nodejs入门学习笔记二——解决阻塞问题

　　在最开始，我们要弄清楚node会什么会存在阻塞？

　　node是这么标榜自己的：“在node中除了代码，所有一切都是并行执行的！”

　　意思是，Node.js可以在不新增额外线程的情况下，依然可以对任务进行并行处理 —— Node.js是单线程的。

　　也就是说，我们启动的web服务器，监听8888端口的start方法，是单线程的。

　　如果某一个请求耗时，那么后面的请求要等上一个请求完成之后才执行，这显然是不合理的！

　　如requestHandlers中start handler：

function start() {
    console.log("Request handler 'start' was called.");

    function sleep(milliSeconds) {
        var startTime = new Date().getTime();
        while (new Date().getTime() < startTime + milliSeconds);
    }

    sleep(10000);
    return "Hello Start";
}

　　我们可以使用child_process模块来实现非阻塞操作，其实就是一个异步操作，强调一点，耗时操作通常都需要通过异步操作来处理。

一种错误的示范：

var exec = require("child_process").exec;

function start() {
  console.log("Request handler 'start' was called.");
  var content = "empty";

  exec("ls -lah", function (error, stdout, stderr) {
    content = stdout;
  });

  return content;
}

function upload() {
  console.log("Request handler 'upload' was called.");
  return "Hello Upload";
}

exports.start = start;
exports.upload = upload;

错误原因，exec异步操作后面的不能再跟同步代码，一个简单的例子，juqery ajax请求成功后的后续操作应该在success中处理，而不应该再ajax整个代码块后面处理。

既然后续操作都要在异步回调函数中实现，所以response的处理就要移步至handler中实现。

server.js

var http = require("http");
var url = require("url");

function start(route, handle) {
  function onRequest(request, response) {
    var pathname = url.parse(request.url).pathname;
    console.log("Request for " + pathname + " received.");

    route(handle, pathname, response);
  }

  http.createServer(onRequest).listen(8888);
  console.log("Server has started.");
}

exports.start = start;

router.js

function route(handle, pathname, response) {
  console.log("About to route a request for " + pathname);
  if (typeof handle[pathname] === 'function') {
    handle[pathname](response);
  } else {
    console.log("No request handler found for " + pathname);
    response.writeHead(404, {"Content-Type": "text/plain"});
    response.write("404 Not found");
    response.end();
  }
}

exports.route = route;

requestHandler.js

var exec = require("child_process").exec;

function start(response) {
  console.log("Request handler 'start' was called.");

  exec("find /",
    { timeout: 10000, maxBuffer: 20000*1024 },
    function (error, stdout, stderr) {
      response.writeHead(200, {"Content-Type": "text/plain"});
      response.write(stdout);
      response.end();
    });
}

function upload(response) {
  console.log("Request handler 'upload' was called.");
  response.writeHead(200, {"Content-Type": "text/plain"});
  response.write("Hello Upload");
  response.end();
}

exports.start = start;
exports.upload = upload;