How can I limit Q promise concurrency?

Question

How do I write a method that limits Q promise concurrency?

For instance, I have a method spawnProcess. It returns a Q promise.
I want no more than 5

Answer 1

This seems to be working for me.

I'm not sure if I could simplify it. The recursion in scheduleNextJob is necessary so the running < limit and limit++ always execute in the same tick.

Also available as a gist.

'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 args = arguments;

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

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

module.exports = {
  limitConcurrency: limitConcurrency
}