如何跨 goroutine 共享地图

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我正在尝试在 go 中编写一个通知结构，该结构将保存一系列键及其各自的值，并且如果值低于阈值，则会触发通知。

当第一个样本低于阈值时，通知应仅触发一次，并且低于该阈值的其他样本不应再次触发，直到该值上升到阈值以上。

例如，假设我的阈值是 10，我发送 15、14、11、10、... 9 的样本。一旦发送 9，就应该触发通知。进一步的样品 8、7、4 不应造成任何影响。以下示例如 5、6、7、9、10、11、14、30 不应执行任何操作。一旦样本再次低于 10：30、20、15、10、7...，则必须发送另一个通知。

当多个 goroutine 操作我的结构时，我遇到了问题。

我尝试使用sync.mutex进行同步，并且还使用了sync.map，但没有运气。我感觉某个地方有一个参考副本或缓存，但我对 go 太陌生，无法找到问题。

为此，我创建了一个如下结构：

type notifier interface {
    send(message string)
}

type notificationboard struct {
    mutex    sync.mutex
    last     sync.map
    notifier notifier
}

func (n *notificationboard) init(notifier notifier) {
    n.notifier = notifier
}

// notifyless ...
func (n *notificationboard) notifyless(key string, value, threshold float64) {
    n.mutex.lock()
    defer n.mutex.unlock()

    if value >= threshold {
        fmt.printf("notificationboard.notifyless %v (value >= threshold): %v >= %v\n", key, value, threshold)
        n.last.store(key, value)
        return
    }

    // value < threshold
    if last, found := n.last.load(key); found == true {
        fmt.printf("notificationboard.notifyless %v (value < threshold): %v < %v : found %v\n", key, value, threshold, last)
        if last.(float64) >= threshold { // first trigger
            n.notifier.send(fmt.sprintf("%s < %v (%v)", key, threshold, value))
        }
    } else {
        fmt.printf("notificationboard.notifyless %v (value < threshold): %v < %v : not found\n", key, value, threshold)
        // not found, started board as less
        n.notifier.send(fmt.sprintf("%s < %v (%v)", key, threshold, value))
    }

    n.last.store(key, value)
    return
}

我知道使用sync.mutex或sync.map应该足够了，但是上面的代码两者都有，因为它是我当前的（损坏的）版本。

为了测试，我设置了以下代码：

type dummy struct{}

func (d *dummy) send(message string) {
    fmt.println("--------------> notifying", message)
}

func newboard() *notificationboard {
    notificationboard := ¬ificationboard{}
    notificationboard.init(&dummy{})
    return notificationboard
}

我还添加了一些 fmt.println 跟踪（为简洁起见，未包含在上面的代码中）并首先准备了一个单例程测试（按预期工作）：

func test1(t *testing.t) {
    board := newboard()
    board.notifyless("k1", 15, 10)
    board.notifyless("k1", 10, 10)
    board.notifyless("k1", 5, 10)
    board.notifyless("k1", 4, 10)
    board.notifyless("k1", 3, 10)
    board.notifyless("k1", 10, 10)
    board.notifyless("k1", 15, 10)
    board.notifyless("k1", 20, 10)
    board.notifyless("k1", 15, 10)
    board.notifyless("k1", 10, 10)
    board.notifyless("k1", 5, 10)
    board.notifyless("k1", 1, 10)
}

输出：

> go test -run test1
notificationboard.notifyless k1 (value >= threshold): 15 >= 10
notificationboard.notifyless k1 (value >= threshold): 10 >= 10
notificationboard.notifyless k1 (value < threshold): 5 < 10 : found 10
--------------> notifying k1 < 10 (5)
notificationboard.notifyless k1 (value < threshold): 4 < 10 : found 5
notificationboard.notifyless k1 (value < threshold): 3 < 10 : found 4
notificationboard.notifyless k1 (value >= threshold): 10 >= 10
notificationboard.notifyless k1 (value >= threshold): 15 >= 10
notificationboard.notifyless k1 (value >= threshold): 20 >= 10
notificationboard.notifyless k1 (value >= threshold): 15 >= 10
notificationboard.notifyless k1 (value >= threshold): 10 >= 10
notificationboard.notifyless k1 (value < threshold): 5 < 10 : found 10
--------------> notifying k1 < 10 (5)
notificationboard.notifyless k1 (value < threshold): 1 < 10 : found 5
pass

我们可以看到输出“notifying ....”发生了两次，仅在样本低于阈值的时刻

但是后来，我创建了一个多 gooutine 测试，然后通知发生多次：

func test3(t *testing.t) {
    preparing := sync.waitgroup{}
    preparing.add(1)

    board := newboard()
    wg := sync.waitgroup{}

    for i := 0; i < 30; i++ {
        wg.add(1)
        go func(x int, not *notificationboard) {
            fmt.printf("routine %v waiting preparation... \n", x)
            preparing.wait()

            for j := 15.0; j > 5; j-- {
                fmt.printf("routine %v notifying %v\n", x, j)
                not.notifyless("keyx", j+float64(x+1)/100, 10)
            }

            wg.done()
        }(i, board)
    }

    preparing.done()
    wg.wait()

}

输出：

> go test -run test3
routine 7 waiting preparation...
routine 2 waiting preparation...
routine 2 notifying 15
notificationboard.notifyless keyx (value >= threshold): 15.03 >= 10
routine 2 notifying 14
notificationboard.notifyless keyx (value >= threshold): 14.03 >= 10
routine 2 notifying 13
notificationboard.notifyless keyx (value >= threshold): 13.03 >= 10
routine 2 notifying 12
notificationboard.notifyless keyx (value >= threshold): 12.03 >= 10
routine 2 notifying 11
notificationboard.notifyless keyx (value >= threshold): 11.03 >= 10
routine 2 notifying 10
notificationboard.notifyless keyx (value >= threshold): 10.03 >= 10
routine 2 notifying 9
notificationboard.notifyless keyx (value < threshold): 9.03 < 10 : found 10.03
--------------> notifying keyx < 10 (9.03)
routine 2 notifying 8
notificationboard.notifyless keyx (value < threshold): 8.03 < 10 : found 9.03
routine 2 notifying 7
notificationboard.notifyless keyx (value < threshold): 7.03 < 10 : found 8.03
routine 2 notifying 6
notificationboard.notifyless keyx (value < threshold): 6.03 < 10 : found 7.03
routine 14 waiting preparation...
routine 14 notifying 15
notificationboard.notifyless keyx (value >= threshold): 15.15 >= 10
routine 14 notifying 14
notificationboard.notifyless keyx (value >= threshold): 14.15 >= 10
routine 14 notifying 13
notificationboard.notifyless keyx (value >= threshold): 13.15 >= 10
routine 14 notifying 12
notificationboard.notifyless keyx (value >= threshold): 12.15 >= 10
routine 14 notifying 11
notificationboard.notifyless keyx (value >= threshold): 11.15 >= 10
routine 14 notifying 10
notificationboard.notifyless keyx (value >= threshold): 10.15 >= 10
routine 14 notifying 9
notificationboard.notifyless keyx (value < threshold): 9.15 < 10 : found 10.15
--------------> notifying keyx < 10 (9.15)
routine 14 notifying 8
notificationboard.notifyless keyx (value < threshold): 8.15 < 10 : found 9.15
routine 14 notifying 7
notificationboard.notifyless keyx (value < threshold): 7.15 < 10 : found 8.15
routine 14 notifying 6
notificationboard.notifyless keyx (value < threshold): 6.15 < 10 : found 7.15
routine 22 waiting preparation...
routine 27 waiting preparation...
routine 27 notifying 15
notificationboard.notifyless keyx (value >= threshold): 15.28 >= 10
routine 27 notifying 14
notificationboard.notifyless keyx (value >= threshold): 14.28 >= 10
routine 27 notifying 13
notificationboard.notifyless keyx (value >= threshold): 13.28 >= 10
routine 27 notifying 12
notificationboard.notifyless keyx (value >= threshold): 12.28 >= 10
routine 27 notifying 11
notificationboard.notifyless keyx (value >= threshold): 11.28 >= 10
routine 27 notifying 10
notificationboard.notifyless keyx (value >= threshold): 10.28 >= 10
routine 27 notifying 9
notificationboard.notifyless keyx (value < threshold): 9.28 < 10 : found 10.28
--------------> notifying keyx < 10 (9.28)
routine 27 notifying 8
notificationboard.notifyless keyx (value < threshold): 8.28 < 10 : found 9.28
routine 27 notifying 7
notificationboard.notifyless keyx (value < threshold): 7.28 < 10 : found 8.28
routine 27 notifying 6
notificationboard.notifyless keyx (value < threshold): 6.28 < 10 : found 7.28
routine 20 waiting preparation...
routine 20 notifying 15
notificationboard.notifyless keyx (value >= threshold): 15.21 >= 10
routine 20 notifying 14
notificationboard.notifyless keyx (value >= threshold): 14.21 >= 10
routine 20 notifying 13
notificationboard.notifyless keyx (value >= threshold): 13.21 >= 10
routine 20 notifying 12
notificationboard.notifyless keyx (value >= threshold): 12.21 >= 10
routine 20 notifying 11
notificationboard.notifyless keyx (value >= threshold): 11.21 >= 10
routine 20 notifying 10
notificationboard.notifyless keyx (value >= threshold): 10.21 >= 10
routine 20 notifying 9
notificationboard.notifyless keyx (value < threshold): 9.21 < 10 : found 10.21
--------------> notifying keyx < 10 (9.21)
routine 20 notifying 8
notificationboard.notifyless keyx (value < threshold): 8.21 < 10 : found 9.21
routine 20 notifying 7
notificationboard.notifyless keyx (value < threshold): 7.21 < 10 : found 8.21
routine 20 notifying 6
notificationboard.notifyless keyx (value < threshold): 6.21 < 10 : found 7.21
routine 19 waiting preparation...
routine 19 notifying 15
notificationboard.notifyless keyx (value >= threshold): 15.2 >= 10
routine 19 notifying 14
notificationboard.notifyless keyx (value >= threshold): 14.2 >= 10
routine 19 notifying 13
notificationboard.notifyless keyx (value >= threshold): 13.2 >= 10
routine 19 notifying 12
notificationboard.notifyless keyx (value >= threshold): 12.2 >= 10
routine 19 notifying 11
notificationboard.notifyless keyx (value >= threshold): 11.2 >= 10
routine 19 notifying 10
notificationboard.notifyless keyx (value >= threshold): 10.2 >= 10
routine 19 notifying 9
notificationboard.notifyless keyx (value < threshold): 9.2 < 10 : found 10.2
--------------> notifying keyx < 10 (9.2)
routine 19 notifying 8
notificationboard.notifyless keyx (value < threshold): 8.2 < 10 : found 9.2
routine 19 notifying 7
notificationboard.notifyless keyx (value < threshold): 7.2 < 10 : found 8.2
routine 19 notifying 6
notificationboard.notifyless keyx (value < threshold): 6.2 < 10 : found 7.2
routine 0 waiting preparation...
routine 0 notifying 15
notificationboard.notifyless keyx (value >= threshold): 15.01 >= 10
routine 0 notifying 14
notificationboard.notifyless keyx (value >= threshold): 14.01 >= 10
routine 0 notifying 13
notificationboard.notifyless keyx (value >= threshold): 13.01 >= 10
routine 0 notifying 12
notificationboard.notifyless keyx (value >= threshold): 12.01 >= 10
routine 0 notifying 11
notificationboard.notifyless keyx (value >= threshold): 11.01 >= 10
routine 0 notifying 10
notificationboard.notifyless keyx (value >= threshold): 10.01 >= 10
routine 0 notifying 9
notificationboard.notifyless keyx (value < threshold): 9.01 < 10 : found 10.01
--------------> notifying keyx < 10 (9.01)
routine 0 notifying 8
notificationboard.notifyless keyx (value < threshold): 8.01 < 10 : found 9.01
routine 0 notifying 7
notificationboard.notifyless keyx (value < threshold): 7.01 < 10 : found 8.01
routine 0 notifying 6
notificationboard.notifyless keyx (value < threshold): 6.01 < 10 : found 7.01
routine 17 waiting preparation...
routine 17 notifying 15
notificationboard.notifyless keyx (value >= threshold): 15.18 >= 10
routine 17 notifying 14
notificationboard.notifyless keyx (value >= threshold): 14.18 >= 10
routine 17 notifying 13
notificationboard.notifyless keyx (value >= threshold): 13.18 >= 10
routine 17 notifying 12
notificationboard.notifyless keyx (value >= threshold): 12.18 >= 10
routine 17 notifying 11
notificationboard.notifyless keyx (value >= threshold): 11.18 >= 10
routine 17 notifying 10
notificationboard.notifyless keyx (value >= threshold): 10.18 >= 10
routine 17 notifying 9
notificationboard.notifyless keyx (value < threshold): 9.18 < 10 : found 10.18
--------------> notifying keyx < 10 (9.18)
routine 17 notifying 8
notificationboard.notifyless keyx (value < threshold): 8.18 < 10 : found 9.18
routine 17 notifying 7
notificationboard.notifyless keyx (value < threshold): 7.18 < 10 : found 8.18
routine 17 notifying 6
notificationboard.notifyless keyx (value < threshold): 6.18 < 10 : found 7.18
routine 15 waiting preparation...
routine 16 waiting preparation...
...... continues

我添加了一个十进制值来表示 goroutine，并查看输出，似乎每个 goroutine 都有自己的映射副本，因为它们正在查找具有相同小数位的先前值。但后来我发现：

...
notificationboard.notifyless keyx (value >= threshold): 10.22 >= 10
routine 21 notifying 9
notificationboard.notifyless keyx (value >= threshold): 10.07 >= 10
routine 6 notifying 9
notificationboard.notifyless keyx (value < threshold): 9.08 < 10 : found 10.07
--------------> notifying keyx < 10 (9.08)
routine 7 notifying 8
notificationboard.notifyless keyx (value < threshold): 9.17 < 10 : found 9.08
routine 16 notifying 8
notificationboard.notifyless keyx (value >= threshold): 10.11 >= 10
routine 10 notifying 9
notificationboard.notifyless keyx (value < threshold): 9.3 < 10 : found 10.11
--------------> notifying keyx < 10 (9.3)
routine 29 notifying 8
notificationboard.notifyless keyx (value < threshold): 9.19 < 10 : found 9.3
routine 18 notifying 8
...

这表明他们还从其他 goroutine 中查找先前的值。

我很确定这是一个基本的并发问题，但我无法发现它。 :(

我正在使用：

> go version
go version go1.10.2 windows/amd64

有什么想法吗？

解决方案

简化逻辑的一种方法可能是运行一个修改映射的 goroutine。然后，它可以侦听通道上的新值（因为如果按顺序处理值应该没问题）。您需要小心地知道您的 goroutine 何时返回，以确保它不会泄漏。一般来说，您不应该在 goroutine 之间共享数据，您应该使用通道在 goroutine 之间进行通信。 https://gobyexample.com/channels 是一个很好的渠道介绍。 https://blog.golang.org/share-memory-by-communicating很好地解释了“不要通过共享内存来通信，而是通过通信来共享内存。”

以下示例说明了如何使用通道而不是共享内存来实现此类应用程序 (playground version)。

package main

import (
    "fmt"
    "sync"
)

type value struct {
    key       string
    value     float64
    threshold float64
}

func main() {
    b := board{
        last: map[string]float64{},
    }
    c := b.start()
    wg := sync.waitgroup{}
    for i := 0; i < 30; i++ {
        wg.add(1)
        go func(i int) {
            for j := 15.0; j > 5; j-- {
                c <- value{"k1", j + float64(i+1)/100, 10}
            }
            wg.done()
        }(i)
    }
    wg.wait()
    close(c)
}

type board struct {
    last map[string]float64
}

func (b *board) start() chan<- value {
    c := make(chan value)
    go func() {
        for v := range c {
            b.notify(v)
        }
    }()
    return c
}

func (b *board) notify(v value) {
    if l, ok := b.last[v.key]; !ok || l >= v.threshold {
        if v.value < v.threshold {
            fmt.printf("%s < %v (%v)\n", v.key, v.threshold, v.value)
        }
    }
    b.last[v.key] = v.value
}

我认为您在设置此类跟踪器时需要设置标志，一个用于值上升时，另一个用于值下降时。我实现了一个

package main

import (
    "fmt"
    "sync"
)

const (
    threshold      int = 10
    upperThreshold int = 20
)

var mu sync.Mutex
var downwatch bool
var upwatch bool

func main() {
    var tracker int = 10
    var temp int = 1
    var sign int = 1
    for i := 1; i < 20; i++ {
        sign = sign * -1
        temp = temp + i
        go UpdateTracker(&tracker, temp*sign)
    }

    for {
    }
    return
}
func SetDownWatch() {
    downwatch = true
}
func SetUpWatch() {
    upwatch = true
}
func UnSetDownWatch() {
    downwatch = false
}
func UnSetUpWatch() {
    upwatch = false
}

func UpdateTracker(tracker *int, val int) {
    mu.Lock()
    defer mu.Unlock()
    if !(upwatch || downwatch) {
        if (*tracker)+val < threshold {
            NotifyOnDrop()
            SetDownWatch()
        }
        if (*tracker + val) > upperThreshold {
            NotifyOnRise()
            SetUpWatch()
        }

    }
    if (*tracker)+val < threshold && upwatch {
        NotifyOnDrop()
        SetDownWatch()
        UnSetUpWatch()
    }

    if (*tracker+val) > upperThreshold && downwatch {
        NotifyOnRise()
        SetUpWatch()
        UnSetDownWatch()
    }

    *tracker = (*tracker) + val
    fmt.Println((*tracker))
    return
}

func NotifyOnDrop() {
    fmt.Println("dropped")
    return
}

func NotifyOnRise() {
    fmt.Println("rose")
    return
}

updatetracker 作为 go 例程运行，并在值超过设置阈值时打印到控制台。我认为这就是您正在寻找的功能，这里缺少的是 last.store 函数，我相信它是您的代码自定义的。我确信还有其他方法可以处理这个问题。这对我来说似乎很简单。

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