Miscellaneous Asynchronous Generators Utilities for ECMAScript NodeJs

• sleep - await the returned promise to sleep the given time
• chain_generators - collect items from multiple async generators from which ever comes first
• bulks - collect items from the given async generator into chunks of given size
• generatorFromEvents - convert emmited events to async generator
• AsyncSemaphore - resource-limiting lock used like this sem.with(async function(){do_expensive_work_here();})
• AsyncLock - behaves like normal thread lock implemented using AsyncSemaphore with special case of n=1
• AsyncEvent - use await event.wait() which will return when event.set()
• AsyncEventAlt - same as AsyncEvent implemented using EventEmitter
• AsyncChannel - a queue of optionally given size that have producers (await ch.push(item)) and cosumers (item=await ch.consume())

Usage

npm install --save '@alsadi/async_utils'

and in your code

import {sleep, chain_generators, bulks} from '@alsadi/async_utils'

Sleep

import { sleep } from "@alsadi/async_utils";
async function main() {
    const p = sleep(5000).then(() =>
        console.log("this will appear after 5 seconds")
    );
    console.log("sleeping for 2 seconds: started ...");
    await sleep(2000);
    console.log("sleeping for 2 seconds: started done");
    console.log("3 seconds remaining: ...");
    await p;
}
main();

Chaining async generators (possibly infinite)

If you have two infinite async generators gen1 and gen2 that produce items you can consume those items as they arrive

for await (const item of chain_generators([gen1, gen2])) {
    console.log(item);
}

Collecting items from an async generator into chunks of given size

for await (const chunk of bulks(3, gen3)) {
    console.log(chunk);
}

generatorFromEvents

Node's built-in http server have events like data on request

const gen = generatorFromEvents(request, ["data"], ["end"], ["error"]);
for await (const [name, [chunk]] of gen) {
    console.log(name, chunk.length);
}

AsyncSemaphore - resource-limiting lock

AsyncSemaphore Background

As Semaphores are important to coordinate threads. This node module gives you a similar feature for your async tasks.

A Semaphore indicates available resources to a given capacity. When this limit is reached, it will sleep until a resource is free.

Think of it like a sound card with limited number of channels, let's say 8 channels, in this case we have a semaphore of size 8, when a process acquire it, there will be 7 free resources available when another process acquire it, there will be 6 free resources available. When all resources are taken the trial to acquire it will sleep until it's available.

AsyncSemaphore Usage

If you have an async generator or you have a producer that generates many tasks and you want a cheap way to limit how many pending tasks

import { SemaphorePromise } from "@alsadi/semaphore";
// indicate that we can have 5 pending tasks
// 5 resources can be allocated without sleep
const sem = new SemaphorePromise(5);
async function main() {
    // ...
    await sem.acquire(); // this will return directly or sleep until resource is available
    const wrapped_promise = myasync_task()
        .then(console.log)
        .finally(() => sem.release()); // when done a resource is available again
    // ...
}

you can use sem.with(...) to do acquire() and release() automatically. sem.with(...) takes a function that returns the promise

import { SemaphorePromise } from "@alsadi/semaphore";
// indicate that we can have 5 pending tasks
// 5 resources can be allocated without sleep
const sem = new SemaphorePromise(5);
async function main() {
    // ...
    const wrapped_promise = sem.with(function () {
        return myasync_task().then(console.log);
    });
    // ...
}

You can use this inside async generators

async function* mygen() {
    while (something()) {
        yield sem.with(function () {
            return myasync_task().then(console.log);
        });
    }
}

AsyncSemaphore Implementation

This implementation only uses an integer counter and a Promise with a callback Other than that it's zero cost. It does not have busy loops or arbtrary sleeps. The size of the semaphore does not affect its cost.

AsyncLock - classical lock behavior

import { sleep, AsyncLock } from "./index.js";

const lock = new AsyncLock();

async function main() {
    const result = await lock.with(my_async_function);
    const result = await lock.with(() => do_something()); // do_something() returns a promise
    const result = await lock.with(async function () {
        await do_something();
    });
}

AsyncEvent

async function main() {
    const event = new AsyncEvent();
    // somewhere event will be set
    sleep(5000).then(() => event.set());
    await event.wait();
}

AsyncChannel - a queue of optionally given size that have producers and cosumers

You can pass an optional queue size, otherwise queue will be unlimited.

const queue_size = 7;
const ch = new AsyncChannel(queue_size);

when you push to the queue you must use await because if queue size limit, push will sleep until consumers do their work

async function producer(name) {
    for (let i = 0; i < 10; ++i) {
        await ch.push(`${name}-${i}`);
    }
}
p = [];
p.push(producer("my-producer1"));
p.push(producer("my-producer2"));
await Promise.all(p);

items in queue are consumed like this

while (true) {
    const item = await ch.consume();
    await do_something(item);
}

you can use async generators like this

async my_consumer() {
    for await (const item of ch.consume_items()) {
        await do_something(item);
    }
}
c=[]
c.push(my_consumer());
c.push(my_consumer());
c.push(my_consumer());
await Promise.all(c);

you can run multiple consumers and producers