MongoDB：单个数据库处理程序的 >5 个打开连接

我对nodejs和mongdb还不熟悉，我将在我的一个项目中使用它们，当我的数据库连接正常工作时，我震惊地发现我的代码实际上有多少数据库连接.下面是一段简单的代码:

var express = require('express');
var mongo = require('mongodb');  
var app = express();

// Further details:
// nodejs: v0.8.18
// mongod: v2.2.2
// node's mongodb driver: v1.2.10

app.get('/', function(req, res){
    res.send('<h1>Ok</h1>');
});

var setUp = function() {   
    // get a handler to the testDB Database
    mongo.Db.connect('mongodb://localhost:27017/testDB', function(err, db) {
        if (err)
            throw err;
        // create a test collection in the database
        db.createCollection('test', function(err, test) {
            if (err)
                throw err;
            // insert a dummy document into the test collection          
            test.insert({'name':'admin', 'pass':'admin'});

            app.listen(3000); 
            console.log('App listening on port 3000');
        });
    });
}

setUp();

当nodejs进程启动时，mongo守护进程输出此日志(log)位:

... connection accepted from 127.0.0.1:40963 #34 (1 connection now open)
... connection accepted from 127.0.0.1:40964 #35 (2 connections now open)
... connection accepted from 127.0.0.1:40965 #36 (3 connections now open)
... connection accepted from 127.0.0.1:40966 #37 (4 connections now open)
... connection accepted from 127.0.0.1:40967 #38 (5 connections now open)
... connection accepted from 127.0.0.1:40968 #39 (6 connections now open)
... end connection 127.0.0.1:40963 (5 connections now open)
... allocating new datafile /var/data/testDB.ns, filling with zeroes...
...

当进程终止时:

... connection 127.0.0.1:40964 (4 connections now open)
... connection 127.0.0.1:40965 (3 connections now open)
... connection 127.0.0.1:40966 (2 connections now open)
... connection 127.0.0.1:40967 (1 connection now open)
... connection 127.0.0.1:40968 (0 connections now open)

mongo驱动程序真的需要与mongod建立那么多连接才能得到一个db处理程序吗？还是我实现这个的方式有问题？我真的希望在那里只看到一个开放的连接...