A logging middleware that captures response, code and duration of call

logging_middleware.go

package main

import (
	"bytes"
	"io"
	"log"
	"net/http"
	"time"
)

type customResponseWriter struct {
	http.ResponseWriter
	statusCode int
}

func (crw *customResponseWriter) WriteHeader(code int) {
	crw.statusCode = code
	crw.ResponseWriter.WriteHeader(code)
}

func loggingMiddleware(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		// Start the timer
		start := time.Now()

		// Create a buffer to capture the response
		buffer := &bytes.Buffer{}

		// Create a custom response writer
		crw := &customResponseWriter{ResponseWriter: w}

		// Create a multiwriter to write to both the response and the buffer
		multi := io.MultiWriter(crw, buffer)

		// Serve the request with the multiwriter
		next.ServeHTTP(multi, r)

		// Calculate the duration
		duration := time.Since(start)

		// Calculate the size
		size := buffer.Len()

		// Log the duration, size, and status code
		log.Printf("Status code: %d, Duration: %v, Size: %d bytes\n", crw.statusCode, duration, size)
	})
}

func main() {
	// Your actual handler
	handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		w.Write([]byte("Hello, World!"))
	})

	// Wrap your handler with the logging middleware, you could also router.Use this with gorilla/mux
	http.Handle("/", loggingMiddleware(handler))

	// Start the server
	log.Fatal(http.ListenAndServe(":8080", nil))
}