synchronous-channel

This package provides synchronous message channels for Node.js Worker Threads and Web Workers in modern browsers.

Synchronous message passing should be avoided if possible, and existing asynchronous APIs such as postMessage should be used instead.

Communication model

We call a thread synchronous if it requires blocking the event loop for large amounts of time. While generally undesirable, it is sometimes difficult to prevent, for example, when running WebAssembly code in a background thread.

Sending thread type	Receiving thread type	Recommended mechanism
Asynchronous	Asynchronous	postMessage
Asynchronous	Synchronous	SynchronousChannel
Synchronous	Asynchronous	postMessage
Synchronous	Synchronous	SynchronousChannel

Examples

Node.js example:

const { Worker, isMainThread, parentPort, workerData } = require('worker_threads');
const { SynchronousChannel } = require('synchronous-channel');
 
if (isMainThread) {
  // Create a new channel that can store up to 16 messages, each having a size
  // of up to 32 bytes.
  const channel = new SynchronousChannel({
    queueLength: 16,
    maxMessageSize: 32
  });
 
  // The main thread will write to the channel.
  const writer = new SynchronousChannel.Writer(channel);
 
  // Start a new thread.
  new Worker(__filename, {
    workerData: {
      channel
    }
  })
  .once('online', () => {
    // As soon as the thread is ready, start sending the current time as a
    // string, and keep doing it forever.
    setInterval(() => {
      const message = Buffer.from(new Date().toISOString());
      if (!writer.write(message)) {
        console.error('Queue is full!');
      }
    }, 500);
  })
  .on('message', message => {
    console.log(`Worker says: ${message}`);
  });
} else {
  // The worker thread will read from the channel.
  const { channel } = workerData;
  const reader = new SynchronousChannel.Reader(channel);
 
  // Keep reading messages... forever.
  let message;
  while ((message = reader.read(true)) !== false) {
    const time = Buffer.from(message).toString('ascii');
    parentPort.postMessage(`The time is ${time}`);
  }
}

API

Class SynchronousChannel

Options:

• queueLength: Maximum numbers of messages to queue.
• maxMessageSize: The maximum size of each message, in bytes.

Each channel uses approximately queueLength * maxMessageSize bytes of memory. It is currently not possible to resize shared memory.

It is not possible to create more than one Writer or more than one Reader for a single SynchronousChannel. Attempting to create additional Writer or Reader instances will throw.

Example:

const channel = new SynchronousChannel({
  queueLength: 64,
  maxMessageSize: 1024
});

Class SynchronousChannel.Writer

Example:

const writer = new SynchronousChannel.Writer(channel);
writer.write(message);

Class SynchronousChannel.Reader

Example:

const reader = new SynchronousChannel.Reader(channel);
const message = reader.read();

Blocking operations and timeouts

This module supports both blocking and non-blocking read and write operations. For blocking operations, a timeout can be specified in milliseconds. Setting a timeout to true is equivalent to setting it to +Infinity, and the operation will potentially never return.

Browsers usually do not allow the main (renderer) thread to use blocking memory operations. Using a timeout will throw an exception in these cases.

Browser support

Currently, the SharedArrayBuffer class is the only way to implement synchronous communication between threads. Due to security concerns, it has been disabled in most browsers, and at the time of writing, only Chrome fully supports SharedArrayBuffer.