go的限流

背景

服务请求下游，oom，排查下来发现是一个下游组件qps陡增导致
但是司内网络框架比较挫，竟然不负责框架内存问题（有内存管理模块，但逻辑又是无限制使用内存）
每个请求一个r、w buffer，请求无限制，内存不够直接就oom，然后就被linux给迁移掉了
所以才有了加限流的必要性（粉饰太平）
所以站在更高维度去考虑这个问题，就变成了一个网络框架是否要去管理内存？

作用

限制请求速率，保护服务，以免服务过载

常用的限流方法

固定窗口、滑动窗口、漏桶、令牌桶

令牌桶：
一个固定大小的桶，系统会以恒定速率向桶中放 Token，桶满则暂时不放
如果桶中有剩余 Token 就可以一直取，如果没有剩余 Token，则需要等到桶中被放置 Token/直接返回失败

本次学习令牌

golang.org/x/time/rate

代码实现

// A Limiter controls how frequently events are allowed to happen.
// Limiter 用来限制发生事件的频率
// 
// It implements a "token bucket" of size b, initially full and refilled
// at rate r tokens per second.
// 初始的时候满的，然后以每秒r tokens的速率来填充，bucket大小为b
// 
// 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.
// r还能设置为 Inf？
// 
// The zero value is a valid Limiter, but it will reject all events.
// Use NewLimiter to create non-zero Limiters.
// 还能有拒绝全部的零值Limiter？妙啊
//
// Limiter has three main methods, Allow, Reserve, and Wait.
// Most callers should use Wait.
// 三个方法：Allow, Reserve, Wait，每个方法都会消耗一个token
//
// 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.
// Allow：非阻塞
// 
// 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.
// Reserve 还能预定？ 牛哇，还能返回如果一定要使用的话，要等多久
// 
// If no token is available, Wait blocks until one can be obtained
// or its associated context.Context is canceled.
// Wait：阻塞
//
// The methods AllowN, ReserveN, and WaitN consume n tokens.
// AllowN、ReserveN、WaitN：消费n个token
type Limiter struct {mu     sync.Mutex// 每秒的事件数，即事件速率limit  Limit// 单次调用（Allow, Reserve, Wait）中消费token的最大数// 更高的 Burst 的值，将会一次允许更多的事件发生（at once）burst  inttokens float64// last is the last time the limiter's tokens field was updated// limiter的tokens字段的前一次被更新的事件last time.Time// lastEvent is the latest time of a rate-limited event (past or future)// 速率受限事件（past or future）的前一次时间lastEvent time.Time
}// A zero Limit allows no events.
type Limit float64

看了结构体就知道如何设计一个很粗糙的限流器了
没有用复杂的结构，就是一个简单的原子计数

使用

// NewLimiter returns a new Limiter that allows events up to rate r and permits
// bursts of at most b tokens.
// 速率r，一次b个突发
func NewLimiter(r Limit, b int) *Limiter {return &Limiter{limit: r,burst: b,}
}

NewLimiter的参数除了用Limit传，还能传间隔

func Every(interval time.Duration) Limit {if interval <= 0 {return Inf}return 1 / Limit(interval.Seconds())
}

Allow、AllowN、Reserve、ReserveN、Wait、WaitN
里面用的都是 reserveN

// Allow reports whether an event may happen now.
func (lim *Limiter) Allow() bool {return lim.AllowN(time.Now(), 1)
}// AllowN reports whether n events may happen at time t.
// Use this method if you intend to drop / skip events that exceed the rate limit.
// Otherwise use Reserve or Wait.
func (lim *Limiter) AllowN(t time.Time, n int) bool {return lim.reserveN(t, n, 0).ok
}// 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.
// The returned Reservation’s OK() method returns false if n exceeds the Limiter's burst size.
// Usage example:
//
//	r := lim.ReserveN(time.Now(), 1)
//	if !r.OK() {
//	  // Not allowed to act! Did you remember to set lim.burst to be > 0 ?
//	  return
//	}
//	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(t time.Time, n int) *Reservation {r := lim.reserveN(t, 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.
// The burst limit is ignored if the rate limit is Inf.
func (lim *Limiter) WaitN(ctx context.Context, n int) (err error) {// The test code calls lim.wait with a fake timer generator.// This is the real timer generator.newTimer := func(d time.Duration) (<-chan time.Time, func() bool, func()) {timer := time.NewTimer(d)return timer.C, timer.Stop, func() {}}return lim.wait(ctx, n, time.Now(), newTimer)
}// wait is the internal implementation of WaitN.
func (lim *Limiter) wait(ctx context.Context, n int, t time.Time, newTimer func(d time.Duration) (<-chan time.Time, func() bool, func())) error {lim.mu.Lock()burst := lim.burstlimit := lim.limitlim.mu.Unlock()// 在限流的情况下，一次请求索要超过 burst 的令牌if n > burst && limit != Inf {return fmt.Errorf("rate: Wait(n=%d) exceeds limiter's burst %d", n, burst)}// Check if ctx is already cancelled// 用这种方式检查ctx是否结束select {case <-ctx.Done():return ctx.Err()default:// 这里有 default，所以不会卡住}// Determine wait limitwaitLimit := InfDuration// 它竟然还照顾了ctx的Deadline，牛哇牛哇if deadline, ok := ctx.Deadline(); ok {// t是当前时间，deadline-t就是等待时长waitLimit = deadline.Sub(t)}// Reserver := lim.reserveN(t, n, waitLimit)if !r.ok {return fmt.Errorf("rate: Wait(n=%d) would exceed context deadline", n)}// Wait if necessary// 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.// 0时长意味着立即执行，不用等limiter// InfDuration时长意味着在此次最大的等待时间里，无法授权这么多的tokendelay := r.DelayFrom(t)if delay == 0 {return nil}// newTimer 中创建了一个定时器，这里用完要停止，不然系统中的定时器越来越多// ch：delay定时器// stop：定时器取消函数// advance：仅用于测试，设置钩子，牛哇牛哇ch, stop, advance := newTimer(delay)defer stop()advance() // only has an effect when testing// 等待谁先来select {case <-ch:// We can proceed.return nilcase <-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()}
}