nodejs入門學習筆記二——解決阻塞問題

　　在最開始，我們要弄清楚node會什么會存在阻塞？

　　node是這么標榜自己的：“在node中除了代碼，所有一切都是並行執行的！”

　　意思是，Node.js可以在不新增額外線程的情況下，依然可以對任務進行並行處理 —— Node.js是單線程的。

　　也就是說，我們啟動的web服務器，監聽8888端口的start方法，是單線程的。

　　如果某一個請求耗時，那么后面的請求要等上一個請求完成之后才執行，這顯然是不合理的！

　　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";
}

　　我們可以使用child_process模塊來實現非阻塞操作，其實就是一個異步操作，強調一點，耗時操作通常都需要通過異步操作來處理。

一種錯誤的示范：

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;

錯誤原因，exec異步操作后面的不能再跟同步代碼，一個簡單的例子，juqery ajax請求成功后的后續操作應該在success中處理，而不應該再ajax整個代碼塊后面處理。

既然后續操作都要在異步回調函數中實現，所以response的處理就要移步至handler中實現。

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;