Go并发模式代码示例

演讲稿：Go Concurrency Patterns

Youtube视频

作者：Rob Pike

练习题目：谷歌搜索：一个虚拟框架

谷歌搜索1.0

PPT从43页开始：https://talks.golang.org/2012/concurrency.slide#43

Google函数接受一个查询并返回一个结果切片(只是字符串)。Google连续调用网页、图片和视频搜索，并将它们附加到搜索结果切片中。

代码如下：

package main

import (
	"fmt"
	"math/rand"
	"time"
)

var (
	Web   = fakeSearch("web")
	Image = fakeSearch("image")
	Video = fakeSearch("video")
)

type Result string

type Search func(query string) Result

func fakeSearch(kind string) Search {
	return func(query string) Result {
		time.Sleep(time.Duration(rand.Intn(100)) * time.Millisecond)
		return Result(fmt.Sprintf("%s result for %q
", kind, query))
	}
}

func Google(query string) (results []Result) {
	results = append(results, Web(query))
	results = append(results, Image(query))
	results = append(results, Video(query))
	return
}

func main() {
	rand.Seed(time.Now().UnixNano())
	start := time.Now()
	results := Google("golang")
	elapsed := time.Since(start)
	fmt.Println(results)
	fmt.Println(elapsed)
}

运行结果如下:

[web result for "golang"
 image result for "golang"
 video result for "golang"
]
153.365484ms

## 谷歌搜索2.0

同时运行网页、图像和视频搜索，并等待所有结果。没有锁，没有条件变量，没有回调。

代码如下，关注Google函数。

package main

import (
	"fmt"
	"math/rand"
	"time"
)

var (
	Web   = fakeSearch("web")
	Image = fakeSearch("image")
	Video = fakeSearch("video")
)

type Result string

type Search func(query string) Result

func fakeSearch(kind string) Search {
	return func(query string) Result {
		time.Sleep(time.Duration(rand.Intn(100)) * time.Millisecond)
		return Result(fmt.Sprintf("%s result for %q
", kind, query))
	}
}

func Google(query string) (results []Result) {
	c := make(chan Result)
	go func() { c <- Web(query) } ()
	go func() { c <- Image(query) } ()
	go func() { c <- Video(query) } ()

	for i := 0; i < 3; i++ {
		result := <-c
		results = append(results, result)
	}
	return
}

func main() {
	rand.Seed(time.Now().UnixNano())
	start := time.Now()
	results := Google("golang")
	elapsed := time.Since(start)
	fmt.Println(results)
	fmt.Println(elapsed)
}

## 谷歌搜索2.1 不要等待缓慢的服务器。没有锁，无条件变量，没有回调。通过select的超时实现，需要把time.After定义的超时通道放在for循环外层。

package main

import (
	"fmt"
	"math/rand"
	"time"
)

var (
	Web   = fakeSearch("web")
	Image = fakeSearch("image")
	Video = fakeSearch("video")
)

type Result string

type Search func(query string) Result

func fakeSearch(kind string) Search {
	return func(query string) Result {
		time.Sleep(time.Duration(rand.Intn(100)) * time.Millisecond)
		return Result(fmt.Sprintf("%s result for %q
", kind, query))
	}
}

func Google(query string) (results []Result) {
	c := make(chan Result)
	go func() { c <- Web(query) } ()
	go func() { c <- Image(query) } ()
	go func() { c <- Video(query) } ()

	timeout := time.After(80 * time.Millisecond)
	for i := 0; i < 3; i++ {
		select {
		case result := <-c:
			results = append(results, result)
		case <-timeout:
			fmt.Println("timed out")
			return
		}
	}
	return
}

func main() {
	rand.Seed(time.Now().UnixNano())
	start := time.Now()
	results := Google("golang")
	elapsed := time.Since(start)
	fmt.Println(results)
	fmt.Println(elapsed)
}

## 谷歌搜索3.0 内容从48页到51页。

使用复制的搜索服务器减少尾部延迟。同样没有锁，没有条件变量，没有回调。

问：我们如何避免因为服务器运行缓慢而丢弃结果？

答: 复制服务器。 向多个副本发送请求，并使用第一个响应。

代码如下：

package main

import (
	"fmt"
	"math/rand"
	"time"
)

var (
	Web1   = fakeSearch("web")
	Web2   = fakeSearch("web")
	Image1 = fakeSearch("image")
	Image2 = fakeSearch("image")
	Video1 = fakeSearch("video")
	Video2 = fakeSearch("video")
)

type Result string

type Search func(query string) Result

func fakeSearch(kind string) Search {
	return func(query string) Result {
		time.Sleep(time.Duration(rand.Intn(100)) * time.Millisecond)
		return Result(fmt.Sprintf("%s result for %q
", kind, query))
	}
}

func Google(query string) (results []Result) {
	c := make(chan Result)
	go func() { c <- First(query, Web1, Web2) } ()
	go func() { c <- First(query, Image1, Image2) } ()
	go func() { c <- First(query, Video1, Video2) } ()
	timeout := time.After(80 * time.Millisecond)
	for i := 0; i < 3; i++ {
		select {
		case result := <-c:
			results = append(results, result)
		case <-timeout:
			fmt.Println("timed out")
			return
		}
	}
	return
}

func First(query string, replicas ...Search) Result {
	c := make(chan Result)
	searchReplica := func(i int) { c <- replicas[i](query) }
	for i := range replicas {
		go searchReplica(i)
	}
	return <-c
}

func main() {
	rand.Seed(time.Now().UnixNano())
	start := time.Now()
	results := Google("golang")
	elapsed := time.Since(start)
	fmt.Println(results)
	fmt.Println(elapsed)
}

执行结果如下：

[image result for "golang"
 web result for "golang"
 video result for "golang"
]
53.605273ms