Go如何实现HTTP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

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。