README.md

channel-queue

A queue management library with channels.

About

This library provides a queue system, organised by channels (or "topics"), that allows for finer-grain control over asynchronous method execution. It allows for queuing tasks by channels (string-based names) - tasks executing sequentially when the previous task is done. It also allows for 3 priority levels : normal, high and tail.

Check out the API documentation.

Installation

Simply run:

npm install @buttercup/channel-queue --save

Usage

The simplest usage is to just queue some tasks:

import { ChannelQueue } from "@buttercup/channel-queue";

const queue = new ChannelQueue();
const work = queue.channel("myChannel").enqueue(() => 123);
// 'work' will resolve with 123

const someChannel = queue.channel("someChannel");
someChannel.enqueue(job1);
someChannel.enqueue(job2);
someChannel.enqueue(job3);

You can push tasks above others by setting them as high-priority:

import { ChannelQueue, TaskPriority } from "@buttercup/channel-queue";

const queue = new ChannelQueue();
const workChannel = queue.channel("work");
// some other tasks added to the queue...
workChannel.enqueue(importantJobMethod, TaskPriority.High).then(result => {
    // 'result' is the resolved result from the 'importantJobMethod' function
});

Tasks can also be run as "low" priority or at the tail end of the queue:

import { ChannelQueue, TaskPriority } from "@buttercup/channel-queue";

const queue = new ChannelQueue();
const workChannel = queue.channel("work");
// some other tasks added to the queue...
workChannel.enqueue(runNearEnd, TaskPriority.Tail);

Error Handling

Regardless whether an enqueued task fails or not, the channel will continue processing after it's done.

By default, if the task enqueued in enqueue throws an error, it will throw at the enqueue call:

// The following throws:
await channel.enqueue(() => Promise.reject(new Error("Fails!")));

You can disable enqueued functions throwing errors by setting tasksThrow to false:

channel.tasksThrow = false;
// The following will not throw:
await channel.enqueue(() => Promise.reject(new Error("Fails!")));

Disabling this is really only useful if you use waitForEmpty with the throwForFailures set to true. This allows you to delay error handling until the channel is waited upon:

channel.tasksThrow = false;
channel.enqueue(() => /* ... */)
channel.enqueue(() => Promise.reject(new Error("Fails!")));

// Later:
await channel.waitForEmpty({ throwForFailures: true });

Stacking

Items can be "stacked", meaning that if specified, items can be limited to only 1 pending item in queue. All items of the same stack name would simply queue on the same item and not create more tasks. The stack can be specified when enqueuing:

import { ChannelQueue } from "@buttercup/channel-queue";

const queue = new ChannelQueue();
const workChannel = queue.channel("work");

const promise1 = workChannel.enqueue(saveWorkFn, undefined, /* Stack ID */ "save");
// work start
const promise2 = workChannel.enqueue(saveWorkFn, undefined, "save");
const promise3 = workChannel.enqueue(saveWorkFn, undefined, "save");
// promise2 and promise3 will be equal, as promise2 was still in the queue when promise3

Parallel Execution

Tasks can be run in parallel using the ParallelChannel class. You can create a parallel channel, in place of a regular channel, by calling ChannelQueue#createParallelChannel:

import { ChannelQueue } from "@buttercup/channel-queue";

const queue = new ChannelQueue();
const workChannel = queue.createParallelChannel("work");
workChannel.enqueue(someTask);

// The same channel can be fetched later using the familiar channel() method:
queue.channel("work"); // The parallel channel

Parallel channels, like their name implies, can run tasks in parallel. Instead of running them head-to-tail like regular channels, parallel channels can execute many tasks side-by-side. You can also limit them to a certain number of threads (default is 2) by calling queue.createParallelChannel("name", 5) (where 5 is the maximum number of simultaneous tasks).

Parallel channels by default do not run tasks of different priorities simultaneously. This means that if the current running tasks are high-priority, no normal priority tasks will be started. This feature can be disabled by running parallelChannel.canRunAcrossTaskTypes = true.

Waiting and Clearing

You can wait for a channel to empty (having all tasks completed) by using waitForEmpty:

channel.enqueue(someTask);
channel.enqueue(someTask);

// Later:
await channel.waitForEmpty();

You can clear all enqueued items in a Channel by calling clear():

queue.channel("myChannel").clear();

Development & Supported Node Versions

This library is intended to be used with NodeJS version 6 and later.

To contribute, clone this project and run npm install before beginning development.

To test, simply run npm test.