Http middlewares in Go

middlewares.go

package main

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

	"github.com/gorilla/mux"
)

// MiddlewareInterceptor intercepts an HTTP handler invocation
type MiddlewareInterceptor func(http.ResponseWriter, *http.Request, http.HandlerFunc)

// MiddlewareHandlerFunc builds on top of http.HandlerFunc, and exposes API to intercept with MiddlewareInterceptor
type MiddlewareHandlerFunc http.HandlerFunc

// MiddlewareChain is a slice of interceptors that will be invoked in their index order
type MiddlewareChain []MiddlewareInterceptor

// Intercept returns back a continuation that will call install middleware to intercept the continuation call
func (cont MiddlewareHandlerFunc) Intercept(mw MiddlewareInterceptor) MiddlewareHandlerFunc {
	return func(writer http.ResponseWriter, request *http.Request) {
		mw(writer, request, http.HandlerFunc(cont))
	}
}

// Handler allows hooking multiple middleware in single call
func (chain MiddlewareChain) Handler(handler http.HandlerFunc) http.Handler {
	current := MiddlewareHandlerFunc(handler)
	for i := len(chain) - 1; i >= 0; i-- {
		mv := chain[i]
		current = current.Intercept(mv)
	}
	return http.HandlerFunc(current)
}

func NewReqIDInterceptor() MiddlewareInterceptor {
	return func(w http.ResponseWriter, r *http.Request, next http.HandlerFunc) {
		if r.Header.Get("X-Request-Id") == "" {
			r.Header.Set("X-Request-Id", "123abc")
			log.Println(r.Header)
		}
		next(w, r)
	}
}

func ElapsedTimeInterceptor() MiddlewareInterceptor {
	return func(w http.ResponseWriter, r *http.Request, next http.HandlerFunc) {
		startTime := time.Now()
		defer func() {
			endTime := time.Now()
			elapsedTime := endTime.Sub(startTime)
			log.Println("Elapsed time:", elapsedTime)
		}()
		next(w, r)
	}
}

func indexHandler(w http.ResponseWriter, r *http.Request) {
	fmt.Fprintf(w, "some work here")
}

func main() {
	// Chain of middleware to be invoked in order of their index
	middlewareChain := MiddlewareChain{
		NewReqIDInterceptor(),
		ElapsedTimeInterceptor(),
	}

	router := mux.NewRouter()
	// Invoke all middlewares for indexHandler
	router.Path("/").Methods("GET").Handler(middlewareChain.Handler(indexHandler))

	srv := &http.Server{
		Addr:    "0.0.0.0:8000",
		Handler: router,
	}

	if err := srv.ListenAndServe(); err != nil {
		fmt.Println(err)
	}
}