Golang实现请求限流的几种办法(小结)

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] (  )，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  ，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 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)
  })
}

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。