Golang HTTP concurrency limiter / throttle for limiting the maximum number of concurrent http handlers running at one time.

http_throttle.go

package main

import (
	"fmt"
	"log"
	"net/http"
	"os"
	"os/exec"
	"strconv"
	"strings"
	"sync/atomic"
	"time"
)

var requestsPerMinute uint64

func main() {

	go func() {
		for {
			select {
			case <-time.After(time.Second):
				num := atomic.SwapUint64(&requestsPerMinute, 0)
				fmt.Printf("%10d requests\n", num)
			}
		}
	}()

	handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		atomic.AddUint64(&requestsPerMinute, 1)
		time.Sleep(time.Second)
		w.Write([]byte("Hello World"))
	})

	http.Handle("/", Throttle(handler, 200))
  
  // You could also use Throttle/Throttler on a mux to limit all endpoints

	http.ListenAndServe(":8080", nil)
}

// Throttle one http.Handler
func Throttle(h http.Handler, n int) http.Handler {
	sema := make(chan struct{}, n)

	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		sema <- struct{}{}
		h.ServeHTTP(w, r)
		<-sema
	})
}

// Throttler creates a re-usable limiter for multiple http.Handlers
func Throttler(n int) func(http.Handler) http.Handler {
	sema := make(chan struct{}, n)

	return func(h http.Handler) http.Handler {
		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			sema <- struct{}{}
			h.ServeHTTP(w, r)
			<-sema
		})
	}
}

echo "GET http://localhost:8080/" | vegeta attack -duration=10s -rate 1000 | vegeta report