How to implement a pseudo blocking async queue in JS/TS?

Question

So here's an oxymoron: I want to create an asynchronous blocking queue in javascript/typescript (if you can implement it without typescript, that's fine). Basically I want to implement something like Java's BlockingQueue expect instead of it actually being blocking, it would be async and I can await dequeues.

Here's the interface I want to implement:

interface AsyncBlockingQueue<T> {
  enqueue(t: T): void;
  dequeue(): Promise<T>;
}

And I'd use it like so:

// enqueue stuff somewhere else

async function useBlockingQueue() {
  // as soon as something is enqueued, the promise will be resolved:
  const value = await asyncBlockingQueue.dequeue();
  // this will cause it to await for a second value
  const secondValue = await asyncBlockingQueue.dequeue();
}

Any ideas?

Bergi · Accepted Answer · 2021-11-22 15:31:41Z

20

It's quite simple actually, dequeue will create a promise that enqueue will resolve. We just have to keep the resolvers in a queue - and also care about the case where values are enqueued before they are dequeued, keeping the already fulfilled promises in a queue.

class AsyncBlockingQueue {
  constructor() {
    // invariant: at least one of the arrays is empty
    this.resolvers = [];
    this.promises = [];
  }
  _add() {
    this.promises.push(new Promise(resolve => {
      this.resolvers.push(resolve);
    }));
  }
  enqueue(t) {
    // if (this.resolvers.length) this.resolvers.shift()(t);
    // else this.promises.push(Promise.resolve(t));
    if (!this.resolvers.length) this._add();
    this.resolvers.shift()(t);
  }
  dequeue() {
    if (!this.promises.length) this._add();
    return this.promises.shift();
  }
  // now some utilities:
  isEmpty() { // there are no values available
    return !this.promises.length; // this.length <= 0
  }
  isBlocked() { // it's waiting for values
    return !!this.resolvers.length; // this.length < 0
  }
  get length() {
    return this.promises.length - this.resolvers.length;
  }
  [Symbol.asyncIterator]() {
    // Todo: Use AsyncIterator.from()
    return {
      next: () => this.dequeue().then(value => ({done: false, value})),
      [Symbol.asyncIterator]() { return this; },
    };
  }
}

I don't know TypeScript, but presumably it's simple to add the the necessary type annotations.

For better performance, use a Queue implementation with circular buffers instead of plain arrays, e.g. this one. You might also use only a single queue and remember whether you currently store promises or resolvers.

edited Nov 22, 2021 at 15:31

answered Nov 7, 2017 at 12:06

Bergi

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5 Comments

Rico Kahler Over a year ago

I'd like to thank stackoverflow for making me add more ! to reach the character minimum

Ben Aston Over a year ago

@Bergi In this solution, why is there the check if (!this.resolvers.length) in enqueue? The logic appears to encode the creation of a Promise (and a resolver), and then the immediate removal of the resolver. What am I missing? Does the length of the array of resolvers ever get above 1?

Bergi Over a year ago

@52d6c6af The length of the resolvers goes above 0 when there were more dequeue() calls than enqueue() calls, the length of the promises goes above 0 when there were more enqueue() calls than dequeue() calls.

Bergi Over a year ago

@52d6c6af No, I don't think this has to do anything with the trampolines that I know

Thomas Halwax Over a year ago

Thank you for your solution! I've added a requeue function that adds a Promise at the head of the array. This way I can do a NO-ACK and still maintain the sequence of enqueued items: ` requeue(t) { if (!this.promises.length) { this._add() } else { this.promises.unshift(new Promise(resolve => { this.resolvingFunctions.unshift(resolve) })) } this.resolvingFunctions.shift()(t) } `

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