npm package: @logeshdas21/channel-queue
Description: A queue management library with channels
Author: Buttercup
Latest version: 0.0.1

# channel-queue
A queue management library with channels.

[![Build Status](https://travis-ci.org/buttercup/channel-queue.svg?branch=master)](https://travis-ci.org/buttercup/channel-queue) [![npm version](https://badge.fury.io/js/%40buttercup%2Fchannel-queue.svg)](https://www.npmjs.com/package/@buttercup/channel-queue)

## 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](API.md).

## Installation

Simply run:

```shell
npm install @buttercup/channel-queue --save
```

## Usage
The simplest usage is to just queue some tasks:

```javascript
const ChannelQueue = require("@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:

```javascript
const ChannelQueue = require("@buttercup/channel-queue");
const { TASK_TYPE_HIGH_PRIORITY } = ChannelQueue.Task;

const queue = new ChannelQueue();
const workChannel = queue.channel("work");
// some other tasks added to the queue...
workChannel.enqueue(importantJobMethod, TASK_TYPE_HIGH_PRIORITY).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:

```javascript
const ChannelQueue = require("@buttercup/channel-queue");
const { TASK_TYPE_TAIL } = ChannelQueue.Task;

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

### 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:

```javascript
const ChannelQueue = require("@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`:

```javascript
const ChannelQueue = require("@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`.

### Clearing

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

```javascript
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`.