Limit concurrency of pending promises

Question

I'm looking for a promise function wrapper that can limit / throttle when a given promise is running so that only a set number of that promise is running at a given time.

In the case below delayPromise should never run concurrently, they should all run one at a time in a first-come-first-serve order.

import Promise from 'bluebird'

function _delayPromise (seconds, str) {
  console.log(str)
  return Promise.delay(seconds)
}

let delayPromise = limitConcurrency(_delayPromise, 1)

async function a() {
  await delayPromise(100, "a:a")
  await delayPromise(100, "a:b")
  await delayPromise(100, "a:c")
}

async function b() {
  await delayPromise(100, "b:a")
  await delayPromise(100, "b:b")
  await delayPromise(100, "b:c")
}

a().then(() => console.log('done'))

b().then(() => console.log('done'))

Any ideas on how to get a queue like this set up?

I have a "debounce" function from the wonderful Benjamin Gruenbaum. I need to modify this to throttle a promise based on it's own execution and not the delay.

export function promiseDebounce (fn, delay, count) {
  let working = 0
  let queue = []
  function work () {
    if ((queue.length === 0) || (working === count)) return
    working++
    Promise.delay(delay).tap(function () { working-- }).then(work)
    var next = queue.shift()
    next[2](fn.apply(next[0], next[1]))
  }
  return function debounced () {
    var args = arguments
    return new Promise(function (resolve) {
      queue.push([this, args, resolve])
      if (working < count) work()
    }.bind(this))
  }
}

Answer 1

I don't think there are any libraries to do this, but it's actually quite simple to implement yourself:

function sequential(fn) { // limitConcurrency(fn, 1)
    let q = Promise.resolve();
    return function(x) {
        const p = q.then(() => fn(x));
        q = p.reflect();
        return p;
    };
}

For multiple concurrent requests it gets a little trickier, but can be done as well.

function limitConcurrency(fn, n) {
    if (n == 1) return sequential(fn); // optimisation
    let q = Promise.resolve();
    const active = new Set();
    const fst = t => t[0];
    const snd = t => t[1];
    return function(x) {
        function put() {
            const p = fn(x);
            const a = p.reflect().then(() => {
                active.delete(a);
            });
            active.add(a);
            return [Promise.race(active), p];
        }
        if (active.size < n) {
            const r = put()
            q = fst(t);
            return snd(t);
        } else {
            const r = q.then(put);
            q = r.then(fst);
            return r.then(snd)
        }
    };
}

Btw, you might want to have a look at the actors model and CSP. They can simplify dealing with such things, there are a few JS libraries for them out there as well.

Example

import Promise from 'bluebird'

function sequential(fn) {
  var q = Promise.resolve();
  return (...args) => {
    const p = q.then(() => fn(...args))
    q = p.reflect()
    return p
  }
}

async function _delayPromise (seconds, str) {
  console.log(`${str} started`)
  await Promise.delay(seconds)
  console.log(`${str} ended`)
  return str
}

let delayPromise = sequential(_delayPromise)

async function a() {
  await delayPromise(100, "a:a")
  await delayPromise(200, "a:b")
  await delayPromise(300, "a:c")
}

async function b() {
  await delayPromise(400, "b:a")
  await delayPromise(500, "b:b")
  await delayPromise(600, "b:c")
}

a().then(() => console.log('done'))
b().then(() => console.log('done'))

// --> with sequential()

// $ babel-node test/t.js
// a:a started
// a:a ended
// b:a started
// b:a ended
// a:b started
// a:b ended
// b:b started
// b:b ended
// a:c started
// a:c ended
// b:c started
// done
// b:c ended
// done

// --> without calling sequential()

// $ babel-node test/t.js
// a:a started
// b:a started
// a:a ended
// a:b started
// a:b ended
// a:c started
// b:a ended
// b:b started
// a:c ended
// done
// b:b ended
// b:c started
// b:c ended
// done