Limiting asynchronous calls in Node.js

As usual, I recommend Caolan McMahon’s async module.

Make your upload_file function take a callback as it’s second parameter:

var async = require("async");

function upload_file(file, callback) {
    // Do funky stuff with file
    callback();
}

var queue = async.queue(upload_file, 10); // Run ten simultaneous uploads

queue.drain = function() {
    console.log("All files are uploaded");
};

// Queue your files for upload
queue.push(files);

queue.concurrency = 20; // Increase to twenty simultaneous uploads