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<ol class="chapter"><li class="chapter-item expanded affix "><a href="../index.html">Go语言圣经</a></li><li class="chapter-item expanded affix "><a href="../preface.html">前言</a></li><li class="chapter-item expanded "><a href="../ch1/ch1.html"><strong aria-hidden="true">1.</strong> 入门</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch1/ch1-01.html"><strong aria-hidden="true">1.1.</strong> Hello, World</a></li><li class="chapter-item expanded "><a href="../ch1/ch1-02.html"><strong aria-hidden="true">1.2.</strong> 命令行参数</a></li><li class="chapter-item expanded "><a href="../ch1/ch1-03.html"><strong aria-hidden="true">1.3.</strong> 查找重复的行</a></li><li class="chapter-item expanded "><a href="../ch1/ch1-04.html"><strong aria-hidden="true">1.4.</strong> GIF动画</a></li><li class="chapter-item expanded "><a href="../ch1/ch1-05.html"><strong aria-hidden="true">1.5.</strong> 获取URL</a></li><li class="chapter-item expanded "><a href="../ch1/ch1-06.html"><strong aria-hidden="true">1.6.</strong> 并发获取多个URL</a></li><li class="chapter-item expanded "><a href="../ch1/ch1-07.html"><strong aria-hidden="true">1.7.</strong> Web服务</a></li><li class="chapter-item expanded "><a href="../ch1/ch1-08.html"><strong aria-hidden="true">1.8.</strong> 本章要点</a></li></ol></li><li class="chapter-item expanded "><a href="../ch2/ch2.html"><strong aria-hidden="true">2.</strong> 程序结构</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch2/ch2-01.html"><strong aria-hidden="true">2.1.</strong> 命名</a></li><li class="chapter-item expanded "><a href="../ch2/ch2-02.html"><strong aria-hidden="true">2.2.</strong> 声明</a></li><li class="chapter-item expanded "><a href="../ch2/ch2-03.html"><strong aria-hidden="true">2.3.</strong> 变量</a></li><li class="chapter-item expanded "><a href="../ch2/ch2-04.html"><strong aria-hidden="true">2.4.</strong> 赋值</a></li><li class="chapter-item expanded "><a href="../ch2/ch2-05.html"><strong aria-hidden="true">2.5.</strong> 类型</a></li><li class="chapter-item expanded "><a href="../ch2/ch2-06.html"><strong aria-hidden="true">2.6.</strong> 包和文件</a></li><li class="chapter-item expanded "><a href="../ch2/ch2-07.html"><strong aria-hidden="true">2.7.</strong> 作用域</a></li></ol></li><li class="chapter-item expanded "><a href="../ch3/ch3.html"><strong aria-hidden="true">3.</strong> 基础数据类型</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch3/ch3-01.html"><strong aria-hidden="true">3.1.</strong> 整型</a></li><li class="chapter-item expanded "><a href="../ch3/ch3-02.html"><strong aria-hidden="true">3.2.</strong> 浮点数</a></li><li class="chapter-item expanded "><a href="../ch3/ch3-03.html"><strong aria-hidden="true">3.3.</strong> 复数</a></li><li class="chapter-item expanded "><a href="../ch3/ch3-04.html"><strong aria-hidden="true">3.4.</strong> 布尔型</a></li><li class="chapter-item expanded "><a href="../ch3/ch3-05.html"><strong aria-hidden="true">3.5.</strong> 字符串</a></li><li class="chapter-item expanded "><a href="../ch3/ch3-06.html"><strong aria-hidden="true">3.6.</strong> 常量</a></li></ol></li><li class="chapter-item expanded "><a href="../ch4/ch4.html"><strong aria-hidden="true">4.</strong> 复合数据类型</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch4/ch4-01.html"><strong aria-hidden="true">4.1.</strong> 数组</a></li><li class="chapter-item expanded "><a href="../ch4/ch4-02.html"><strong aria-hidden="true">4.2.</strong> Slice</a></li><li class="chapter-item expanded "><a href="../ch4/ch4-03.html"><strong aria-hidden="true">4.3.</strong> Map</a></li><li class="chapter-item expanded "><a href="../ch4/ch4-04.html"><strong aria-hidden="true">4.4.</strong> 结构体</a></li><li class="chapter-item expanded "><a href="../ch4/ch4-05.html"><strong aria-hidden="true">4.5.</strong> JSON</a></li><li class="chapter-item expanded "><a href="../ch4/ch4-06.html"><strong aria-hidd
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<!-- 头部 -->
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<ul dir="auto">
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<li><em>KusonStack一站式可编程配置技术栈: <a href="https://github.com/KusionStack/kusion">https://github.com/KusionStack/kusion</a></em></li>
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<li><em>凹语言™: <a href="https://github.com/wa-lang/wa">https://github.com/wa-lang/wa</a></em></li>
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<h2 id="86-示例-并发的web爬虫"><a class="header" href="#86-示例-并发的web爬虫">8.6. 示例: 并发的Web爬虫</a></h2>
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<p>在5.6节中，我们做了一个简单的web爬虫，用bfs(广度优先)算法来抓取整个网站。在本节中，我们会让这个爬虫并行化，这样每一个彼此独立的抓取命令可以并行进行IO，最大化利用网络资源。crawl函数和gopl.io/ch5/findlinks3中的是一样的。</p>
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<p><u><i>gopl.io/ch8/crawl1</i></u></p>
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<pre><code class="language-go">func crawl(url string) []string {
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fmt.Println(url)
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list, err := links.Extract(url)
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if err != nil {
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log.Print(err)
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}
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return list
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}
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</code></pre>
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<p>主函数和5.6节中的breadthFirst(广度优先)类似。像之前一样，一个worklist是一个记录了需要处理的元素的队列，每一个元素都是一个需要抓取的URL列表，不过这一次我们用channel代替slice来做这个队列。每一个对crawl的调用都会在他们自己的goroutine中进行并且会把他们抓到的链接发送回worklist。</p>
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<pre><code class="language-go">func main() {
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worklist := make(chan []string)
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// Start with the command-line arguments.
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go func() { worklist <- os.Args[1:] }()
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// Crawl the web concurrently.
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seen := make(map[string]bool)
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for list := range worklist {
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for _, link := range list {
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if !seen[link] {
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seen[link] = true
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go func(link string) {
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worklist <- crawl(link)
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}(link)
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}
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}
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}
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}
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</code></pre>
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<p>注意这里的crawl所在的goroutine会将link作为一个显式的参数传入，来避免“循环变量快照”的问题（在5.6.1中有讲解）。另外注意这里将命令行参数传入worklist也是在一个另外的goroutine中进行的，这是为了避免channel两端的main goroutine与crawler goroutine都尝试向对方发送内容，却没有一端接收内容时发生死锁。当然，这里我们也可以用buffered channel来解决问题，这里不再赘述。</p>
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<p>现在爬虫可以高并发地运行起来，并且可以产生一大坨的URL了，不过还是会有俩问题。一个问题是在运行一段时间后可能会出现在log的错误信息里的：</p>
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<pre><code>$ go build gopl.io/ch8/crawl1
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$ ./crawl1 http://gopl.io/
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http://gopl.io/
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https://golang.org/help/
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https://golang.org/doc/
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https://golang.org/blog/
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...
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2015/07/15 18:22:12 Get ...: dial tcp: lookup blog.golang.org: no such host
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2015/07/15 18:22:12 Get ...: dial tcp 23.21.222.120:443: socket: too many open files
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...
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</code></pre>
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<p>最初的错误信息是一个让人莫名的DNS查找失败，即使这个域名是完全可靠的。而随后的错误信息揭示了原因：这个程序一次性创建了太多网络连接，超过了每一个进程的打开文件数限制，既而导致了在调用net.Dial像DNS查找失败这样的问题。</p>
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<p>这个程序实在是太他妈并行了。无穷无尽地并行化并不是什么好事情，因为不管怎么说，你的系统总是会有一些个限制因素，比如CPU核心数会限制你的计算负载，比如你的硬盘转轴和磁头数限制了你的本地磁盘IO操作频率，比如你的网络带宽限制了你的下载速度上限，或者是你的一个web服务的服务容量上限等等。为了解决这个问题，我们可以限制并发程序所使用的资源来使之适应自己的运行环境。对于我们的例子来说，最简单的方法就是限制对links.Extract在同一时间最多不会有超过n次调用，这里的n一般小于文件描述符的上限值，比如20。这和一个夜店里限制客人数目是一个道理，只有当有客人离开时，才会允许新的客人进入店内。</p>
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<p>我们可以用一个有容量限制的buffered channel来控制并发，这类似于操作系统里的计数信号量概念。从概念上讲，channel里的n个空槽代表n个可以处理内容的token（通行证），从channel里接收一个值会释放其中的一个token，并且生成一个新的空槽位。这样保证了在没有接收介入时最多有n个发送操作。（这里可能我们拿channel里填充的槽来做token更直观一些，不过还是这样吧。）由于channel里的元素类型并不重要，我们用一个零值的struct{}来作为其元素。</p>
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<p>让我们重写crawl函数，将对links.Extract的调用操作用获取、释放token的操作包裹起来，来确保同一时间对其只有20个调用。信号量数量和其能操作的IO资源数量应保持接近。</p>
|
|||
|
|
<p><u><i>gopl.io/ch8/crawl2</i></u></p>
|
|||
|
|
<pre><code class="language-go">// tokens is a counting semaphore used to
|
|||
|
|
// enforce a limit of 20 concurrent requests.
|
|||
|
|
var tokens = make(chan struct{}, 20)
|
|||
|
|
|
|||
|
|
func crawl(url string) []string {
|
|||
|
|
fmt.Println(url)
|
|||
|
|
tokens <- struct{}{} // acquire a token
|
|||
|
|
list, err := links.Extract(url)
|
|||
|
|
<-tokens // release the token
|
|||
|
|
if err != nil {
|
|||
|
|
log.Print(err)
|
|||
|
|
}
|
|||
|
|
return list
|
|||
|
|
}
|
|||
|
|
</code></pre>
|
|||
|
|
<p>第二个问题是这个程序永远都不会终止，即使它已经爬到了所有初始链接衍生出的链接。（当然，除非你慎重地选择了合适的初始化URL或者已经实现了练习8.6中的深度限制，你应该还没有意识到这个问题。）为了使这个程序能够终止，我们需要在worklist为空或者没有crawl的goroutine在运行时退出主循环。</p>
|
|||
|
|
<pre><code class="language-go">func main() {
|
|||
|
|
worklist := make(chan []string)
|
|||
|
|
var n int // number of pending sends to worklist
|
|||
|
|
|
|||
|
|
// Start with the command-line arguments.
|
|||
|
|
n++
|
|||
|
|
go func() { worklist <- os.Args[1:] }()
|
|||
|
|
|
|||
|
|
// Crawl the web concurrently.
|
|||
|
|
seen := make(map[string]bool)
|
|||
|
|
|
|||
|
|
for ; n > 0; n-- {
|
|||
|
|
list := <-worklist
|
|||
|
|
for _, link := range list {
|
|||
|
|
if !seen[link] {
|
|||
|
|
seen[link] = true
|
|||
|
|
n++
|
|||
|
|
go func(link string) {
|
|||
|
|
worklist <- crawl(link)
|
|||
|
|
}(link)
|
|||
|
|
}
|
|||
|
|
}
|
|||
|
|
}
|
|||
|
|
}
|
|||
|
|
</code></pre>
|
|||
|
|
<p>这个版本中，计数器n对worklist的发送操作数量进行了限制。每一次我们发现有元素需要被发送到worklist时，我们都会对n进行++操作，在向worklist中发送初始的命令行参数之前，我们也进行过一次++操作。这里的操作++是在每启动一个crawler的goroutine之前。主循环会在n减为0时终止，这时候说明没活可干了。</p>
|
|||
|
|
<p>现在这个并发爬虫会比5.6节中的深度优先搜索版快上20倍，而且不会出什么错，并且在其完成任务时也会正确地终止。</p>
|
|||
|
|
<p>下面的程序是避免过度并发的另一种思路。这个版本使用了原来的crawl函数，但没有使用计数信号量，取而代之用了20个常驻的crawler goroutine，这样来保证最多20个HTTP请求在并发。</p>
|
|||
|
|
<pre><code class="language-go">func main() {
|
|||
|
|
worklist := make(chan []string) // lists of URLs, may have duplicates
|
|||
|
|
unseenLinks := make(chan string) // de-duplicated URLs
|
|||
|
|
|
|||
|
|
// Add command-line arguments to worklist.
|
|||
|
|
go func() { worklist <- os.Args[1:] }()
|
|||
|
|
|
|||
|
|
// Create 20 crawler goroutines to fetch each unseen link.
|
|||
|
|
for i := 0; i < 20; i++ {
|
|||
|
|
go func() {
|
|||
|
|
for link := range unseenLinks {
|
|||
|
|
foundLinks := crawl(link)
|
|||
|
|
go func() { worklist <- foundLinks }()
|
|||
|
|
}
|
|||
|
|
}()
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
// The main goroutine de-duplicates worklist items
|
|||
|
|
// and sends the unseen ones to the crawlers.
|
|||
|
|
seen := make(map[string]bool)
|
|||
|
|
for list := range worklist {
|
|||
|
|
for _, link := range list {
|
|||
|
|
if !seen[link] {
|
|||
|
|
seen[link] = true
|
|||
|
|
unseenLinks <- link
|
|||
|
|
}
|
|||
|
|
}
|
|||
|
|
}
|
|||
|
|
}
|
|||
|
|
</code></pre>
|
|||
|
|
<p>所有的爬虫goroutine现在都是被同一个channel - unseenLinks喂饱的了。主goroutine负责拆分它从worklist里拿到的元素，然后把没有抓过的经由unseenLinks channel发送给一个爬虫的goroutine。</p>
|
|||
|
|
<p>seen这个map被限定在main goroutine中；也就是说这个map只能在main goroutine中进行访问。类似于其它的信息隐藏方式，这样的约束可以让我们从一定程度上保证程序的正确性。例如，内部变量不能够在函数外部被访问到；变量（§2.3.4）在没有发生变量逃逸（译注：局部变量被全局变量引用地址导致变量被分配在堆上）的情况下是无法在函数外部访问的；一个对象的封装字段无法被该对象的方法以外的方法访问到。在所有的情况下，信息隐藏都可以帮助我们约束我们的程序，使其不发生意料之外的情况。</p>
|
|||
|
|
<p>crawl函数爬到的链接在一个专有的goroutine中被发送到worklist中来避免死锁。为了节省篇幅，这个例子的终止问题我们先不进行详细阐述了。</p>
|
|||
|
|
<p><strong>练习 8.6：</strong> 为并发爬虫增加深度限制。也就是说，如果用户设置了depth=3，那么只有从首页跳转三次以内能够跳到的页面才能被抓取到。</p>
|
|||
|
|
<p><strong>练习 8.7：</strong> 完成一个并发程序来创建一个线上网站的本地镜像，把该站点的所有可达的页面都抓取到本地硬盘。为了省事，我们这里可以只取出现在该域下的所有页面（比如golang.org开头，译注：外链的应该就不算了。）当然了，出现在页面里的链接你也需要进行一些处理，使其能够在你的镜像站点上进行跳转，而不是指向原始的链接。</p>
|
|||
|
|
<p><strong>译注：</strong>
|
|||
|
|
拓展阅读 <a href="http://marcio.io/2015/07/handling-1-million-requests-per-minute-with-golang/">Handling 1 Million Requests per Minute with Go</a>。</p>
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