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 mainimport ( "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 mainimport ( "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 rateimport ( "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 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) })}

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。

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。