Golang实现请求限流的几种办法(小结)

Go  /  管理员 发布于 7年前   436

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。那么何为限流呢？顾名思义，限流就是限制流量，就像你宽带包了1个G的流量，用完了就没了。

简单的并发控制

利用 channel 的缓冲设定，我们就可以来实现并发的限制。我们只要在执行并发的同时，往一个带有缓冲的 channel 里写入点东西（随便写啥，内容不重要）。让并发的 goroutine在执行完成后把这个 channel 里的东西给读走。这样整个并发的数量就讲控制在这个 channel的缓冲区大小上。

比如我们可以用一个 bool 类型的带缓冲 channel 作为并发限制的计数器。

chLimit := make(chan bool, 1)

然后在并发执行的地方，每创建一个新的 goroutine，都往 chLimit 里塞个东西。

for i, sleeptime := range input {  chs[i] = make(chan string, 1)  chLimit <- true  go limitFunc(chLimit, chs[i], i, sleeptime, timeout)}

这里通过 go 关键字并发执行的是新构造的函数。他在执行完后，会把 chLimit的缓冲区里给消费掉一个。

limitFunc := func(chLimit chan bool, ch chan string, task_id, sleeptime, timeout int) {  Run(task_id, sleeptime, timeout, ch)  <-chLimit}

这样一来，当创建的 goroutine 数量到达 chLimit 的缓冲区上限后。主 goroutine 就挂起阻塞了，直到这些 goroutine 执行完毕，消费掉了 chLimit 缓冲区中的数据，程序才会继续创建新的 goroutine 。我们并发数量限制的目的也就达到了。

例子

package main import (  "fmt"  "time") func Run(task_id, sleeptime, timeout int, ch chan string) {  ch_run := make(chan string)  go run(task_id, sleeptime, ch_run)  select {  case re := <-ch_run:    ch <- re  case <-time.After(time.Duration(timeout) * time.Second):    re := fmt.Sprintf("task id %d , timeout", task_id)    ch <- re  }} func run(task_id, sleeptime int, ch chan string) {   time.Sleep(time.Duration(sleeptime) * time.Second)  ch <- fmt.Sprintf("task id %d , sleep %d second", task_id, sleeptime)  return} func main() {  input := []int{3, 2, 1}  timeout := 2  chLimit := make(chan bool, 1)  chs := make([]chan string, len(input))  limitFunc := func(chLimit chan bool, ch chan string, task_id, sleeptime, timeout int) {    Run(task_id, sleeptime, timeout, ch)    <-chLimit  }  startTime := time.Now()  fmt.Println("Multirun start")  for i, sleeptime := range input {    chs[i] = make(chan string, 1)    chLimit <- true    go limitFunc(chLimit, chs[i], i, sleeptime, timeout)  }   for _, ch := range chs {    fmt.Println(<-ch)  }  endTime := time.Now()  fmt.Printf("Multissh finished. Process time %s. Number of task is %d", endTime.Sub(startTime), len(input))}

运行结果：

Multirun start
     task id 0 , timeout
     task id 1 , timeout
     task id 2 , sleep 1 second
     Multissh finished. Process time 5s. Number of task is 3

如果修改并发限制为2：

chLimit := make(chan bool, 2)

运行结果：

Multirun start
    task id 0 , timeout
    task id 1 , timeout
    task id 2 , sleep 1 second
    Multissh finished. Process time 3s. Number of task is 3

使用计数器实现请求限流

限流的要求是在指定的时间间隔内，server 最多只能服务指定数量的请求。实现的原理是我们启动一个计数器，每次服务请求会把计数器加一，同时到达指定的时间间隔后会把计数器清零；这个计数器的实现代码如下所示：

type RequestLimitService struct {  Interval time.Duration  MaxCount int  Lock   sync.Mutex  ReqCount int} func NewRequestLimitService(interval time.Duration, maxCnt int) *RequestLimitService {  reqLimit := &RequestLimitService{    Interval: interval,    MaxCount: maxCnt,  }   go func() {    ticker := time.NewTicker(interval)    for {      <-ticker.C      reqLimit.Lock.Lock()      fmt.Println("Reset Count...")      reqLimit.ReqCount = 0      reqLimit.Lock.Unlock()    }  }()   return reqLimit} func (reqLimit *RequestLimitService) Increase() {  reqLimit.Lock.Lock()  defer reqLimit.Lock.Unlock()   reqLimit.ReqCount += 1} func (reqLimit *RequestLimitService) IsAvailable() bool {  reqLimit.Lock.Lock()  defer reqLimit.Lock.Unlock()   return reqLimit.ReqCount < reqLimit.MaxCount}

在服务请求的时候, 我们会对当前计数器和阈值进行比较，只有未超过阈值时才进行服务：

var RequestLimit = NewRequestLimitService(10 * time.Second, 5) func helloHandler(w http.ResponseWriter, r *http.Request) {  if RequestLimit.IsAvailable() {    RequestLimit.Increase()    fmt.Println(RequestLimit.ReqCount)    io.WriteString(w, "Hello world!\n")  } else {    fmt.Println("Reach request limiting!")    io.WriteString(w, "Reach request limit!\n")  }} func main() {  fmt.Println("Server Started!")  http.HandleFunc("/", helloHandler)  http.ListenAndServe(":8000", nil)}

完整代码 url

使用golang官方包实现httpserver频率限制

使用golang来编写httpserver时，可以使用官方已经有实现好的包：

import(  "fmt"  "net"  "golang.org/x/net/netutil") func main() {  l, err := net.Listen("tcp", "127.0.0.1:0")  if err != nil {    fmt.Fatalf("Listen: %v", err)  }  defer l.Close()  l = LimitListener(l, max)    http.Serve(l, http.HandlerFunc())    //bla bla bla.................}

源码[url] ( https://github.com/golang/net/blob/master/netutil/listen.go )，基本思路就是为连接数计数，通过make chan来建立一个最大连接数的channel, 每次accept就+1，close时候就-1. 当到达最大连接数时，就等待空闲连接出来之后再accept。

// Copyright 2013 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 netutil provides network utility functions, complementing the more// common ones in the net package.package netutil // import "golang.org/x/net/netutil" import (  "net"  "sync") // LimitListener returns a Listener that accepts at most n simultaneous// connections from the provided Listener.func LimitListener(l net.Listener, n int) net.Listener {  return &limitListener{    Listener: l,    sem:   make(chan struct{}, n),    done:   make(chan struct{}),  }} type limitListener struct {  net.Listener  sem    chan struct{}  closeOnce sync.Once   // ensures the done chan is only closed once  done   chan struct{} // no values sent; closed when Close is called} // acquire acquires the limiting semaphore. Returns true if successfully// accquired, false if the listener is closed and the semaphore is not// acquired.func (l *limitListener) acquire() bool {  select {  case <-l.done:    return false  case l.sem <- struct{}{}:    return true  }}func (l *limitListener) release() { <-l.sem } func (l *limitListener) Accept() (net.Conn, error) {  //如果sem满了，就会阻塞在这  acquired := l.acquire()  // If the semaphore isn't acquired because the listener was closed, expect  // that this call to accept won't block, but immediately return an error.  c, err := l.Listener.Accept()  if err != nil {    if acquired {      l.release()    }    return nil, err  }  return &limitListenerConn{Conn: c, release: l.release}, nil} func (l *limitListener) Close() error {  err := l.Listener.Close()  l.closeOnce.Do(func() { close(l.done) })  return err} type limitListenerConn struct {  net.Conn  releaseOnce sync.Once  release   func()} func (l *limitListenerConn) Close() error {  err := l.Conn.Close()  //close时释放占用的sem  l.releaseOnce.Do(l.release)  return err}

使用Token Bucket（令牌桶算法）实现请求限流

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

Demo代码的实现

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)  })} 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"))}

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

// 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 {  //maximum token, token num per second  limit Limit  //burst field, max token num  burst int  mu  sync.Mutex  //tokens num, change  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  //This is the time to action  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)}

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 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)  })}

以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持。