Node js - how to handle multiple asynchronous tasks - node.js

Question

I'm new to node and got stuck with handling multiple async tasks.
Except from node, I've got another server (S1) which doesn't return data immediately to requests, it can returns multiple types of data and also can send notifications without requesting them specifically, so node have to listen to data from it , parse it and act accordingly.
The connection to this server (S1) is done by using:
S1 = net.createConnection({'host':S1Host, 'port': S1Port});
And node listens to data with:
S1.on('data', function(data){
S1DataParse(data);
});
I have to route the correct data (after parsing it) to a specific POST request.
app.post('/GetFooFromS1', function(req, res){
// Send request to S1
S1.write({'type':'foo'});
// If got the correct data sometime in the future, send response to the browser
res.setHeader('Content-Type', 'application/json');
res.json({'status':'success', 'value':S1FooData});
});
I tried to use the async module for that, but with no success.
What I was trying to do:
var asyncTasks = [];
app.post('/GetFooFromS1', function(req, res){
asyncTasks.push(function(callback){
// Send request to S1
S1.write({'type':'foo'});
});
async.parallel(asyncTasks, function(response){
res.setHeader('Content-Type', 'application/json');
res.json({'status':'success', 'value':response});
});
});
and another task in S1DataParse:
function S1DataParse(){
if(data.type='foo'){
asyncTasks.push(function(callback){
callback(data);
});
}
}
But, of course, the second task never added to the asyncTasks array. I really got stuck with that.
Can you please help me with that?
Thanks
-=-=-=- Edit -=-=-=-
Eventually, I came accross with events and EventEmitter().
From the POST request I call the function that sends requests to the data server (DataServerClientGet).
In this function I register a listener which will get the future data.
eventEmitter.on('getData', returnDataServerData);
It all works great except for one thing. Whenever I refresh the page or add other POST requests, I get an error:
Error: Can't set headers after they are sent.
It would be great if I solve this problem. Help me, please.
Thanks ;)
The whole code looks like this:
var express = require('express');
var app = express();
var http = require('http').Server(app);
var bodyParser = require('body-parser')
var net = require('net');
var events = require('events');
var dataServerHost = '127.0.0.1';
var dataServerPort = 12345;
var dataServerClient;
var logMsg;
var eventEmitter = new events.EventEmitter();
/*******************************************/
// Init
/*******************************************/
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({extended: true}));
app.use(express.static(__dirname + '/public'));
/*******************************************/
// Connect to the data server
/*******************************************/
DataServerConnect();
/*******************************************/
// Open listener on port 3000 (to browser)
/*******************************************/
http.listen(3000, function(){
logMsg = 'listening on *:3000';
console.log(logMsg);
});
/*******************************************/
// Routing
/*******************************************/
app.get('/', function(req, res){
res.sendFile(__dirname + '/index.html');
});
app.post('/GetDataFoo', function(req, res){
var msg;
var size;
msg ='\n{"Type":"Query", "SubType":"GetDataFoo","SearchFilter":""}';
size = msg.length;
logMsg = 'Client to DataServer: GetDataFoo';
console.log(logMsg);
DataServerClientGet('GetDataFoo', size, msg, res);
});
/*******************************************/
// Functions
/*******************************************/
function DataServerConnect(){
dataServerClient = net.createConnection({'host':dataServerHost, 'port': dataServerPort}, function(){
logMsg = 'Connected to DataServer ['+dataServerHost+':'+dataServerPort+']';
console.log(logMsg);
});
dataServerClient.on('data', function(data){
logMsg = 'DataServerData>>>\n'+data.toString()+'DataServerData<<<';
console.log(logMsg);
DataServerDataParse(data.toString());
});
dataServerClient.on('end', function(){
logMsg = 'Disconnected from DataServer';
console.log(logMsg);
});
}
function DataServerClientGet(type, size, msg, res){
dataServerClient.write('Type: Json\nSize: '+size+'\n\n'+msg, function(err){
var returnDataServerData = function returnDataServerData(results){
res.setHeader('Content-Type', 'application/json');
res.json({'status':'success', 'value':results});
}
eventEmitter.on('getData', returnDataServerData);
}
function DataServerDataParse(json){
if(json.Type=='GetDataFoo')
{
var MessageList = json.MessageList;
eventEmitter.emit('getData', MessageList);
}
}
-=-=-=- Edit -=-=-=-
The Error: Can't set headers after they are sent. caused by adding the same listener of the same type each time the DataServerClientGet was called and the res was sending multiple times.
I solved this one by adding: removeListener(event, listener)
right after the res, inside the function. Anyway, I think it's wrong and can cause problems if there will be multiple calling to DataServerClientGet with the same type etc.

Answer 1

There is a optional callback parameter that you can pass to write function(docs), something like :
S1.write({'type':'foo'},function(err){
if(err){
//Handle error
}else{
res.setHeader('Content-Type', 'application/json');
res.json({'status':'success', 'value':response});
}
})
This can work with post route , but in your 'data' listener ,you cant send data from server to client when there is not connection initialized by client (it is not bidireccional ) if you want bidireccional behavior you can check socket.io