我有以下使用 Node.js 的简单 http 服务器:

var http = require('http');

var server = http.createServer(function(req, res) {
    var counter = 0;

    for(var i = 1; i <= 30; i++) {
        http.get({ host: "www.google.com" }, function(r) {
            counter++;
            res.write("Response " + counter + ": " + r.statusCode + "\n");
            if(counter == 30) res.end();                                                                                                                                   
        });
    }
});

server.listen(8000);

当我在 8000 端口 curl 进入本地主机时，我得到了预期的结果:

Response 1: 200
Response 2: 200
Response 3: 200
...
Response 30: 200

但是当我尝试在第一个进程运行时从另一个终端 curl in 时，我看到控制台挂起并等待第一个进程完全完成，然后它才开始接收相同的输出。

我的理解是，由于这是使用回调的异步代码，因此 Node 可以通过在事件循环的下一个时钟周期处理它们来同步处理多个请求。事实上，我什至观看了 Ryan Dahl 的视频，视频中他用 hello world 示例做了类似的事情。我的代码中有什么导致服务器阻塞？

最佳答案

您的问题与阻止调用无关；这与您一次只能打开一定数量的连接到单个主机的事实有关。一旦达到最大打开连接数，其他对 http.get 的异步调用必须等到打开连接数再次下降，这发生在其他请求完成并触发回调时.由于您创建新请求的速度快于它们耗尽的速度，因此您会得到看似阻塞的结果。

这是我为测试此程序而创建的程序的修改版本。 (请注意，有一种更简单的方法可以解决您的问题，如 mtomis 所指出的——更多内容见下文。)我添加了一些 console.log 日志记录，因此更容易判断事情的顺序正在处理；我还拒绝所有对 / 以外的任何请求，因此 favicon.ico 请求将被忽略。最后，我向许多不同的网站提出请求。

var http = require('http');

// http://mostpopularwebsites.net/1-50/
var sites = [
  "www.google.com", "www.facebook.com", "www.youtube.com",
  "www.yahoo.com", "www.blogspot.com", "www.baidu.com", "www.live.com",
  "www.wikipedia.org", "www.twitter.com", "www.qq.com", "www.msn.com",
  "www.yahoo.co.jp", "www.sina.com.cn", "www.google.co.in", "www.taobao.com",
  "www.amazon.com", "www.linkedin.com", "www.google.com.hk",
  "www.wordpress.com", "www.google.de", "www.bing.com", "www.google.co.uk",
  "www.yandex.ru", "www.ebay.com", "www.google.co.jp", "www.microsoft.com",
  "www.google.fr", "www.163.com", "www.google.com.br",
  "www.googleusercontent.com", "www.flickr.com"
];

var server = http.createServer(function(req, res) {
  console.log("Got a connection.");
  if(req.url != "/") {
    console.log("But returning because the path was not '/'");
    res.end();
    return;
  }

  var counter = 0;

  for(var i = 1; i <= 30; i++) {
    http.get({ host: sites[i] }, function(index, host, r) {
      counter++;
      console.log("Response " + counter + " from # " + index + " (" + host + ")");
      res.write("Response " + counter + " from # " + index + " (" + host + ")\n");
      if(counter == 30) res.end();
    }.bind(this, i, sites[i]));
  }
  console.log("Done with for loop.");
});

server.listen(8000);

我运行这个程序并很快在两个不同的浏览器中访问了该页面(我还刷新了我的 DNS 缓存，因为测试运行得太快而无法获得良好的输出)。这是输出:

Got a connection.
Done with for loop.
Response 1 from # 8 (www.twitter.com)
Response 2 from # 1 (www.facebook.com)
Response 3 from # 12 (www.sina.com.cn)
Response 4 from # 4 (www.blogspot.com)
Response 5 from # 13 (www.google.co.in)
Response 6 from # 19 (www.google.de)
Response 7 from # 26 (www.google.fr)
Response 8 from # 28 (www.google.com.br)
Response 9 from # 17 (www.google.com.hk)
Response 10 from # 6 (www.live.com)
Response 11 from # 20 (www.bing.com)
Response 12 from # 29 (www.googleusercontent.com)
Got a connection.
Done with for loop.
Response 13 from # 10 (www.msn.com)
Response 14 from # 2 (www.youtube.com)
Response 15 from # 18 (www.wordpress.com)
Response 16 from # 16 (www.linkedin.com)
Response 17 from # 7 (www.wikipedia.org)
Response 18 from # 3 (www.yahoo.com)
Response 19 from # 15 (www.amazon.com)
Response 1 from # 6 (www.live.com)
Response 2 from # 1 (www.facebook.com)
Response 3 from # 8 (www.twitter.com)
Response 4 from # 4 (www.blogspot.com)
Response 20 from # 11 (www.yahoo.co.jp)
Response 21 from # 9 (www.qq.com)
Response 5 from # 2 (www.youtube.com)
Response 6 from # 13 (www.google.co.in)
Response 7 from # 10 (www.msn.com)
Response 8 from # 24 (www.google.co.jp)
Response 9 from # 17 (www.google.com.hk)
Response 10 from # 18 (www.wordpress.com)
Response 11 from # 16 (www.linkedin.com)
Response 12 from # 3 (www.yahoo.com)
Response 13 from # 12 (www.sina.com.cn)
Response 14 from # 11 (www.yahoo.co.jp)
Response 15 from # 7 (www.wikipedia.org)
Response 16 from # 15 (www.amazon.com)
Response 17 from # 9 (www.qq.com)
Response 22 from # 5 (www.baidu.com)
Response 23 from # 27 (www.163.com)
Response 24 from # 14 (www.taobao.com)
Response 18 from # 5 (www.baidu.com)
Response 19 from # 14 (www.taobao.com)
Response 25 from # 24 (www.google.co.jp)
Response 26 from # 30 (www.flickr.com)
Response 20 from # 29 (www.googleusercontent.com)
Response 21 from # 22 (www.yandex.ru)
Response 27 from # 23 (www.ebay.com)
Response 22 from # 19 (www.google.de)
Response 23 from # 21 (www.google.co.uk)
Response 24 from # 28 (www.google.com.br)
Response 25 from # 25 (www.microsoft.com)
Response 26 from # 20 (www.bing.com)
Response 27 from # 30 (www.flickr.com)
Response 28 from # 22 (www.yandex.ru)
Response 28 from # 27 (www.163.com)
Response 29 from # 25 (www.microsoft.com)
Response 29 from # 26 (www.google.fr)
Response 30 from # 21 (www.google.co.uk)
Response 30 from # 23 (www.ebay.com)
Got a connection.
But returning because the path was not '/'

如您所见，除了我按下 Alt+Tab Enter 所花的时间外，回调是完全混合在一起的——最好的异步、非阻塞 I/O。

[编辑]

正如 mtomis 提到的，每个主机可以打开的最大连接数可以通过全局 http.globalAgent.maxSockets 配置。只需将其设置为您希望每台主机能够处理的并发连接数，您观察到的问题就会消失。

关于javascript - 为什么这个 javascript 会在 Node.js 中阻塞？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/7087650/