kokoro-p-queue
v6.2.0
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Promise queue with concurrency control
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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…