GO middleware example (with gorilla/mux)

go_middleware.go

package main

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

	"github.com/gorilla/context"
	"github.com/gorilla/mux"
)

func main() {
	// gorilla/mux router
	r := mux.NewRouter()

	// use the logger middleware on complete router
	r.Use(logMW)

	// serving routes
	r.HandleFunc("/hello", helloHandler)
	r.HandleFunc("/secret", authMW(secretHandler)) // this route using the authentication middleware

	// start server on port 3333
	http.ListenAndServe("localhost:3333", r)
}

// (open) hello route
func helloHandler(w http.ResponseWriter, r *http.Request) {
	fmt.Fprintln(w, "<h1>Hello World!</h1>")
}

// restricted secret route
func secretHandler(w http.ResponseWriter, r *http.Request) {
	// getting isAdmin from context and convert to bool
	adm := context.Get(r, "isAdmin").(bool)

	// creating response, depending on isAdmin status
	body := "<h1>Hello on secret route.</h1>\n<p>%s</p>"
	var response string
	if adm {
		response = fmt.Sprintf(body, "You are admin.")
	} else {
		response = fmt.Sprintf(body, "You are user.")
	}

	fmt.Fprintln(w, response)
}

// compare the two forms of writing the middleware

// for global use (using a http.Handler!)
func logMW(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		log.Printf("%s - %s (%s)", r.Method, r.URL.Path, r.RemoteAddr)

		// compare the return-value to the authMW
		next.ServeHTTP(w, r)
	})
}

// for use on route (using a http.HandlerFunc)
func authMW(next http.HandlerFunc) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		// read basic auth information
		usr, _, ok := r.BasicAuth()

		// if there is no basic auth (no matter which credentials)
		if !ok {
			errMsg := "Authentication error!"
			// return a 403 forbidden
			http.Error(w, errMsg, http.StatusForbidden)
			log.Println(errMsg)

			// stop processing route
			return
		}

		// let's assume we check the user against a database to get
		// his admin-right and put this to the request context
		context.Set(r, "isAdmin", true)

		// else continue processing
		log.Printf("User %s logged in.", usr)
		next(w, r)
	}
}