Go如何实现HTTP请求限流示例

在开发高并发系统时有三把利器用来保护系统：缓存、降级和限流!为了保证在业务高峰期，线上系统也能保证一定的弹性和稳定性，最有效的方案就是进行服务降级了，而限流就是降级系统最常采用的方案之一。

这里为大家推荐一个开源库 https://github.com/didip/tollbooth 但是，如果您想要一些简单的、轻量级的或者只是想要学习的东西，实现自己的中间件来处理速率限制并不困难。今天我们就来聊聊如何实现自己的一个限流中间件

首先我们需要安装一个提供了 Token bucket (令牌桶算法)的依赖包，上面提到的toolbooth 的实现也是基于它实现的

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

好了我们先看Demo代码的实现：

limit.go

package main

import (
 "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)
 })
}

main.go

package main

import (
 "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"))
}

我们看看 rate.NewLimiter的源码：

// 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 rate

import (
"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)
}

算法描述：

用户配置的平均发送速率为r，则每隔1/r秒一个令牌被加入到桶中（每秒会有r个令牌放入桶中），桶中最多可以存放b个令牌。如果令牌到达时令牌桶已经满了，那么这个令牌会被丢弃；

实现用户粒度的限流

虽然在某些情况下使用单个全局速率限制器非常有用，但另一种常见情况是基于IP地址或API密钥等标识符为每个用户实施速率限制器。我们将使用IP地址作为标识符。简单实现代码如下：

package main
import (
 "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)
 })
}

当然这只是一个简单的实现方案，如果我们要在微服务的API-GateWay中去实现限流还是要考虑很多东西的。建议大家可以看看 https://github.com/didip/tollbooth 的源码。

来源：https://xiequan.info/go如何实现http请求限流/