req-limiter-2.go

package main

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

const maxOutstanding = 2

var counter = 0

func process(r *http.Request) {
	time.Sleep(time.Second * 3)
}

func handle(queue <-chan *http.Request) {
	for r := range queue {
		process(r)
		counter++
		fmt.Println(counter)
	}
}

func serve(queue <-chan *http.Request) {
	for i := 0; i < maxOutstanding; i++ {
		go handle(queue)
	}
}

func main() {
	var queue = make(chan *http.Request)

	go serve(queue)

	http.HandleFunc("/go", func(res http.ResponseWriter, req *http.Request) {
		queue <- req
	})

	http.ListenAndServe(":3001", nil)
}

req-limiter.go

package main

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

const maxOutstanding = 2

var counter = 0

var sem = make(chan int, maxOutstanding)
var queue = make(chan *http.Request)

func process(r *http.Request) {
	time.Sleep(time.Second * 3)
}

func handle(r *http.Request) {
	process(r)
	counter++
	fmt.Println(counter)

	<-sem
}

func serve(queue <-chan *http.Request) {
	for req := range queue {
		sem <- 1
		go handle(req)
	}
}

func goHandler(res http.ResponseWriter, req *http.Request) {
	queue <- req
}

func main() {
	go serve(queue)

	http.HandleFunc("/go", goHandler)

	http.ListenAndServe(":3001", nil)
}