@mekwall/p-queue
v7.1.0
Published
Promise queue with concurrency control in a hybrid package
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@mekwall/p-queue
This is a fork of sindresorhus/p-queue that provides both CommonJS and ES modules as a hybrid package.
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 @mekwall/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.
import PQueue from "p-queue";
import got from "got";
const queue = new PQueue({ concurrency: 1 });
(async () => {
await queue.add(() => got("https://sindresorhus.com"));
console.log("Done: sindresorhus.com");
})();
(async () => {
await queue.add(() => got("https://avajs.dev"));
console.log("Done: avajs.dev");
})();
(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
If true
, specifies that any pending Promises, should be carried over into the next interval and counted against the intervalCap
. If false
, any of those pending Promises will not count towards the next intervalCap
.
queue
PQueue
instance.
.add(fn, options?)
Adds a sync or async task to the queue. Always returns a promise.
Note: If your items can potentially throw an exception, you must handle those errors from the returned Promise or they may be reported as an unhandled Promise rejection and potentially cause your process to exit immediately.
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.
.onSizeLessThan(limit)
Returns a promise that settles when the queue size is less than the given limit: queue.size < limit
.
If you want to avoid having the queue grow beyond a certain size you can await queue.onSizeLessThan()
before adding a new item.
Note that this only limits the number of items waiting to start. There could still be up to concurrency
jobs already running that this call does not include in its calculation.
.clear()
Clear the queue.
.size
Size of the queue, the number of queued items waiting to run.
.sizeBy(options)
Size of the queue, filtered by the given options.
For example, this can be used to find the number of items remaining in the queue with a specific priority level.
import PQueue from "p-queue";
const queue = new PQueue();
queue.add(async () => "🦄", { priority: 1 });
queue.add(async () => "🦄", { priority: 0 });
queue.add(async () => "🦄", { priority: 1 });
console.log(queue.sizeBy({ priority: 1 }));
//=> 2
console.log(queue.sizeBy({ priority: 0 }));
//=> 1
.pending
Number of running items (no longer in the queue).
.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.
import delay from "delay";
import PQueue from "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));
completed
Emitted when an item completes without error.
import delay from "delay";
import PQueue from "p-queue";
const queue = new PQueue({ concurrency: 2 });
queue.on("completed", (result) => {
console.log(result);
});
queue.add(() => Promise.resolve("hello, world!"));
error
Emitted if an item throws an error.
import delay from "delay";
import PQueue from "p-queue";
const queue = new PQueue({ concurrency: 2 });
queue.on("error", (error) => {
console.error(error);
});
queue.add(() => Promise.reject(new Error("error")));
idle
Emitted every time the queue becomes empty and all promises have completed; queue.size === 0 && queue.pending === 0
.
import delay from "delay";
import PQueue from "p-queue";
const queue = new PQueue();
queue.on("idle", () => {
console.log(`Queue is idle. Size: ${queue.size} Pending: ${queue.pending}`);
});
const job1 = queue.add(() => delay(2000));
const job2 = queue.add(() => delay(500));
await job1;
await job2;
// => 'Queue is idle. Size: 0 Pending: 0'
await queue.add(() => delay(600));
// => 'Queue is idle. Size: 0 Pending: 0'
The idle
event is emitted every time the queue reaches an idle state. On the other hand, the promise the onIdle()
function returns resolves once the queue becomes idle instead of every time the queue is idle.
add
Emitted every time the add method is called and the number of pending or queued tasks is increased.
next
Emitted every time a task is completed and the number of pending or queued tasks is decreased. This is emitted regardless of whether the task completed normally or with an error.
import delay from "delay";
import PQueue from "p-queue";
const queue = new PQueue();
queue.on("add", () => {
console.log(`Task is added. Size: ${queue.size} Pending: ${queue.pending}`);
});
queue.on("next", () => {
console.log(
`Task is completed. Size: ${queue.size} Pending: ${queue.pending}`
);
});
const job1 = queue.add(() => delay(2000));
const job2 = queue.add(() => delay(500));
await job1;
await job2;
//=> 'Task is added. Size: 0 Pending: 1'
//=> 'Task is added. Size: 0 Pending: 2'
await queue.add(() => delay(600));
//=> 'Task is completed. Size: 0 Pending: 1'
//=> 'Task is completed. Size: 0 Pending: 0'
Advanced example
A more advanced example to help you understand the flow.
import delay from "delay";
import PQueue from "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:
import PQueue from "p-queue";
class QueueClass {
constructor() {
this._queue = [];
}
enqueue(run, options) {
this._queue.push(run);
}
dequeue() {
return this._queue.shift();
}
get size() {
return this._queue.length;
}
filter(options) {
return this._queue;
}
}
const queue = new PQueue({ queueClass: QueueClass });
p-queue
will call corresponding methods to put and get operations from this queue.
FAQ
How do the concurrency
and intervalCap
options affect each other?
They are just different constraints. The concurrency
option limits how many things run at the same time. The intervalCap
option limits how many things run in total during the interval (over time).
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…