How to join synchronous and asynchronous world? - node.js

Question

I'm beginning with NodeJs and I'm facing a problem I'm sure a lot of people may already have solved.
I have a basic NodeJs web server that serves files if a file is found, or 404:
var http = require('http'),
url = require('url'),
fs = require('fs'),
mongoose = require('mongoose'),
fileSystem = require('fs'),
path = require('path'),
util = require('util'),
EventEmitter = require('events').EventEmitter;
var mimeTypes = {
"html": "text/html",
"jpeg": "image/jpeg",
"jpg": "image/jpeg",
"png": "image/png",
"js": "text/javascript",
"css": "text/css"};
var server;
var Schema = mongoose.Schema;
mongoose.connect('mongodb://localhost/test');
var db = mongoose.connection;
var personneSchema = new Schema({
nom: String,
prenom: String
});
var Personne = db.model('Personne', personneSchema);
var personneAdresseSchema = new Schema({
idPersonne: String,
idAdresse: String
});
var PersonneAdresse = db.model('PersonneAdresse', personneAdresseSchema);
var adresseSchema = new Schema({
ligne1: String,
ligne2: String,
codePostal: String,
ville: String
});
var Adresse = db.model('Adresse', adresseSchema);
db.on('error', console.error.bind(console, 'connection error:'));
db.once('open', function () {
server = http.createServer(function (request, response) {
if (request.url=='/persons') {
console.log('> Persons request');
/* IMPLEMENTATION PROBLEM HERE */
var retour='[]';
retour=retour.substr(0, retour.length-1)+']';
response.writeHead(200, {
'Cache-Control': 'no-cache, must-revalidate',
'Expires': 'Mon, 26 Jul 1997 05:00:00 GMT',
'Content-type': 'application/json'
});
response.end(retour);
console.log('> end of main function');
return;
}
var uri = url.parse(request.url).pathname;
var filename = path.join(process.cwd(), uri);
console.log("> " + filename);
fs.exists(filename, function(exists) {
if ((!exists) || (fs.lstatSync(filename).isDirectory())) {
console.log(">> fichier inexistant : " + filename);
response.writeHead(200, {'Content-Type': 'text/plain'});
response.write('404 Not Found\n');
response.end();
// Stopper tout traitement :
return;
}
var mimeType = mimeTypes[path.extname(filename).split(".")[1]];
response.writeHead(200, {'Content-Type':mimeType});
var fileStream = fs.createReadStream(filename);
fileStream.pipe(response);
});
});
// Listen on port 8000, IP defaults to 127.0.0.1
server.listen(8000);
// Put a friendly message on the terminal
console.log("Server running at http://127.0.0.1:8000/");
});
The problem is: the Webserver needs to get a response ready to be sent before the end of the "main" function. But if you try to do a mongoose call, it's not synchronous. If you put the following code in the main function:
Personne.find({}).select('nom prenom').exec(function (err, p) {
console.log('> Persons request finished');
});
The log shows something like:
> Persons request
> end of main function
> Persons request finished
So the main function that needs "response" to be filled can't have it filled with Personne.find({}) because Personne.find({}) finished after. What is the way to handle this? I cant find a very simple, self explaining example on the Web (nodejs + mongoose always gives me solutions with either nodejs alone, mongoose alone, or using full Web frameworks, whereas I just need a simple working example).

Answer 1

Any code that you want to be executed after the database call has finished has to be in the callback function for the database call. So you can't just put the database call in your main function -- you have to move all of the code from your main function that needs to be executed after the database call into the callback function instead.

Answer 2

If you want the server to start after your mongo query finishes, you can either use a callback:
db.once('open', function () {
server = http.createServer(function (request, response) {
Personne.find({}).select('nom prenom').exec(function (err, persons) {
// Log the result of the query
console.log(persons)
// After the query is executed, you can use the results anywhere else inside your logic
if (request.url=='/persons') {
console.log('> Persons request');
/* IMPLEMENTATION PROBLEM HERE */
var retour='[]';
retour=retour.substr(0, retour.length-1)+']';
response.writeHead(200, {
'Cache-Control': 'no-cache, must-revalidate',
'Expires': 'Mon, 26 Jul 1997 05:00:00 GMT',
'Content-type': 'application/json'
});
response.end(retour);
console.log('> end of main function');
return;
}
var uri = url.parse(request.url).pathname;
var filename = path.join(process.cwd(), uri);
console.log("> " + filename);
fs.exists(filename, function(exists) {
if ((!exists) || (fs.lstatSync(filename).isDirectory())) {
console.log(">> fichier inexistant : " + filename);
response.writeHead(200, {'Content-Type': 'text/plain'});
response.write('404 Not Found\n');
response.end();
// Stopper tout traitement :
return;
}
var mimeType = mimeTypes[path.extname(filename).split(".")[1]];
response.writeHead(200, {'Content-Type':mimeType});
var fileStream = fs.createReadStream(filename);
fileStream.pipe(response);
});
});
});
// Listen on port 8000, IP defaults to 127.0.0.1
server.listen(8000);
// Put a friendly message on the terminal
console.log("Server running at http://127.0.0.1:8000/");
});