bthreads

bthreads

A worker_threads wrapper for node.js. Provides transparent fallback for pre-11.7.0 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 (with node@<11.7.0):

$ 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.

Multiple Entry Points

require('bthreads') will automatically pick the backend depending on what is available, but in some cases that may not be what you want. Because of this, there are also more explicit entry points:

• require('bthreads/process') - Always the child_process backend, regardless of node version.
• require('bthreads/threads') - Always the worker_threads backend, regardless of node version.
• require('bthreads/stable') - Points to the worker_threads backend once it is considered "stable", child_process otherwise. The current "stable" node version for worker_threads is considered to be 11.11.0. May change in the future.

Caveats

Some caveats for the child_process backend:

• The transfer list only works for MessagePorts. ArrayBuffers 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.
• In order to avoid memory leaks, MessagePorts (all aside from the parentPort) do not hold event loop references (ref() and unref() are noops).
• Prior to node 10, objects like Proxys can be serialized and cloned as they cannot be detected from regular javascript.
• SHARE_ENV does not work and will throw an error if passed.

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 requires a "bootstrap" file for 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. By default, bthreads will pull in its own bundle as an npm package from unpkg.com. If using the default bootstrap file, you must have blob: and/or data: set as a Content-Security-Policy source (see content-security-policy.com for a guide). 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).
• 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.
• SHARE_ENV does not work and will throw an error if passed.
• workerData is serialized as json instead of using the structured clone algorithm. This limits what can be sent as workerData. This was done to reduce code size since serializing structured data is non-trivial.
• The stdio, stdin, and stdout options will throw an error if passed. STDIO streams do not exist in the browser. This is done to reduce code size.
• To make sure bthreads is aware of the Buffer object in the browser, you must assign bthreads.Buffer like so: bthreads.Buffer = Buffer;. Once again, this was done to reduce code size.

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. This means they are restricted by the connect-src Content-Security-Policy directive specifically (instead of perhaps the worker-src directive).
• All transferred ArrayBuffers behave as if they were SharedArrayBuffers (i.e. they're not neutered). Be careful!
• Uncaught errors will not be caught and emitted as error events on worker objects.
• Worker scripts cannot be executed as ES modules.
• Exotic objects like Proxys can be serialized and cloned as they cannot be detected from regular javascript.

Caveats for all of the above:

• For a number of reasons, bthreads has to walk the objects you pass in to send. Note that the cloning function may get confused if you attempt to send the raw prototype of a built-in object (for example worker.postMessage(Buffer.prototype)).

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(__filename);

  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 {
  const {parent} = threads;

  Buffer.poolSize = 1; // Make buffers easily transferrable.

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

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

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

Writing code for node and the browser

One of the remarkable features of bthreads is that it allows for static analysis when bundling. The threads.Pool and threads.Thread objects resolve their filename argument as if it was a require() from the calling file.

const thread = new threads.Thread('./worker.js');

The above line will resolve to ${__dirname}/worker.js in node.js and ${window.location}/worker.js in the browser. In node.js, it is not relative to the current working directory! We accomplish this through various forms of sorcery.

Why does this matter? Because it allows for browserify and/or webpack to do static analysis on your code and ship your code (including workers) as a single bundled file! Of course, this would require an extra browserify or webpack plugin which adds some more initialization code for choosing the proper entry point.

How this works behind the scenes (for plugin implementers)

Statically analyzing the line above, the compiler should replace './worker.js' with 'bthreads-worker@[id]'. When initializing the code, bthreads should be implicitly required. bthreads will set an environment variable called process.env.BTHREADS_WORKER_INLINE which contains the [id] you generated previously, allowing you to determine which function to run inside the worker thread.

In other words, when the compiler comes across:

const thread = new threads.Thread('./worker.js');

./worker.js should be included in the bundled and mapped to an ID (in our case, we include it in the bundle with an ID of 1).

Our line becomes:

const thread = new threads.Thread('bthreads-worker@1');

The bundle's main entry point should include some initialization code like:

requireBthreads();

if (process.env.BTHREADS_WORKER_INLINE)
  requireWorker(process.env.BTHREADS_WORKER_INLINE);
else
  requireMain();

importScripts

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

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

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

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 module.require instead of require.

const threads = require('bthreads');

function myWorker() {
  const threads = module.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.browser - true if a browser backend is being used.
  • threads.location - The current module URL (cross-platform import.meta.url).
  • threads.filename - The current module filename (cross-platform __filename).
  • threads.dirname - The current module dirname (cross-platform __dirname).
  • threads.require(location) - importScripts() wrapper (browser+worker only).
  • threads.resolve(location) - Resolve a URL or path to a filename. This is what threads.require calls internally.
  • threads.exit(code) - A reference to process.exit.
  • threads.cores - Number of CPU cores available.
• Options
  • threads.Buffer - In the browser, this must be set to the Buffer object in order for bthreads to be aware of buffers.
  • threads.bufferify - A boolean indicating whether to cast Uint8Arrays to Buffer objects after receiving. Only affects the high-level API. This option is on by default.
• 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.
  • Socket#closed (read only) - A boolean representing whether the socket is closed.
• 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#read() (async) - Wait for and read the next message event.
  • 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#online (read only) - A boolean representing whether the thread is online.
  • 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#open() (async) - Wait for the online event to be emitted.
  • Thread#close() (async) - Terminate the thread and wait for exit event but also listen for errors and reject the promise if any occur (in other words, a better async version of Thread#terminate).
  • Thread#wait() (async) - Wait for thread to exit, but do not invoke close(). Also listen for errors and reject the promise if any occur.
• 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() (async) - Close the port and wait for close event, but
  • Port#wait() (async) - Wait for port to exit, but do not invoke close(). Also listen for errors and reject the promise if any occur.
• 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() (async) - Populate and wait until all threads are online (otherwise threads will be lazily spawned).
  • Pool#close() (async) - Close all threads in pool, reject on errors.
  • Pool#populate() - Populate the pool with this.size threads (otherwise threads will be lazily spawned).
  • Pool#next() - Return the next thread in queue (this may spawn a new thread).
  • 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 is 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.
• options.bootstrap is a valid option in the browser when used in combination with options.eval. Its value should be the URL of a compiled bundle file. For security, it's recommended to serve your own bootstrap file. This can be set to false to do a raw eval (you must inline your own initialization code, presumably by using importScripts).
• 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.
• options.dirname allows you to set the __dirname of an eval'd module. This makes require more predictable in eval'd modules (note this is not necessary with the Thread and Pool objects -- it is done automatically).

Worker Data

In the browser, workerData is serialized as JSON instead of structured data. To force usage of the structured clone algorithm, it's possible to require ./lib/encoding (note that this will increase your code size greatly).

const encoding = require('bthreads/encoding');
const thread = new threads.Thread('./worker.js', {
  workerData: encoding.stringify({ foo: 'bar' })
});

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.