTP(k, httptest.NewRequest("GET", "/yok", nil))
    fmt.Println(k.kod)
}

Test yourself

If the handler never calls <code>WriteHeader</code>, what is the status code?
- 200
- 0
- 404
- 500
Show answer

200 — The first Write implicitly writes 200; that's why the recorder starts its code field at 200.

Capture the code

func (k *kayitci) ____(c int) {
    k.kod = c
    k.ResponseWriter.WriteHeader(c)
}

Show answer

func (k *kayitci) WriteHeader(c int) {
    k.kod = c
    k.ResponseWriter.WriteHeader(c)
}

Log fields

r.Method

GET, POST...
r.URL.Path

Request path
k.kod

Status code
time.Since

Duration

Next lesson: Auth middleware

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