Golang實(shí)現(xiàn)請(qǐng)求限流的幾種辦法

在開發(fā)高并發(fā)系統(tǒng)時(shí)，有三把利器用來保護(hù)系統(tǒng)：緩存、降級(jí)和限流。那么何為限流呢？顧名思義，限流就是限制流量，就像你寬帶包了1個(gè)G的流量，用完了就沒了。

簡(jiǎn)單的并發(fā)控制

利用 channel 的緩沖設(shè)定，我們就可以來實(shí)現(xiàn)并發(fā)的限制。我們只要在執(zhí)行并發(fā)的同時(shí)，往一個(gè)帶有緩沖的 channel 里寫入點(diǎn)東西（隨便寫啥，內(nèi)容不重要）。讓并發(fā)的 goroutine在執(zhí)行完成后把這個(gè) channel 里的東西給讀走。這樣整個(gè)并發(fā)的數(shù)量就講控制在這個(gè) channel的緩沖區(qū)大小上。

比如我們可以用一個(gè) bool 類型的帶緩沖 channel 作為并發(fā)限制的計(jì)數(shù)器。

   chLimit := make(chan bool, 1)

然后在并發(fā)執(zhí)行的地方，每創(chuàng)建一個(gè)新的 goroutine，都往 chLimit 里塞個(gè)東西。

    for i, sleeptime := range input {
    chs[i] = make(chan string, 1)
    chLimit <- true
    go limitFunc(chLimit, chs[i], i, sleeptime, timeout)
}

這里通過 go 關(guān)鍵字并發(fā)執(zhí)行的是新構(gòu)造的函數(shù)。他在執(zhí)行完后，會(huì)把 chLimit的緩沖區(qū)里給消費(fèi)掉一個(gè)。

     limitFunc := func(chLimit chan bool, ch chan string, task_id, sleeptime, timeout int) {
    Run(task_id, sleeptime, timeout, ch)
    <-chLimit
}

這樣一來，當(dāng)創(chuàng)建的 goroutine 數(shù)量到達(dá) chLimit 的緩沖區(qū)上限后。主 goroutine 就掛起阻塞了，直到這些 goroutine 執(zhí)行完畢，消費(fèi)掉了 chLimit 緩沖區(qū)中的數(shù)據(jù)，程序才會(huì)繼續(xù)創(chuàng)建新的 goroutine 。我們并發(fā)數(shù)量限制的目的也就達(dá)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))
}

運(yùn)行結(jié)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

如果修改并發(fā)限制為2：

     chLimit := make(chan bool, 2)

運(yùn)行結(jié)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

使用計(jì)數(shù)器實(shí)現(xiàn)請(qǐng)求限流

限流的要求是在指定的時(shí)間間隔內(nèi)，server 最多只能服務(wù)指定數(shù)量的請(qǐng)求。實(shí)現(xiàn)的原理是我們啟動(dòng)一個(gè)計(jì)數(shù)器，每次服務(wù)請(qǐng)求會(huì)把計(jì)數(shù)器加一，同時(shí)到達(dá)指定的時(shí)間間隔后會(huì)把計(jì)數(shù)器清零；這個(gè)計(jì)數(shù)器的實(shí)現(xiàn)代碼如下所示：

    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
}

在服務(wù)請(qǐng)求的時(shí)候, 我們會(huì)對(duì)當(dāng)前計(jì)數(shù)器和閾值進(jìn)行比較，只有未超過閾值時(shí)才進(jìn)行服務(wù)：

    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)
}

完整代碼url

使用golang官方包實(shí)現(xiàn)httpserver頻率限制

使用golang來編寫httpserver時(shí)，可以使用官方已經(jīng)有實(shí)現(xiàn)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.................
}

源碼[url] ( https://github.com/golang/net/blob/master/netutil/listen.go)，基本思路就是為連接數(shù)計(jì)數(shù)，通過make chan來建立一個(gè)最大連接數(shù)的channel, 每次accept就+1，close時(shí)候就-1. 當(dāng)?shù)竭_(dá)最大連接數(shù)時(shí)，就等待空閑連接出來之后再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滿了，就會(huì)阻塞在這
    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時(shí)釋放占用的sem
    l.releaseOnce.Do(l.release)
    return err
}

使用Token Bucket（令牌桶算法）實(shí)現(xiàn)請(qǐng)求限流

在開發(fā)高并發(fā)系統(tǒng)時(shí)有三把利器用來保護(hù)系統(tǒng)：緩存、降級(jí)和限流!為了保證在業(yè)務(wù)高峰期，線上系統(tǒng)也能保證一定的彈性和穩(wěn)定性，最有效的方案就是進(jìn)行服務(wù)降級(jí)了，而限流就是降級(jí)系統(tǒng)最常采用的方案之一。

這里為大家推薦一個(gè)開源庫https://github.com/didip/tollbooth，但是，如果您想要一些簡(jiǎn)單的、輕量級(jí)的或者只是想要學(xué)習(xí)的東西，實(shí)現(xiàn)自己的中間件來處理速率限制并不困難。今天我們就來聊聊如何實(shí)現(xiàn)自己的一個(gè)限流中間件

首先我們需要安裝一個(gè)提供了 Token bucket (令牌桶算法)的依賴包，上面提到的toolbooth 的實(shí)現(xiàn)也是基于它實(shí)現(xiàn)的：

     $ go get golang.org/x/time/rate

Demo代碼的實(shí)現(xiàn)

     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：用戶配置的平均發(fā)送速率為r，則每隔1/r秒一個(gè)令牌被加入到桶中（每秒會(huì)有r個(gè)令牌放入桶中），桶中最多可以存放b個(gè)令牌。如果令牌到達(dá)時(shí)令牌桶已經(jīng)滿了，那么這個(gè)令牌會(huì)被丟棄；

實(shí)現(xiàn)

      // 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)
}

雖然在某些情況下使用單個(gè)全局速率限制器非常有用，但另一種常見情況是基于IP地址或API密鑰等標(biāo)識(shí)符為每個(gè)用戶實(shí)施速率限制器。我們將使用IP地址作為標(biāo)識(shí)符。簡(jiǎn)單實(shí)現(xiàn)代碼如下：

     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)
    })
}
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