Async Cancellables

Cancellation token and example/utility classes that support cancellation

Table of contents

• Async Cancellables
  • Prerequisites
  • Table of contents
  • Installation
  • API
  • Examples
    • Basic usage
    • Using improved race/any methods
    • File download function limiting concurrent downloads
    • Cancellable async calls via socket
    • Cancellable sleep promise
  • Authors
  • License

Prerequisites

This project requires NodeJS (version 18 or later) and NPM.

Installation

To install and set up the library, run:

$ npm install async-cancellables

API

Exports basic classes (any of them can be used via standalone packages)

• CancellationToken from @async-cancellables/ct
• AsyncLock from @async-cancellables/async-lock
• AsyncState from @async-cancellables/async-state

The rest of the classes should be installed separately

• AsyncCooldownQueue from @async-cancellables/async-cooldown-queue
• AsyncHybridLimit from @async-cancellables/async-hybrid-limit

Examples

Basic usage

Creating independent cancellation tokens:

// cancelled manually by calling parent1.cancel()
const parent1 = CT.manual();

// cancelled after 5 seconds
const parent2 = CT.timeout(5000);

// cancelled when 'event' event is emitted on target
const parent3 = CT.event(target, 'event');

Creating child cancellation tokens:

// cancelled after 10 seconds or when parent1 is cancelled (manually)
const child1 = parent1.timeout(10000);

// cancelled when 'event' event is emitted on target or when parent2 is cancelled (after 5 seconds)
const child2 = parent2.event(target, 'event');

// cancelled manually by calling child3.cancel() or when parent3 is cancelled (when 'event' event is emitted on target)
const child3 = parent3.manual();

Using tokens:

const token = CT.manual();

// waits for asyncCall() to finish, but throws an error if token is cancelled
const asyncCallResult = await token.waitPromise(asyncCall());

// waits for target.event event to be emitted, but throws an error if token is cancelled, returns array of event arguments
const eventArgumentsArray = await token.waitEvent(target, 'event');

// waits for 10 seconds, but throws an error if token is cancelled
await token.sleep(10000);

Using improved race/any methods

// every lock imitates remote resource
const locks = [new AsyncLock(1), new AsyncLock(1), new AsyncLock(1)];

// waits for the first lock to be available, cancels waiting for other locks
const waitSuccess = await CT.any(locks.map((lock) => lock.waitOne()));

// it's possible to wait with a timeout
const waitSuccess = await CT.timeout(5000).any(locks.map((lock) => lock.waitOne()));

// waitSuccess contains index of the lock and the ticket
const index = waitSuccess.index;
const ticket = waitSuccess.value;

File download function limiting concurrent downloads

const asyncLock = new AsyncLock(5);

async function downloadFile(url, ct = null) {
    const ticket = await asyncLock.waitOne(ct);
    try {
        return await ct.waitPromise(got(url));
    }
    finally {
        ticket.release();
    }
}

const content = await downloadFile('https://example.com/', CT.timeout(5000));

Downloads file from url with 5 concurrent downloads. If it takes more than 5 seconds, and the function still waits for a slot, it will exit the queue and throw an error. If it is already downloading, the function will throw an error immediately, but actual download will continue in the background. asyncLock.waitOne call supports cancellation, so if ct is cancelled, the function will exit the queue.

Cancellable async calls via socket

socket.on('connection', (client) => {
    const clientCT = CT.event(client, 'disconnect');

    client.on('startProcessing', (...args) => {
        const key = randomKey();
        const requestCT = CT.event(client, `cancelProcessing_${key}`).attachTo(clientCT);
        processing(requestCT, ...args).then(result => client.emit(`finishProcessing_${key}`, result), error => !CT.isCancellationError(error) && console.log(error));
    });
});

Socket processes async calls of processing() function. If client disconnects, all pending calls are cancelled. If client sends cancelProcessing_${key} event, the call with the same key is cancelled. clientCT is getting cancelled when client disconnects, it is created once per connection. requestCT is getting cancelled when client disconnects or sends cancelProcessing_${key} event, it is created once per call and attached to clientCT to cancel all calls when client disconnects.

Cancellable sleep promise

async function sleep(ms, ct = null) {
    return new Promise((resolve, reject) => {
        let timeout;
        if (ct) {
          ct.throwIfCancelled();
          [resolve, reject] = ct.processCancel(resolve, reject, clearTimeout.bind(null, timeout));
        }
        timeout = setTimeout(resolve, ms);
    });
}

Example of binding cancellation token to a promise. If ct is cancelled, the promise will be rejected with ct error. If ct is not cancelled, the promise will be resolved after ms milliseconds. ct.throwIfCancelled() checks if ct is already cancelled and throws an error if it is. ct.processCancel() returns a new resolve and reject functions that will prevent calling cancel function (in this case it is clearTimeout.bind(null, timeout)) after promise is resolved or rejected. On cancelling ct, onCancel function will be called, it should stop the operation that is being waited for. In this case, it is clearing existing timeout.

Authors

• vuwuv - Initial work - vuwuv

License

[MIT License] © vuwuv