Promise Queues

Consider an asynchronous queue. With a conventional queue, you must put a value in before you can take it out. That is not the case for a promise queue. Just as you can attach an observer to a promise before it is resolved, with a promise queue, you can get a promise for the next value in order before that value has been given.

var queue = new Queue();
queue.get().then(function (value) {
    console.log(value);
})
queue.put("Hello, World!");

Likewise of course you can add a value to the queue before observing it.

var queue = new Queue();
queue.put("Hello, World!");
queue.get().then(function (value) {
    console.log(value);
})

Although promises come out of the queue in the same order their corresponding resolutions enter, a promise obtained later may settle sooner than another promise. The values you put in the queue may themselves be promises.

var queue = new Queue();
queue.get().then(function () {
    console.log("Resolves later");
});
queue.get().then(function () {
    console.log("Resolves sooner");
});
queue.put(Promise.delay(100));
queue.put();

A promise queue qualifies as an asynchronous collection, specifically a collection of results: values or thrown errors captured by promises. The queue is not particular about what those values mean and is a suitable primitive for many more interesting tools.

The implementation of a promise queue is sufficiently succinct that there’s no harm in embedding it here. This comes from Mark Miller's Concurrency Strawman for ECMAScript and is a part of the Q library, exported by the q/queue module. Internally, a promise queue is an asynchronous linked list that tracks the head promise and tail deferred. get advances the head promise and put advances the tail deferred.

module.exports = PromiseQueue;
function PromiseQueue() {
    if (!(this instanceof PromiseQueue)) {
        return new PromiseQueue();
    }
    var ends = Promise.defer();
    this.put = function (value) {
        var next = Promise.defer();
        ends.resolve({
            head: value,
            tail: next.promise
        });
        ends.resolve = next.resolve;
    };
    this.get = function () {
        var result = ends.promise.get("head");
        ends.promise = ends.promise.get("tail");
        return result;
    };
}

The implementation uses methods defined in a closure. Regardless of how this is accomplished, it is important that it be possible to pass the free get function to a consumer and put to a producer to preserve the principle of least authority and the unidirectional flow of data from producer to consumer.

See the accompanying sketch of a promise queue implementation.

A promise queue does not have a notion of termination, graceful or otherwise. We will later use a pair of promise queues to transport iterations between streams.