Q promise concurrency limiter

promise_scheduler.js

'use strict';

var q = require('q');

/**
 * Constructs a function that proxies to promiseFactory
 * limiting the count of promises that can run simultaneously.
 * @param promiseFactory function that returns promises.
 * @param limit how many promises are allowed to be running at the same time.
 * @returns function that returns a promise that eventually proxies to promiseFactory.
 */
function limitConcurrency(promiseFactory, limit) {
  var running = 0,
      semaphore;

  function scheduleNextJob() {
    if (running < limit) {
      running++;
      return q();
    }

    if (!semaphore) {
      semaphore = q.defer();
    }

    return semaphore.promise
      .finally(scheduleNextJob);
  }

  function processScheduledJobs() {
    running--;

    if (semaphore && running < limit) {
      semaphore.resolve();
      semaphore = null;
    }
  }

  return function () {
    var _this = this,
        args = arguments;

    function runJob() {
      return promiseFactory.apply(_this, args);
    }

    return scheduleNextJob()
      .then(runJob)
      .finally(processScheduledJobs);
  };
}

module.exports = {
  limitConcurrency: limitConcurrency
};