p-queue

Promise queue with concurrency control

Useful for rate-limiting async (or sync) operations. For example, when interacting with a REST API or when doing CPU/memory intensive tasks.

Install

$ npm install p-queue

Usage

Here we run only one promise at the time. For example, set concurrency to 4 to run four promises at the same time.

const {default: PQueue} = require('p-queue');
const got = require('got');

const queue = new PQueue({concurrency: 1});

(async () => {
	await queue.add(() => got('sindresorhus.com'));
	console.log('Done: sindresorhus.com');
})();

(async () => {
	await queue.add(() => got('ava.li'));
	console.log('Done: ava.li');
})();

(async () => {
	const task = await getUnicornTask();
	await queue.add(task);
	console.log('Done: Unicorn task');
})();

API

PQueue(options?)

Returns a new queue instance, which is an EventEmitter3 subclass.

options

Type: object

concurrency

Type: number Default: Infinity Minimum: 1

Concurrency limit.

timeout

Type: number

Per-operation timeout in milliseconds. Operations fulfill once timeout elapses if they haven't already.

throwOnTimeout

Type: boolean Default: false

Whether or not a timeout is considered an exception.

autoStart

Type: boolean Default: true

Whether queue tasks within concurrency limit, are auto-executed as soon as they're added.

queueClass

Type: Function

Class with a enqueue and dequeue method, and a size getter. See the Custom QueueClass section.

intervalCap

Type: number Default: Infinity Minimum: 1

The max number of runs in the given interval of time.

interval

Type: number Default: 0 Minimum: 0

The length of time in milliseconds before the interval count resets. Must be finite.

carryoverConcurrencyCount

Type: boolean Default: false

Whether the task must finish in the given interval or will be carried over into the next interval count.

queue

PQueue instance.

.add(fn, [options])

Adds a sync or async task to the queue. Always returns a promise.

fn

Type: Function

Promise-returning/async function.

options

Type: Object

priority

Type: number Default: 0

Priority of operation. Operations with greater priority will be scheduled first.

.addAll(fns, [options])

Same as .add(), but accepts an array of sync or async functions and returns a promise that resolves when all functions are resolved.

.pause()

Put queue execution on hold.

.start()

Start (or resume) executing enqueued tasks within concurrency limit. No need to call this if queue is not paused (via options.autoStart = false or by .pause() method.)

Returns this (the instance).

.onEmpty()

Returns a promise that settles when the queue becomes empty.

Can be called multiple times. Useful if you for example add additional items at a later time.

.onIdle()

Returns a promise that settles when the queue becomes empty, and all promises have completed; queue.size === 0 && queue.pending === 0.

The difference with .onEmpty is that .onIdle guarantees that all work from the queue has finished. .onEmpty merely signals that the queue is empty, but it could mean that some promises haven't completed yet.

.clear()

Clear the queue.

.size

Size of the queue.

.pending

Number of pending promises.

.timeout

.concurrency

.isPaused

Whether the queue is currently paused.

Events

active

Emitted as each item is processed in the queue for the purpose of tracking progress.

const delay = require('delay');
const {default: PQueue} = require('p-queue');

const queue = new PQueue({concurrency: 2});

let count = 0;
queue.on('active', () => {
	console.log(`Working on item #${++count}.  Size: ${queue.size}  Pending: ${queue.pending}`);
});

queue.add(() => Promise.resolve());
queue.add(() => delay(2000));
queue.add(() => Promise.resolve());
queue.add(() => Promise.resolve());
queue.add(() => delay(500));

Advanced example

A more advanced example to help you understand the flow.

const delay = require('delay');
const {default: PQueue} = require('p-queue');

const queue = new PQueue({concurrency: 1});

(async () => {
	await delay(200);

	console.log(`8. Pending promises: ${queue.pending}`);
	//=> '8. Pending promises: 0'

	(async () => {
		await queue.add(async () => '🐙');
		console.log('11. Resolved')
	})();

	console.log('9. Added 🐙');

	console.log(`10. Pending promises: ${queue.pending}`);
	//=> '10. Pending promises: 1'

	await queue.onIdle();
	console.log('12. All work is done');
})();

(async () => {
	await queue.add(async () => '🦄');
	console.log('5. Resolved')
})();
console.log('1. Added 🦄');

(async () => {
	await queue.add(async () => '🐴');
	console.log('6. Resolved')
})();
console.log('2. Added 🐴');

(async () => {
	await queue.onEmpty();
	console.log('7. Queue is empty');
})();

console.log(`3. Queue size: ${queue.size}`);
//=> '3. Queue size: 1`

console.log(`4. Pending promises: ${queue.pending}`);
//=> '4. Pending promises: 1'

$ node example.js
1. Added 🦄
2. Added 🐴
3. Queue size: 1
4. Pending promises: 1
5. Resolved 🦄
6. Resolved 🐴
7. Queue is empty
8. Pending promises: 0
9. Added 🐙
10. Pending promises: 1
11. Resolved 🐙
12. All work is done

Custom QueueClass

For implementing more complex scheduling policies, you can provide a QueueClass in the options:

class QueueClass {
	constructor() {
		this._queue = [];
	}
	enqueue(run, options) {
		this._queue.push(run);
	}
	dequeue() {
		return this._queue.shift();
	}
	get size() {
		return this._queue.length;
	}
}

p-queue will call corresponding methods to put and get operations from this queue.

Related

• p-limit - Run multiple promise-returning & async functions with limited concurrency
• p-throttle - Throttle promise-returning & async functions
• p-debounce - Debounce promise-returning & async functions
• p-all - Run promise-returning & async functions concurrently with optional limited concurrency
• More…