详解Golang实现请求限流的几种办法

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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 

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)
    

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。
以下是完整代码：

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 := 

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 {
   

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：https://github.com/hiberabyss/JustDoIt/blob/master/RequestLimit/request_limit.go
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.................
}


源码如下(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 

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


先看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"))
}


然后看看 rate.NewLimiter的源码：
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；

// 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  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  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 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)
    })
}


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