Concurrent workers.

workers.js

"use strict";

var d3 = require("d3");

var WORKERS = 5;

var worker = function(complete) {
  var doWork = function() {
    return done();
  };

  function done() {
    process.nextTick(doWork);
  }

  return doWork;
};

var running = 0;

var startWorker = function() {
  worker(function() {
    running--;
  })();

  running++;
};

d3.range(0, WORKERS).forEach(function(i) {
  startWorker();
});

var progress = setInterval(function() {
  process.stdout.write("\n");
  console.log("----------------------");
  console.log("Workers: %d", running);
  console.log("----------------------");

  // start workers back up if they've stopped
  while (running < WORKERS) {
    startWorker();
  }

}, 30000);