$ go get golang.org/x/time/rate

package mainimport (  "net/http"  "golang.org/x/time/rate")var limiter = rate.NewLimiter(2, 5)func limit(next http.Handler) http.Handler {  return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {    if limiter.Allow() == false {      http.Error(w, http.StatusText(429), http.StatusTooManyRequests)      return    }    next.ServeHTTP(w, r)  })}

package mainimport (  "net/http")func main() {  mux := http.NewServeMux()  mux.HandleFunc("/", okHandler)  // Wrap the servemux with the limit middleware.  http.ListenAndServe(":4000", limit(mux))}func okHandler(w http.ResponseWriter, r *http.Request) {  w.Write([]byte("OK"))}

// Copyright 2015 The Go Authors. All rights reserved.// Use of this source code is governed by a BSD-style// license that can be found in the LICENSE file.// Package rate provides a rate limiter.package rateimport ( "fmt" "math" "sync" "time" "golang.org/x/net/context")// Limit defines the maximum frequency of some events.// Limit is represented as number of events per second.// A zero Limit allows no events.type Limit float64// Inf is the infinite rate limit; it allows all events (even if burst is zero).const Inf = Limit(math.MaxFloat64)// Every converts a minimum time interval between events to a Limit.func Every(interval time.Duration) Limit { if interval <= 0 {  return Inf } return 1 / Limit(interval.Seconds())}// A Limiter controls how frequently events are allowed to happen.// It implements a "token bucket" of size b, initially full and refilled// at rate r tokens per second.// Informally, in any large enough time interval, the Limiter limits the// rate to r tokens per second, with a maximum burst size of b events.// As a special case, if r == Inf (the infinite rate), b is ignored.// See https://en.wikipedia.org/wiki/Token_bucket for more about token buckets.//// The zero value is a valid Limiter, but it will reject all events.// Use NewLimiter to create non-zero Limiters.//// Limiter has three main methods, Allow, Reserve, and Wait.// Most callers should use Wait.//// Each of the three methods consumes a single token.// They differ in their behavior when no token is available.// If no token is available, Allow returns false.// If no token is available, Reserve returns a reservation for a future token// and the amount of time the caller must wait before using it.// If no token is available, Wait blocks until one can be obtained// or its associated context.Context is canceled.//// The methods AllowN, ReserveN, and WaitN consume n tokens.type Limiter struct { limit Limit burst int mu   sync.Mutex tokens float64 // last is the last time the limiter's tokens field was updated last time.Time // lastEvent is the latest time of a rate-limited event (past or future) lastEvent time.Time}// Limit returns the maximum overall event rate.func (lim *Limiter) Limit() Limit { lim.mu.Lock() defer lim.mu.Unlock() return lim.limit}// Burst returns the maximum burst size. Burst is the maximum number of tokens// that can be consumed in a single call to Allow, Reserve, or Wait, so higher// Burst values allow more events to happen at once.// A zero Burst allows no events, unless limit == Inf.func (lim *Limiter) Burst() int { return lim.burst}// NewLimiter returns a new Limiter that allows events up to rate r and permits// bursts of at most b tokens.func NewLimiter(r Limit, b int) *Limiter { return &Limiter{  limit: r,  burst: b, }}// Allow is shorthand for AllowN(time.Now(), 1).func (lim *Limiter) Allow() bool { return lim.AllowN(time.Now(), 1)}// AllowN reports whether n events may happen at time now.// Use this method if you intend to drop / skip events that exceed the rate limit.// Otherwise use Reserve or Wait.func (lim *Limiter) AllowN(now time.Time, n int) bool { return lim.reserveN(now, n, 0).ok}// A Reservation holds information about events that are permitted by a Limiter to happen after a delay.// A Reservation may be canceled, which may enable the Limiter to permit additional events.type Reservation struct { ok    bool lim    *Limiter tokens  int timeToAct time.Time // This is the Limit at reservation time, it can change later. limit Limit}// OK returns whether the limiter can provide the requested number of tokens// within the maximum wait time. If OK is false, Delay returns InfDuration, and// Cancel does nothing.func (r *Reservation) OK() bool { return r.ok}// Delay is shorthand for DelayFrom(time.Now()).func (r *Reservation) Delay() time.Duration { return r.DelayFrom(time.Now())}// InfDuration is the duration returned by Delay when a Reservation is not OK.const InfDuration = time.Duration(1<<63 - 1)// DelayFrom returns the duration for which the reservation holder must wait// before taking the reserved action. Zero duration means act immediately.// InfDuration means the limiter cannot grant the tokens requested in this// Reservation within the maximum wait time.func (r *Reservation) DelayFrom(now time.Time) time.Duration { if !r.ok {  return InfDuration } delay := r.timeToAct.Sub(now) if delay < 0 {  return 0 } return delay}// Cancel is shorthand for CancelAt(time.Now()).func (r *Reservation) Cancel() { r.CancelAt(time.Now()) return}// CancelAt indicates that the reservation holder will not perform the reserved action// and reverses the effects of this Reservation on the rate limit as much as possible,// considering that other reservations may have already been made.func (r *Reservation) CancelAt(now time.Time) { if !r.ok {  return } r.lim.mu.Lock() defer r.lim.mu.Unlock() if r.lim.limit == Inf || r.tokens == 0 || r.timeToAct.Before(now) {  return } // calculate tokens to restore // The duration between lim.lastEvent and r.timeToAct tells us how many tokens were reserved // after r was obtained. These tokens should not be restored. restoreTokens := float64(r.tokens) - r.limit.tokensFromDuration(r.lim.lastEvent.Sub(r.timeToAct)) if restoreTokens <= 0 {  return } // advance time to now now, _, tokens := r.lim.advance(now) // calculate new number of tokens tokens += restoreTokens if burst := float64(r.lim.burst); tokens > burst {  tokens = burst } // update state r.lim.last = now r.lim.tokens = tokens if r.timeToAct == r.lim.lastEvent {  prevEvent := r.timeToAct.Add(r.limit.durationFromTokens(float64(-r.tokens)))  if !prevEvent.Before(now) {   r.lim.lastEvent = prevEvent  } } return}// Reserve is shorthand for ReserveN(time.Now(), 1).func (lim *Limiter) Reserve() *Reservation { return lim.ReserveN(time.Now(), 1)}// ReserveN returns a Reservation that indicates how long the caller must wait before n events happen.// The Limiter takes this Reservation into account when allowing future events.// ReserveN returns false if n exceeds the Limiter's burst size.// Usage example://  r, ok := lim.ReserveN(time.Now(), 1)//  if !ok {//   // Not allowed to act! Did you remember to set lim.burst to be > 0 ?//  }//  time.Sleep(r.Delay())//  Act()// Use this method if you wish to wait and slow down in accordance with the rate limit without dropping events.// If you need to respect a deadline or cancel the delay, use Wait instead.// To drop or skip events exceeding rate limit, use Allow instead.func (lim *Limiter) ReserveN(now time.Time, n int) *Reservation { r := lim.reserveN(now, n, InfDuration) return &r}// Wait is shorthand for WaitN(ctx, 1).func (lim *Limiter) Wait(ctx context.Context) (err error) { return lim.WaitN(ctx, 1)}// WaitN blocks until lim permits n events to happen.// It returns an error if n exceeds the Limiter's burst size, the Context is// canceled, or the expected wait time exceeds the Context's Deadline.func (lim *Limiter) WaitN(ctx context.Context, n int) (err error) { if n > lim.burst {  return fmt.Errorf("rate: Wait(n=%d) exceeds limiter's burst %d", n, lim.burst) } // Check if ctx is already cancelled select { case <-ctx.Done():  return ctx.Err() default: } // Determine wait limit now := time.Now() waitLimit := InfDuration if deadline, ok := ctx.Deadline(); ok {  waitLimit = deadline.Sub(now) } // Reserve r := lim.reserveN(now, n, waitLimit) if !r.ok {  return fmt.Errorf("rate: Wait(n=%d) would exceed context deadline", n) } // Wait t := time.NewTimer(r.DelayFrom(now)) defer t.Stop() select { case <-t.C:  // We can proceed.  return nil case <-ctx.Done():  // Context was canceled before we could proceed. Cancel the  // reservation, which may permit other events to proceed sooner.  r.Cancel()  return ctx.Err() }}// SetLimit is shorthand for SetLimitAt(time.Now(), newLimit).func (lim *Limiter) SetLimit(newLimit Limit) { lim.SetLimitAt(time.Now(), newLimit)}// SetLimitAt sets a new Limit for the limiter. The new Limit, and Burst, may be violated// or underutilized by those which reserved (using Reserve or Wait) but did not yet act// before SetLimitAt was called.func (lim *Limiter) SetLimitAt(now time.Time, newLimit Limit) { lim.mu.Lock() defer lim.mu.Unlock() now, _, tokens := lim.advance(now) lim.last = now lim.tokens = tokens lim.limit = newLimit}// reserveN is a helper method for AllowN, ReserveN, and WaitN.// maxFutureReserve specifies the maximum reservation wait duration allowed.// reserveN returns Reservation, not *Reservation, to avoid allocation in AllowN and WaitN.func (lim *Limiter) reserveN(now time.Time, n int, maxFutureReserve time.Duration) Reservation { lim.mu.Lock() defer lim.mu.Unlock() if lim.limit == Inf {  return Reservation{   ok:    true,   lim:    lim,   tokens:  n,   timeToAct: now,  } } now, last, tokens := lim.advance(now) // Calculate the remaining number of tokens resulting from the request. tokens -= float64(n) // Calculate the wait duration var waitDuration time.Duration if tokens < 0 {  waitDuration = lim.limit.durationFromTokens(-tokens) } // Decide result ok := n <= lim.burst && waitDuration <= maxFutureReserve // Prepare reservation r := Reservation{  ok:  ok,  lim:  lim,  limit: lim.limit, } if ok {  r.tokens = n  r.timeToAct = now.Add(waitDuration) } // Update state if ok {  lim.last = now  lim.tokens = tokens  lim.lastEvent = r.timeToAct } else {  lim.last = last } return r}// advance calculates and returns an updated state for lim resulting from the passage of time.// lim is not changed.func (lim *Limiter) advance(now time.Time) (newNow time.Time, newLast time.Time, newTokens float64) { last := lim.last if now.Before(last) {  last = now } // Avoid making delta overflow below when last is very old. maxElapsed := lim.limit.durationFromTokens(float64(lim.burst) - lim.tokens) elapsed := now.Sub(last) if elapsed > maxElapsed {  elapsed = maxElapsed } // Calculate the new number of tokens, due to time that passed. delta := lim.limit.tokensFromDuration(elapsed) tokens := lim.tokens + delta if burst := float64(lim.burst); tokens > burst {  tokens = burst } return now, last, tokens}// durationFromTokens is a unit conversion function from the number of tokens to the duration// of time it takes to accumulate them at a rate of limit tokens per second.func (limit Limit) durationFromTokens(tokens float64) time.Duration { seconds := tokens / float64(limit) return time.Nanosecond * time.Duration(1e9*seconds)}// tokensFromDuration is a unit conversion function from a time duration to the number of tokens// which could be accumulated during that duration at a rate of limit tokens per second.func (limit Limit) tokensFromDuration(d time.Duration) float64 { return d.Seconds() * float64(limit)}

package mainimport (  "net/http"  "sync"  "time"  "golang.org/x/time/rate")// Create a custom visitor struct which holds the rate limiter for each// visitor and the last time that the visitor was seen.type visitor struct {  limiter *rate.Limiter  lastSeen time.Time}// Change the the map to hold values of the type visitor.var visitors = make(map[string]*visitor)var mtx sync.Mutex// Run a background goroutine to remove old entries from the visitors map.func init() {  go cleanupVisitors()}func addVisitor(ip string) *rate.Limiter {  limiter := rate.NewLimiter(2, 5)  mtx.Lock()  // Include the current time when creating a new visitor.  visitors[ip] = &visitor{limiter, time.Now()}  mtx.Unlock()  return limiter}func getVisitor(ip string) *rate.Limiter {  mtx.Lock()  v, exists := visitors[ip]  if !exists {    mtx.Unlock()    return addVisitor(ip)  }  // Update the last seen time for the visitor.  v.lastSeen = time.Now()  mtx.Unlock()  return v.limiter}// Every minute check the map for visitors that haven't been seen for// more than 3 minutes and delete the entries.func cleanupVisitors() {  for {    time.Sleep(time.Minute)    mtx.Lock()    for ip, v := range visitors {      if time.Now().Sub(v.lastSeen) > 3*time.Minute {        delete(visitors, ip)      }    }    mtx.Unlock()  }}func limit(next http.Handler) http.Handler {  return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {    limiter := getVisitor(r.RemoteAddr)    if limiter.Allow() == false {      http.Error(w, http.StatusText(429), http.StatusTooManyRequests)      return    }    next.ServeHTTP(w, r)  })}