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))
}
}
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