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Cloth

Cloth is a simple thread pool and task queue for Node. It's a lightweight abstraction of Node's child_process and EventEmitter modules.

// index.js
 
const Pool = require('cloth/pool');
 
const pool = new Pool(`${__dirname}/worker`);
 
pool.run('Hello, world!').on('end', message => {
  console.log(message);
});

// worker.js
 
const worker = require('cloth/worker');
 
worker.run(command => {
  console.log(command);
  return 'Goodbye, world!';
});

Getting Started

Installation:

npm install cloth --save

Usage:

Instantiate a pool with the path to the worker file:

const Pool = require('cloth/pool');
 
const pool = new Pool(`${__dirname}/worker`);

Run

const task = pool.run('Hello, world!');

The argument we're passing is the task's command. Here, the command is a string, but it can be an array, object or any other serializable data structure.

Since this is the first task we're running, all the workers are idle and the task will be run immediately. If we try to run tasks faster than our workers finish them, tasks will be put into a queue where they'll run as soon as a worker finishes its current task.

What if we want information back from a task? We can listen to events it sends back:

task.on('end', message => {
  console.log(message);
});

The end event occurs when the worker finishes processing the task. But what happens if an error prevents it from finishing?

task
  .on('end', message => {
    console.log(message);
  })
  .on('error', err => {
    console.log(err);
  });

We can chain together as many on calls as we'd like this way — and other than start, end and error, we can have our worker send back custom events as well:

task
  .on('end', message => {
    console.log(message);
  })
  .on('error', err => {
    console.log(err);
  })
  .on('test', err => {
    console.log(err);
  });

All of which brings us to:

// worker.js
 
const worker = require('cloth/worker');
 
worker.run(command => {
  console.log(command);
  return 'Goodbye, world!';
});

Remember how we instantiated the pool with the path to the worker file? This is that file!

The most important worker method is run. Every time the worker picks up a task, the callback supplied to this method is invoked with the task's command. We process the command however we want and return the result, and the worker will take care of sending the results back to the main process:

worker.run(command => {
  console.log(command);
  return 'Goodbye, world!';
});

If we run into problems, we just have to throw an error and the worker will let the main process know:

worker.run(command => {
  if (!command) {
    throw new Error('No command!');
  }
  console.log(command);
  return 'Goodbye, world!';
});

If our task is asynchronous, we can add a callback parameter and deal with things in traditional Node style:

worker.run((command, callback) => {
  if (!command) {
    callback(new Error('No command!'));
  }
  console.log(command);
  callback(null, 'Goodbye, world!');
});

Finally, if we need to, we can also send intermediate results back to the main process:

worker.run((command, callback) => {
  if (!command) {
    callback(new Error('No command!'));
  }
  worker.send('test', 'message');
  console.log(command);
  callback(null, 'Goodbye, world!');
});

That's the quick intro to Cloth! Check out the API docs below for more information. Happy parallelism!

API

Pool

A Pool is the main process's interface to the child processes. It manages the worker pool and task queue to optimize concurrency.

constructor(worker, [options])

Creates a new pool with a given worker file. By default, it will create the same number of workers as CPU cores (require('os').cpus().length) but this can be overridden in the options.

options

• workers: the number of workers to create
• arguments: an array of string arguments to be sent to each worker as it's created

run(command)

Creates a task with the given command to be run as soon as a worker is available. Returns the task.

on(event, listener)

Calls the supplied listener with the event message and task whenever an event of the specified type is triggered by any task.

total()

Returns the number of workers in the pool.

available()

Returns the number of workers in the pool that are not currently processing tasks.

drain()

Kills all the worker processes and removes all the tasks from the queue.

worker

A worker is the children processes' interface to the main process. It invokes a user-defined function to process tasks and manages task lifecycle events and error handling.

run(fn)

Invokes the supplied function with the task's command whenever the worker picks up a task. The function signature should be fn(command, [callback]). If it doesn't take a callback, returning any value will cause the task to end successfully; if it does take a callback, errors can be sent with callback(err) and successful results with callback(null, results). Throwing an error will also make the worker send an error back to the main process.

send(event, message)

Sends an event with the supplied type and message back to the main process. Any listeners for that event type on either the task or the pool will be triggered.

arguments

The arguments passed to the Pool when it was instantiated in the main process.

Task

A set of instructions that are run as soon as a worker frees up. Tasks are picked up in the order they're sent.

on(event, listener)

Calls the supplied listener with the event message and task whenever an event of the specified type is triggered by this specific task.

state

The state of a task with regard to the queue; one of four possibilities:

• queued: the task hasn't yet been picked up
• processing: the task is currently running
• finished: the task completed successfully
• error: the task couldn't complete because of an error