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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 4he 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。
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社区。 | 
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