async和eventproxy同樣都是控制并發(fā)的，eventproxy會(huì)同時(shí)發(fā)起所有請(qǐng)求，所以在上一篇博客中最后出現(xiàn)了503的錯(cuò)誤；使用async則可以很好地解決這個(gè)問題，使用async能夠控制并發(fā)線程的數(shù)量

一、async

• async在github : https://github.com/caolan/async
• async使用示例 : https://github.com/alsotang/async_demo
• async常用的方法有好幾個(gè)，這里使用新增加的mapLimit(collection,iteratee(item,callback),callback(err,results))方法解決遺留的問題

二、使用async限制并發(fā)訪問數(shù)量

1. demo

首先來個(gè)預(yù)熱程序看看如何使用mapList方法：

var async=require("async");
var currentCount=0;
console.log("will create a url list size 10 !");
var urls=[];
for(var i=0;i<10;i++){
    urls.push('http://www.example.com/'+i+".html");
}
async.mapLimit(urls,2,function(url,callback){
    var delay=parseInt(500);
    currentCount++;
    console.log("currentCount is :"+currentCount+",current url is :"+url+",use time is :"+delay+" mm");
    setTimeout(function(){
        currentCount--;
        callback(null,url+' html content ');
    },delay);
},function(err,result){
    console.log("result:");
    console.log(result);
});

運(yùn)行結(jié)果：

will create a url list size 10 !
currentCount is :1,current url is :http://www.example.com/0.html,use time is :500 mm
currentCount is :2,current url is :http://www.example.com/1.html,use time is :500 mm
currentCount is :2,current url is :http://www.example.com/2.html,use time is :500 mm
currentCount is :2,current url is :http://www.example.com/3.html,use time is :500 mm
currentCount is :2,current url is :http://www.example.com/4.html,use time is :500 mm
currentCount is :2,current url is :http://www.example.com/5.html,use time is :500 mm
currentCount is :2,current url is :http://www.example.com/6.html,use time is :500 mm
currentCount is :2,current url is :http://www.example.com/7.html,use time is :500 mm
currentCount is :2,current url is :http://www.example.com/8.html,use time is :500 mm
currentCount is :2,current url is :http://www.example.com/9.html,use time is :500 mm
result:
[ 'http://www.example.com/0.html html content ',
  'http://www.example.com/1.html html content ',
  'http://www.example.com/2.html html content ',
  'http://www.example.com/3.html html content ',
  'http://www.example.com/4.html html content ',
  'http://www.example.com/5.html html content ',
  'http://www.example.com/6.html html content ',
  'http://www.example.com/7.html html content ',
  'http://www.example.com/8.html html content ',
  'http://www.example.com/9.html html content ' ]

2.mapLimit方法參數(shù)的含義說明

mapLimit方法的完整定義：
mapLimit(collection,iteratee(item,callback),callback(err,results))

collection : 一個(gè)數(shù)組
item : collection的一個(gè)元素
callback : 回調(diào)函數(shù)，該回調(diào)函數(shù)比較特殊，理解起來也比較困難，在iteratee方法中一定要調(diào)用該回調(diào)函數(shù)，一般來說會(huì)有三種常用的使用方法：

3.使用async限制并發(fā)訪問數(shù)量完整代碼

var express=require("express");
var superagent=require("superagent");
var cheerio=require("cheerio");
var url=require("url");
var eventproxy=require("eventproxy");
var async=require("async");
var app=express();
var baseUrl='https://cnodejs.org/';
function output(arr){
    for(var i=0;i<arr.length;i++){
        console.log(arr[i]);
    }
}
superagent.get(baseUrl).end(function(err,resp){
    if(err){
        return console.error(err);
    }
    var arr=[];
    var $=cheerio.load(resp.text);
    $("#topic_list .topic_title").each(function(idx,element){
        $element=$(element);
        var _url=url.resolve(baseUrl,$element.attr("href"));
        arr.push(_url);
    });
    //驗(yàn)證得到的所有文章鏈接集合
    output(arr);
    //接下來遍歷arr，解析每一個(gè)頁面中需要的信息
    async.mapLimit(arr,3,function(url,callback){
        superagent.get(url).end(function(err,mes){
            if(err){
                console.log("get \""+url+"\" error !"+err);
                console.log("message info:"+JSON.stringify(mes));
            }
            console.log('fetch '+url+" succeful !");
            //console.log("mess info:"+JSON.stringify(mes));
            var $=cheerio.load(mes.text);
            var jsonData={
                title:$(".topic_full_title").text().trim(),
                href:url,
                firstcomment:$("#reply1 .markdown-text").text().trim()
            };
            console.log("aim data is :"+JSON.stringify(jsonData));
            callback(null,jsonData);
        });
    },function(error,results){
        console.log("result :");
        console.log(results);   
    });
});

這樣上一篇博客中的503錯(cuò)誤就解決了。