使用node-mongodb-native连接mongodb，连接自动关闭的问题！

问题就是，我用node-mongodb-native连接上mongodb之后，默认打开了一个有五个连接的连接池，但是这5个连接在几分钟后自动就关闭了 这个是mongodb的配置文件 这个是mongodb的log日志

这个是我的base_mongodb.js文件

var MongoClient = require('mongodb').MongoClient
   , ObjectId = require('mongodb').ObjectID
   , assert = require('assert')
   , Util = require('./util')
   , db;
module.exports = function(){
   var self = this;

   this.findOneById = function(tableName,id,callback){
       connection(function (db) {
           db.collection(tableName,function(err,collection){
               var mongoId = ObjectId(id);
               var cursor = collection.find({'_id':mongoId});
               cursor.toArray(function(err,docs){
                   if(err){
                       callback(false);
                   }else{
                       callback(docs);
                   }
                   cursor.rewind();
               });
           });
       })
   };
   /**
    * 连接mongodb
    * @param callback
    */
   function connection(callback){
       if(!db){
           var dbConfig = Util.get('config.json','db');
           var host = dbConfig['host']
               , user = dbConfig['user']
               , password = dbConfig['password']
               , port = dbConfig['port']
               , db_name = dbConfig['db_name']
               , authSource = dbConfig['authSource'];
           var url = 'mongodb://'+user+':'+password+'@'+host+':'+port+'/'+db_name+'?authSource='+authSource;
           MongoClient.connect(url,function(err,dbObject){
               assert.equal(null,err);
               console.log('Connect to MongoDB success~');
               db = dbObject;
               callback(db);
           });
       }else{
           callback(db);
       }
   }
}

这个是我的http.js文件

var http=require('http')
    , BaseMongodb = require('./base_mongodb')
    , baseMongodb = new BaseMongodb();
http.createServer(function(req,res){
    res.writeHead(200,{"Content-type":"text/html;charset=utf-8"});
    switch (req.url)
    {
        case '/':
            baseMongodb.findOneById('node_booki','5574fc1055608daf0b789f38',function(ret){
                if(ret)
                    res.end(JSON.stringify(ret));
                else
                    res.end('222');
            });

            break;
    }
}).listen(3000);
console.log("浏览器打开http://127.0.0.1:3000");