Go并發模式代碼示例

演講稿：Go Concurrency Patterns

Youtube視頻

作者：Rob Pike

練習題目：谷歌搜索：一個虛擬框架

谷歌搜索1.0

PPT從43頁開始：https://talks.golang.org/2012/concurrency.slide#43

Google函式接受一個查詢并回傳一個結果切片(只是字串)，Google連續呼叫網頁、圖片和視頻搜索，并將它們附加到搜索結果切片中，

代碼如下：

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\n", 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)
}

運行結果如下:

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

同時運行網頁、影像和視頻搜索，并等待所有結果，沒有鎖，沒有條件變數，沒有回呼，

代碼如下，關注Google函式，

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\n", 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)
}

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\n", 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)
}

使用復制的搜索服務器減少尾部延遲，同樣沒有鎖，沒有條件變數，沒有回呼，

問：我們如何避免因為服務器運行緩慢而丟棄結果？

答: 復制服務器， 向多個副本發送請求，并使用第一個回應，

代碼如下：

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\n", 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)
}

執行結果如下：

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

標籤：Go

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