TCP communication with nodeJS

Question

I'm trying to make a communication between nodejs server and a red pitaya card. The client connect to the web server, use a form to choose some settings , they are send to a red pitaya who send back a big string of numbers.

Then, the web server write them in a downloadable file. Its working randomly.

here is the server :

var settings = require('./settings_model');
var fs = require('fs');
var net = require('net');
var msg = [];

module.exports = function(app) {
net.createServer(function (socket) {

  socket.on('data', function (data) {
      /*    test console  */     
         var msg = JSON.stringify(data);
       console.log("data received :" + msg);

          tab = new Array();
          for(i = 0; i < nbPix; i++){
              tab[i] = new Array();
              fs.appendFile('./public/img.txt', '\n');
                for(var j=0 ; j < len; j++){
                  tab[i][j]= data[i+(j*nbPix)];
                    if(!isNaN(tab[i][j])){
                      fs.appendFile('./public/img.txt',  tab[i][j]+ " ");
                    }; 
                };
          }; 
  });

 app.post('/api/sendSettings', function(req, res) {

      // creation of the file
        fs.writeFile("./public/img.txt", "", function(err){
             if(err){console.log(err)};
             console.log("the file was saved");
         });

    // here we send settings to the red pitaya          
       socket.write(input); 
       res.end();
  });
 }).listen(9000);
};

For tiny values, sometimes it works. I can see the log :

data received :{"type":"Buffer","data":[1,1,....2]}

But, for the same values, it can be divided and bug my 2d array

data receive :{"type":"Buffer","data":[1,1,....1]}

data receive :{"type":"Buffer","data":[2,2,....2]}

And if the data received is too big, system crash with an :

Error : EMFILE: too many open files, open './public/img.txt'

Is there a way to set the received stream size ? i think i need all the data to build my 2d array.

For the EMFILE error, ive tried to change settings like "open file" size. Still not working. It should not try to open more than one file anyway ??

i'm new to this kind of project so please if u can give me hints or docs links, ill gladly accept!