bthreads

bthreads

A worker_threads wrapper for node.js. Provides transparent fallback for pre-v11.7.0 node.js (via child_process) as well as browser web workers. Browserifiable, webpack-able.

Usage

const threads = require('bthreads');

if (threads.isMainThread) {
  const worker = new threads.Worker(__filename, {
    workerData: 'foo'
  });

  worker.on('message', console.log);
  worker.on('error', console.error);

  worker.on('exit', (code) => {
    if (code !== 0)
      console.error(`Worker stopped with exit code ${code}.`);
  });
} else {
  threads.parentPort.postMessage(threads.workerData + 'bar');
}

Output:

$ node --experimental-worker threads.js
foobar
$ node threads.js
foobar

Backends

bthreads has 4 backends and a few layers of fallback:

• worker_threads - Uses the still experimental worker_threads module in node.js. Only usable prior to node.js v11.7.0 if --experimental-worker is passed on the command line.
• child_process - Leverages the child_process module in node.js to emulate worker threads.
• web_workers - Web Workers API (browser only).
• polyfill - A polyfill for the web workers API.

The current backend is exposed as threads.backend. Note that the current backend can be set with the BTHREADS_BACKEND environment variable.

Caveats

Some caveats for the child_process backend:

• The transfer list only works for MessagePorts. Array buffers won't actually be transferred.
• options.workerData probably has a limited size depending on platform (the maximum size of an environment variable).
• SharedArrayBuffer does not work and will throw an error if sent.
• The object serializer does not yet support circular references. Any circular reference will be replaced with undefined on the other side.

Caveats for the web_workers backend:

• options.workerData possibly has a limited size depending on the browser (the maximum size of options.name).
• options.eval will create a data URI and execute a new worker from it. When using a bundler, note that the bundler will not be able to compile the eval'd code. This means that require will have limited usability (restricted to only core browserify modules and bthreads itself).
• Furthermore, options.eval requires that data: be set for the worker-src Content-Security-Policy. See content-security-policy.com for a guide.
• The close event for MessagePorts only has partial support (if a thread suddenly terminates, close will not be emitted for any remote ports). This is because the close event is not yet a part of the standard Web Worker API. See https://github.com/whatwg/html/issues/1766 for more info.

Caveats for the polyfill backend:

• Code will not actually run in a separate context (obviously).
• importScripts will perform a synchronous XMLHttpRequest and potentially freeze the UI. Additionally, XHR is bound to certain cross-origin rules that importScripts is not.
• Similarly, worker scripts are also spawned using XHR. The same cross-origin limitations apply.
• Blob, File, FileList, and ImageBitmap cannot be cloned due to limitations of the DOM.

Finally, caveats for the worker_threads backend:

• It is somewhat unstable and crashes a lot with assertion failures, particularly when there is an uncaught exception or the thread is forcefully terminated. Note that worker_threads is still experimental in node.js!
• Native modules will be unusable if they are not built as context-aware addons.

High-level API

The low-level node.js API is not very useful on its own. bthreads optionally provides an API similar to bsock.

Example (for brevity, the async wrapper is not included below):

const threads = require('bthreads');

if (threads.isMainThread) {
  const thread = new threads.Thread(__filename);

  thread.bind('event', (x, y) => {
    console.log(x + y);
  });

  console.log(await thread.call('job', ['hello']));
} else {
  const {parent} = threads;

  parent.hook('job', async (arg) => {
    return arg + ' world';
  });

  parent.fire('event', ['foo', 'bar']);
}

Output:

foobar
hello world

Creating a thread pool

You may find yourself wanting to parallelize the same worker jobs. The high-level API offers a thread pool object (threads.Pool) which will automatically load balance and scale to the number of CPU cores.

if (threads.isMainThread) {
  const pool = new threads.Pool(threads.source);

  const results = await Promise.all([
    pool.call('job1'), // Runs on thread 1.
    pool.call('job2'), // Runs on thread 2.
    pool.call('job3')  // Runs on thread 3.
  ]);

  console.log(results);
} else {
  Buffer.poolSize = 1; // Make buffers easily transferrable.

  pool.hook('job1', async () => {
    const buf = Buffer.from('job1 result');
    return [buf, [buf.buffer]]; // Transfer the array buffer.
  });

  pool.hook('job2', async () => {
    return 'job2 result';
  });

  pool.hook('job3', async () => {
    return 'job3 result';
  });
}

Writing code for node and the browser

It's good to be aware of browserify and how it sets __filename and __dirname.

For example:

const worker = new threads.Worker(`${__dirname}/worker.js`);

If your code resides in /root/project/lib/main.js, the browserify generated path will ultimately be /lib/worker.js. Meaning /root/project/lib/worker.js should exist for node and http://[host]/lib/worker.js should exist for the browser.

The browser backend also exposes a browser flag for this situation.

Example:

const worker = new threads.Worker(threads.browser
                                ? 'http://.../' + path.basename(file)
                                : file);

To make self-execution easier, bthreads also exposes a threads.source property which refers to the main module's filename in node.js and the current script URL in the browser.

importScripts

In the browser, bthreads exposes a more useful version of importScripts.

const threads = require('bthreads');
const _ = threads.importScripts('https://unpkg.com/underscore/underscore.js');

This should work for any library exposed as UMD or CommonJS. Note that threads.importScripts behaves more like require in that it caches modules by URL. The cache is accessible through threads.importScripts.cache.

More about eval'd browser code

Note that if you are eval'ing some code inside a script you plan to bundle with browserify or webpack, require may get unintentionally transformed or overridden. This generally happens when you are calling toString on a defined function.

const threads = require('bthreads');

function myWorker() {
  const threads = require('bthreads');

  threads.parentPort.postMessage('foo');
}

const code = `(${myWorker})();`;
const worker = new threads.Worker(code, { eval: true });

The solution is to access global.require instead of require.

const threads = require('bthreads');

function myWorker() {
  const threads = global.require('bthreads');

  threads.parentPort.postMessage('foo');
}

const code = `(${myWorker})();`;
const worker = new threads.Worker(code, { eval: true });

API

• Default API
  • threads.isMainThread - See worker_threads documentation.
  • threads.parentPort - See worker_threads documentation (worker only).
  • threads.threadId - See worker_threads documentation.
  • threads.workerData - See worker_threads documentation (worker only).
  • threads.MessagePort - See worker_threads documentation.
  • threads.MessageChannel - See worker_threads documentation.
  • threads.Worker - See worker_threads documentation.
• Helpers
  • threads.backend - A string indicating the current backend (worker_threads, child_process, web_workers, or polyfill).
  • threads.source - The current main module filename or script URL (null if in eval'd thread).
  • threads.browser - true if a browser backend is being used.
  • threads.process - Reference to the child_process backend. This is present to explicitly use the child_process backend instead of the worker_threads backend.
  • threads.exit(code) - A reference to process.exit (worker only).
  • threads.stdin - A reference to process.stdin (worker only).
  • threads.stdout - A reference to process.stdout (worker only).
  • threads.stderr - A reference to process.stderr (worker only).
  • threads.console - A reference to global.console (worker only).
  • threads.importScripts(url) - importScripts() wrapper (browser+worker only).
  • threads.cores - Number of CPU cores available (this value will always be 2 in the browser).
• High-Level API
  • threads.Thread - Thread Class (see below).
  • threads.Port - Port Class (see below).
  • threads.Channel - Channel Class (see below).
  • threads.Pool - Pool Class (see below).
  • threads.parent - A reference to the parent Port (worker only, see below).

Socket Class (abstract, extends EventEmitter)

• Constructor
  • new Socket() - Not meant to be called directly.
• Properties
  • Socket#events (read only) - A reference to the bind EventEmitter.
• Methods
  • Socket#bind(name, handler) - Bind remote event.
  • Socket#unbind(name, handler) - Unbind remote event.
  • Socket#hook(name, handler) - Add hook handler.
  • Socket#unhook(name) - Remove hook handler.
  • Socket#send(msg, [transferList]) - Send message, will be emitted as a message event on the other side.
  • Socket#fire(name, args, [transferList]) - Fire bind event.
  • Socket#call(name, args, [transferList], [timeout]) (async) - Call remote hook.
  • Socket#hasRef() - Test whether socket has reference.
  • Socket#ref() - Reference socket.
  • Socket#unref() - Clear socket reference.
• Events
  • Socket@message(msg) - Emitted on message received.
  • Socket@error(err) - Emitted on error.
  • Socket@event(event, args) - Emitted on bind event.

Thread Class (extends Socket)

• Constructor
  • new Thread(filename, [options]) - Instantiate thread with module.
  • new Thread(code, [options]) - Instantiate thread with code.
  • new Thread(function, [options]) - Instantiate thread with function.
• Properties
  • Thread#stdin (read only) - A writable stream representing stdin (only present if options.stdin was passed).
  • Thread#stdout (read only) - A readable stream representing stdout.
  • Thread#stderr (read only) - A readable stream representing stderr.
  • Thread#threadId (read only) - An integer representing the thread ID.
• Methods
  • Thread#terminate([callback]) - Terminate the thread and optionally bind to the exit event.
  • Thread#close() (async) - Terminate the thread and wait for exit but also listen for errors and reject the promise if any occur (in other words, a better async version of Thread#terminate).
• Events
  • Thread@online() - Emitted once thread is online.
  • Thread@exit(code) - Emitted on exit.

Port Class (extends Socket)

• Constructor
  • new Port() - Not meant to be called directly.
• Methods
  • Port#start() - Open and bind port (usually automatic).
  • Port#close() - Close port.
• Events
  • Port@close() - Emitted on port close.

Channel Class

• Constructor
  • new Channel() - Instantiate channel.
• Properties
  • Channel#port1 (read only) - A Port object.
  • Channel#port2 (read only) - A Port object.

Pool Class (extends EventEmitter)

• Constructor
  • new Pool(filename, [options]) - Instantiate pool with module.
  • new Pool(code, [options]) - Instantiate pool with code.
  • new Pool(function, [options]) - Instantiate pool with function.
• Properties
  • Pool#file (read only) - A reference to the filename, function, or code that was passed in.
  • Pool#options (read only) - A reference to the options passed in.
  • Pool#size (read only) - Number of threads to spawn.
  • Pool#events (read only) - A reference to the bind EventEmitter.
  • Pool#threads (read only) - A Set containing all spawned threads.
• Methods
  • Pool#open() - Open and populate the pool with this.size threads (otherwise threads will be lazily spawned).
  • Pool#close() (async) - Close all threads in pool, reject on errors.
  • Pool#next() - Return the next thread in queue (this may spawn a new thread).
  • Pool#terminate(callback) - Terminate all threads in pool, optionally execute a callback once exit has been emitted for all threads.
  • Pool#bind(name, handler) - Bind remote event for all threads.
  • Pool#unbind(name, handler) - Unbind remote event for all threads.
  • Pool#hook(name, handler) - Add hook handler for all threads.
  • Pool#unhook(name) - Remove hook handler for all threads.
  • Pool#send(msg) - Send message to all threads, will be emitted as a message event on the other side (this will populate the pool with threads on the first call).
  • Pool#fire(name, args) - Fire bind event to all threads (this will populate the pool with threads on the first call).
  • Pool#call(name, args, [transferList], [timeout]) (async) - Call remote hook on next thread in queue (this may spawn a new thread).
  • Pool#hasRef() - Test whether pool has reference.
  • Pool#ref() - Reference pool.
  • Pool#unref() - Clear pool reference.
• Events
  • Pool@message(msg, thread) - Emitted on message received.
  • Pool@error(err, thread) - Emitted on error.
  • Pool@event(event, args, thread) - Emitted on bind event.
  • Pool@spawn(thread) - Emitted immediately after thread is spawned.
  • Pool@online(thread) - Emitted once thread is online.
  • Pool@exit(code, thread) - Emitted on thread exit.

Thread, Pool, and Worker Options

The options object accepted by the Thread, Pool, and Worker classes nearly identical to the worker_threads worker options with some differences:

• options.type and options.credentials are valid options when using the browser backend (see web_workers). Note that options.type = 'module' will not work with the polyfill backend. If a file extension is .mjs, options.type is automatically set to module for consistency with node.js.
• The browser backend requires a header or "prelude" file for eval'd code. This is essentially a bundle which provides all the necessary browserify modules (such that require('path') works, for example), as well as bthreads itself. When using a browser backend options.header is a valid option. It should be the URL to a bundle. By default, bthreads imports the bthreads-bundle package from unpkg.com.
• The Pool class accepts size option. This allows you to manually set the pool size instead of determining it by the number of CPU cores.

Contribution and License Agreement

If you contribute code to this project, you are implicitly allowing your code to be distributed under the MIT license. You are also implicitly verifying that all code is your original work. </legalese>

License

• Copyright (c) 2019, Christopher Jeffrey (MIT License).

See LICENSE for more info.