Module: queue

A blocking queue implementation with a fixed capacity. The enqueue and dequeue methods will block if the queue is full or empty, respectively, until space is available or an item is enqueued. This allows for producers being throttled by back-pressure from the availability of consumers and a very simple implementation of a continuous consumer.

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Example
async function consume(q, id) {
  while (true) {
    const msg = await q.dequeue();
    console.log(id, "started processing", msg);
    await new Promise((resolve) => setTimeout(resolve, 100)); // Pretend to be busy
    console.log(id, "finished processing", msg);
  }
}

const q = new Queue(2);
consume(q, "Worker 1");
consume(q, "Worker 2");

await q.enqueue("Hello, World 1!");  // Consumed immediately
await q.enqueue("Hello, World 2!");  // Consumed immediately
await q.enqueue("Hello, World 3!");  // Held in queue
await q.enqueue("Hello, World 4!");  // Held in queue
await q.enqueue("Hello, World 5!");  // Blocked until worker available
await q.enqueue("Hello, World 6!");  // Blocked until worker available

Methods

(async) dequeue()

Get the next item from the queue. If the queue is empty, this will block until an item is enqueued. Always await this method to ensure you get the proper result out.

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Returns:

The next item from the queue

(async) enqueue(item)

Put an item in the queue. If the queue is full and you await this function it will block until space is available, allowing for back-pressure to throttle producers. If you don't await this function, you effectively have an "infinite" capacity, since the enqueue functions will wait in the JavaScript event loop Promise queue. This is probably not what you want.

Parameters:
Name Type Description
item *

The item to put in the queue
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isEmpty()

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Returns:

true if the queue is empty, false otherwise

isFull()

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Returns:

true if the queue is full, false otherwise

peek()

Peek at the next item in the queue without dequeuing it. This will return undefined if the queue is empty, but it will not block.

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Returns:

The next item in the queue, or undefined if the queue is empty

size()

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Returns:

The number of items currently in the queue