使用Node.js和ajax进行长轮询

时间：2013-09-25 16:22:36

标签： javascript mysql ajax node.js

我有以下服务器代码。

var http = require('http');
var mysql = require('mysql');
var querystring = require('request');
var util = require('util');
var url = require('url');

var singer_name;
var currentmodif, lastmodif;
var requests=[];
var response;

var connection = mysql.createConnection({
  host     : 'localhost',
  user     : 'someone',
  password : 'xxxxxxx',
  database : 'rest',  //mysql database to work with (optional)
});
connection.connect(); //connect to mysql

connection.query('SELECT * FROM musics WHERE id=1', function(err, rows, fields) {
  if (err) throw err;

  singer_name=rows[0].singer_name;
  currentmodif=rows[0].time_added;
});


http.createServer(function (req, res) {
    console.log('request received');

    requests.push({
        response: res,
        timestamp: new Date().getTime()
    });

    if(req.method=='GET'){
        var url_parts = url.parse(req.url,true);
        lastmodif = url_parts.query.timestamp;
    }

    //check_update(req, res);

}).listen(9000);


setInterval(function() {

    var expiration = new Date().getTime() - 30000;

    for (var i = requests.length - 1; i >= 0; i--) {
        //console.log("Request timestamp: "+requests[i].timestamp+" Expiration : "+expiration);
        response = requests[i].response;
        if (requests[i].timestamp < expiration) {
            console.log("The condition is met");
            response.writeHead(200, {
            'Content-Type'   : 'text/plain',
            'Access-Control-Allow-Origin' : '*'
            });

            // return response
            response.write('_testcb(\'ok\')', 'utf8');
            response.end();
            //break;
        }
    }

    connection.query('SELECT * FROM musics WHERE id=1', function(err, rows, fields) {
        if (err) throw err;
        currentmodif=rows[0].time_added;        
            //console.log("currentmodif: "+currentmodif+" lastmodif: "+lastmodif);
        if (currentmodif > lastmodif){
            singer_name=rows[0].singer_name; 
            var _arrays = {'singer_name': singer_name, 'time': currentmodif}
            var data = "_testcb"+"("+JSON.stringify(_arrays)+")";
            response.writeHead(200, {
            'Content-Type'   : 'text/plain',
            'Access-Control-Allow-Origin' : '*'
            });
            if (response.end(data))
            console.log("Response successfully sent");
            //return false;
        }

    });
}, 2000);

和客户代码： 

<html>
<head>
    <script src="http://code.jquery.com/jquery-1.10.1.min.js"></script>
    <title>Node.js Ajax test</title>
</head>
<body>

</body>
<script>
var timestamp = "1380020402";
function callNode() {

var time = "1380020402";
    $.ajax({
        url: 'http://xx.xxx.xx.xxx:9000/',
        dataType: "jsonp",
        data: {"timestamp":timestamp},
        type: 'POST',
        jsonpCallback: "_testcb",
        cache: false,
        timeout: 35000,
        success: function(response, code, xhr) {
            if ('ok' == response) {
                callNode();
                return false;
            }

            console.log(response);

            timestamp = response.time;
            // make new call
            callNode();
        },
        error: function(jqXHR, textStatus, errorThrown) {
            console.log('error ' + textStatus + " " + errorThrown);
        }
    });
}
$(function () {
    callNode();
});
</script>
</html>

我正在尝试进行长时间的民意调查。因此，在更新数据库中的数据之前，应暂停对ajax请求的响应，但上述代码不起作用。我正在从不同的域发出ajax请求，因此使用jsonp。

确切的问题是，当前在数据库中更改数据时，响应不会被发送。它时不时地起作用，但它并不始终可靠。

另一个问题是超时的代码块无法正常工作。如果请求是30秒，那么应该发送一个空白响应，以避免从ajax超时。

如果有人可以提供帮助，我会很感激。

干杯。

1 个答案:

答案 0 :(得分：5)

我已经弄明白了。修改后的代码如下：

客户方：

<html>
<head>
    <script src="http://code.jquery.com/jquery-1.10.1.min.js"></script>
    <title>Node.js Ajax test</title>
</head>
<body>

</body>
<script>
var timestamp;
function callNode() {

    $.ajax({
        url: 'http://xx.xxx.xx.xxx:9000/',
        dataType: "jsonp",
        data: {"timestamp":timestamp},
        //type: 'POST', //don't need this with jsonp
        jsonpCallback: "_testcb",
        cache: false,
        timeout: 35000,
        success: function(response, code, xhr) {
            if ('ok' == response) {
                console.log(response);
                callNode();
                return false;
            }

            console.log(response);

            timestamp = response.time;
            // make new call
            callNode();
        },
        error: function(jqXHR, textStatus, errorThrown) {
            console.log('error ' + textStatus + " " + errorThrown);
        }
    });
}
$(function () {
    setTimeout(callNode, 1); //call function with setTimeout in order to avoid ugly constant browser loading 
});
</script>
</html>

服务器端（server.js）：

var http = require('http');
var mysql = require('mysql');
var util = require('util');
var url = require('url');

var singer_name, currentmodif, lastmodif, request, response, time_of_request;

//var requests=[];

var connection = mysql.createConnection({
  host     : 'localhost',
  user     : 'someone',
  password : 'xxxxxx',
  database : 'rest',  //mysql database to work with (optional)
});
connection.connect(); //connect to mysql

connection.query('SELECT * FROM musics WHERE id=1', function(err, rows, fields) {
  if (err) throw err;

  singer_name=rows[0].singer_name;
  currentmodif=rows[0].time_added;
});


http.createServer(function (req, res) {
    request = req;
    response = res;
    time_of_request = new Date().getTime();
    console.log('request received');


    if(req.method=='GET'){
        var url_parts = url.parse(req.url,true);
        lastmodif = url_parts.query.timestamp;
    }

    req.on('error', function(e) {
        console.log('problem with request: ' + e.message);
    });

    //checkupdate();     

}).listen(9000);

var response;

function checkupdate() { 

    var expiration = new Date().getTime() - 30000;

    //for (var i = requests.length - 1; i >= 0; i--) {
        //console.log("Request timestamp: "+time_of_request+" Expiration : "+expiration);
        if (time_of_request < expiration) {
            console.log("The condition is met");
            // return response
            response.write('_testcb(\'ok\')', 'utf8');
            response.end();
        }
    //}

    connection.query('SELECT * FROM musics WHERE id=1', function(err, rows, fields) {
        if (err) throw err;
        currentmodif=rows[0].time_added;

        if (lastmodif == undefined)
            lastmodif = 0;

        console.log("currentmodif: "+currentmodif+" lastmodif: "+lastmodif);

        if (currentmodif > lastmodif){
            singer_name=rows[0].singer_name;  
            var _arrays = {'singer_name': singer_name, 'time': currentmodif} 
            var data = "_testcb"+"("+JSON.stringify(_arrays)+")";

            //response.writeHead(200, { 'content-type':'application/json',
                                    //'Access-Control-Allow-Origin' : '*'});
            //response.write(data); 
            response.end(data);
            console.log("Response successfully sent");
            //return false;
        }

    });
};

setInterval(checkupdate, 2000);

问题在于服务器端。当服务器想要回复并因此没有发送响应时，响应对象不可用（它是未定义的）。我可能忽略了node.js控制台中的错误。

这几乎是使用MYSQL数据库对node.js进行长轮询的完整示例。在回复ajax请求之前，此脚本将等待新数据可用。如果新请求（在MYSQL中）在请求的30秒内不可用，则会进行虚假回复，以便请求不会超时。 ajax的成功回调中存在一个条件，即在收到此演示响应时重新启动此ajax请求，从而使其成为无限循环。

我已经成功测试了上面的代码，似乎工作正常。我运行了脚本然后更新了我的数据库中的数据（主要是time_added字段），这触发了对来自node.js服务器的新数据的等待ajax调用的回复。

我希望这段代码可以帮助那些人。

这里有结帐教程以获得进一步说明：http://www.sahilsaid.com/blog/long-polling-node-js-mysql-database-ajax/

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