middleware_full.go

package main

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

func init() {
	log.SetFlags(log.Lshortfile)
}

// middleware provides a convenient mechanism for filtering HTTP requests
// entering the application. It returns a new handler which performs various
// operations and finishes with calling the next HTTP handler.
type middleware func(http.HandlerFunc) http.HandlerFunc

// chainMiddleware provides syntactic sugar to create a new middleware
// which will be the result of chaining the ones received as parameters.
func chainMiddleware(mw ...middleware) middleware {
	return func(final http.HandlerFunc) http.HandlerFunc {
		return func(w http.ResponseWriter, r *http.Request) {
			last := final
			for i := len(mw) - 1; i >= 0; i-- {
				last = mw[i](last)
			}
			last(w, r)
		}
	}
}

func withLogging(next http.HandlerFunc) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		log.Printf("Logged connection from %s", r.RemoteAddr)
		next.ServeHTTP(w, r)
	}
}

func withTracing(next http.HandlerFunc) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		log.Printf("Tracing request for %s", r.RequestURI)
		next.ServeHTTP(w, r)
	}
}

func home(w http.ResponseWriter, r *http.Request) {
	log.Println("reached home")
	fmt.Fprintf(w, "welcome")
}

func main() {
	mw := chainMiddleware(withLogging, withTracing)
	http.Handle("/", mw(home))
	log.Fatal(http.ListenAndServe(":8080", nil))
}

@gbbr thanks for smallest middleware chaining example.

One minor feedback, in chainMiddleware, for each new request, the for_loop will get executed. We can avoid this by taking for_loop just one level up (outer func) so that on every new request this loop will not run. This for_loop run once when new handler is registered. Every time new request arrives, it just call last and the magic of middleware chaining will happen. Here is modified version link