Using connection in mongoDB with Node.js

I have node.js file that is connecting to the mongoDB and getting the number of the documents in the collection then it's running in for loop and updating each document with a value them I'm getting from an external server, the thing is that i have a lot of connections open at a time and the program is crashing, my question is how can i use 1 connection at a time and multiple connections this is my code:

var request = require('request'); var cheerio = require ('cheerio'); var fs = require('fs'); var MongoClient = require('mongodb').MongoClient; //var config = JSON.parse(fs.readFileSync("config.json")); //var host = config.host; //var port = config.port; // Variable Deceleration //var dbName = "ystocks"; var port = "27017"; var host = "localhost"; var collection = "stocks" var dbName = "yahooStocks"; function updateStock(){ // getting the number of docs in the collection getNumOfDocs("stocks", host, port, dbName, function(err, count) { if (err) { return console.log(err.message); } console.log('number of documents', count); for(i = 0 ; i <= 20; i++) { // Finding each document by _id with a callbak GetDocumentByID("stocks", host, port, dbName, i, function(err, findOne) { if (err) { return console.log(err.message); } console.log('Here is the doc that was found', findOne.ticker); var stockName = findOne.ticker; console.log("\n* stock name : " +stockName + "\n"); var yUrl = "http://finance.yahoo.com/q/ks?s=" + stockName; getStockValue(stockName, yUrl); }); // end of GetDocumentByID }//end of for loop }); // end of getNumOfDocs call }// end of update function ////passed arguments //stockName = for printing stock name to console //yUrl = the specific url for the current stock ticker function getStockValue(stockName, yUrl){ request(yUrl, stockName, function (error, response, body) { var date = setTimeToLocal(new Date(), 3); if (!error && response.statusCode == 200) { var $ = cheerio.load(body); // the keys - We get them from a certain class attribute var span = $('.time_rtq_ticker>span'); var stockValue = $(span).text(); console.log("checking the type of the stockValue " + typeof(stockValue)); //************************************* // parsing the value to int in case it was a String var parsedValue = parseFloat(stockValue); console.log("checking the type of the stockValue " + typeof(parsedValue) + " " + parsedValue); // Calling the setStockValue function which will update the stock value setStockValue(stockName, parsedValue); //writehead ??? //response.writeHead(200, {'Content-Type':'text/plain'}); console.log("Response received from -> " + yUrl); console.log(date); console.log("Stock - " + stockName + " --> " + stockValue ); //response.write is for printing in browser when we have a server running //response.write("Welcome to stockwatch\n"); //response.write("Stock - " + ticker + " current value--> " + stockValue + "\n" ); //TODO #1 //ALEX : lines 61-72 -> should be replaced with a function call that // connects to the DB and updates the specified stock //i think it sould look like this : funcName( tickerName , currenValue ){...} var json = {stockName : "", price : "", date : ""}; json.price = stockValue; json.stockName = stockName; json.date = date; var tempName = json.stockName +'.json'; fs.writeFile( tempName ,JSON.stringify(json, null, 4) , function(err) { if(!err) { console.log("File successfully written"); } });//end of filewrite //end of TODO #1 //response.end("res end"); }//end of !error && response.statusCode == 200 else if (response.statusCode == 404){ console.log("Response failed from " + yUrl + " --> error code: " + response.statusCode); }//end of statusCode == 400 });// end of request }//end of getStockValue // updating a stock with a provided ticker and value and the MongoClient // edited by Alex Brodov on : AUG 21 2014 function setStockValue(ticker, value) { // open the connection the DB server MongoClient.connect("mongodb://localhost:27017/yahooStocks", function (error, db){ if(error) throw error; var query = {'ticker' : ticker}; operator ={'$set' : {'value' : value}}; // find one document in our collection db.collection('stocks').update(query, operator, function(error, updated){ if (error) throw error; //print the result console.dir("Successfully updated " + updated + " document"); // close the DB db.close(); });// end of update }); // Connection to the DB } // end of setStockValue // Gets the local date and the desired offset time // set function setTimeToLocal(date, offset ) { // getting the local tome in millseconds var localtime = date.getTime(); // getting the local offset in millseconds var localOffset = date.getTimezoneOffset()*60000; var utc = localOffset + localtime; // Jerusalem offset // var offset = 3; // Jerusalem time in millseconds var jerusalem = utc + (offset*3600000); var d = new Date(jerusalem); console.log("Jerusalem Local Time: " + d.toLocaleString()); return d; } // end of SetTimeToLocal /** * Gets the number of documents from a collection * With a provided collection name, DB host, DB port, DB name and a callback function * Edited by Alex */ function getNumOfDocs (collectionName, host, port, dbName, callback) { var mongoConnection = MongoClient.connect("mongodb://" + host + ":" + port + "/" + dbName, function (error, db){ if(error) return callback(err); db.collection(collectionName).count({}, function(error, numOfDocs){ if(error) return callback(err); db.close(); callback(null, numOfDocs); }); // end of count }); // end of connection to DB } // end of getNumOfDocs /** * Gets a document * With a provided collection name, DB host, DB port, DB name, the document ID and a callback * Edited by Alex Brodov on Aug 24th 2014 */ function GetDocumentByID (collectionName, host, port, dbName, docID, callback) { // open the connection to DB server MongoClient.connect("mongodb://" + host + ":" + port + "/" + dbName, function (error, db){ if(error) return callback(error); var query = {'_id' : docID}; // find one document in our collection db.collection(collectionName).findOne(query, function(error, doc) { if (error) return callback(error); console.log(doc); // close the DB db.close(); callback(null, doc); });// end of findOne }); // Connection to the DB } var refreshInterval = setInterval(updateStock, 30000);