在開發高併發系統時，有三把利器用來保護系統：快取、降級和限流。那麼何為限流呢？顧名思義，限流就是限制流量，就像你寬頻包了1個G的流量，用完了就沒了。

簡單的併發控制

利用 channel 的緩衝設定，我們就可以來實現併發的限制。我們只要在執行併發的同時，往一個帶有緩衝的 channel 裡寫入點東西（隨便寫啥，內容不重要）。讓併發的 goroutine在執行完成後把這個 channel 裡的東西給讀走。這樣整個併發的數量就講控制在這個 channel的緩衝區大小上。

比如我們可以用一個 bool 型別的帶緩衝 channel 作為併發限制的計數器。

chLimit := make(chan bool,1)

然後在併發執行的地方，每建立一個新的 goroutine，都往 chLimit 裡塞個東西。

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

這裡通過 go 關鍵字併發執行的是新構造的函式。他在執行完後，會把 chLimit的緩衝區裡給消費掉一個。

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

這樣一來，當建立的 goroutine 數量到達 chLimit 的緩衝區上限後。主 goroutine 就掛起阻塞了，直到這些 goroutine 執行完畢，消費掉了 chLimit 緩衝區中的資料，程式才會繼續建立新的 goroutine 。我們併發數量限制的目的也就達到了。

例子

package main
 
import (
  "fmt"
  "time"
)
 
func Run(task_id,timeout int,ch chan string) {
  ch_run := make(chan string)
  go run(task_id,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",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,timeout int) {
    Run(task_id,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,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))
}

執行結果：

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

如果修改併發限制為2：

chLimit := make(chan bool,2)

執行結果：

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

使用計數器實現請求限流

限流的要求是在指定的時間間隔內，server 最多隻能服務指定數量的請求。實現的原理是我們啟動一個計數器，每次服務請求會把計數器加一，同時到達指定的時間間隔後會把計數器清零；這個計數器的實現程式碼如下所示：

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
}

在服務請求的時候,我們會對當前計數器和閾值進行比較，只有未超過閾值時才進行服務：

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)
}

完整程式碼url

使用golang官方包實現httpserver頻率限制

使用golang來編寫httpserver時，可以使用官方已經有實現好的包：

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.................
}

原始碼[url] (https://github.com/golang/net/blob/master/netutil/listen.go )，基本思路就是為連線數計數，通過make chan來建立一個最大連線數的channel,每次accept就+1，close時候就-1. 當到達最大連線數時，就等待空閒連接出來之後再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滿了，就會阻塞在這
  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時釋放佔用的sem
  l.releaseOnce.Do(l.release)
  return err
}

使用Token Bucket（令牌桶演算法）實現請求限流

在開發高併發系統時有三把利器用來保護系統：快取、降級和限流!為了保證在業務高峰期，線上系統也能保證一定的彈性和穩定性，最有效的方案就是進行服務降級了，而限流就是降級系統最常採用的方案之一。

這裡為大家推薦一個開源庫https://github.com/didip/tollbooth ，但是，如果您想要一些簡單的、輕量級的或者只是想要學習的東西，實現自己的中介軟體來處理速率限制並不困難。今天我們就來聊聊如何實現自己的一個限流中介軟體

首先我們需要安裝一個提供了 Token bucket (令牌桶演算法)的依賴包，上面提到的toolbooth 的實現也是基於它實現的：

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

Demo程式碼的實現

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"))
}

演算法描述：使用者配置的平均傳送速率為r，則每隔1/r秒一個令牌被加入到桶中（每秒會有r個令牌放入桶中），桶中最多可以存放b個令牌。如果令牌到達時令牌桶已經滿了，那麼這個令牌會被丟棄；

實現

// 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,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,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",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,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,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,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)
}

雖然在某些情況下使用單個全域性速率限制器非常有用，但另一種常見情況是基於IP地址或API金鑰等識別符號為每個使用者實施速率限制器。我們將使用IP地址作為識別符號。簡單實現程式碼如下：

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,r)
  })
}

以上就是本文的全部內容，希望對大家的學習有所幫助，也希望大家多多支援我們。