@travetto/worker

travetto: Worker

Install: primary

$ npm install @travetto/worker

This module provides the necessary primitives for handling dependent workers. A worker can be an individual actor or could be a pool of workers. Node provides ipc (inter-process communication) functionality out of the box. This module builds upon that by providing enhanced event management, richer process management, as well as constructs for orchestrating a conversation between two processes.

Execution Pools

With respect to managing multiple executions, WorkPool is provided to allow for concurrent operation, and processing of jobs concurrently. To manage the flow of jobs, there are various InputSource implementation that allow for a wide range of use cases.

The supported InputSources are

• Array is a list of jobs, will execute in order until list is exhausted.

• Queue is similar to list but will execute forever waiting for new items to be added to the queue.
• Iterator is a generator function that will continue to produce jobs until the iterator is exhausted.

Below is a pool that will convert images on demand, while queuing as needed.

Code: Image processing queue, with a fixed batch/pool size

class ImageProcessor {
  active = false;
  proc: ChildProcess;

  destroy() {
    this.proc.destroy();
  }

  async execute(path: string) {
    this.active = true;
    try {
      this.proc = ...convert ...
      await this.proc;
    } catch (e) {

    }
    this.active = false;
  }
}

class ImageCompressor extends WorkerPool {

  pendingImages = new QueueInputSource<string>();

  constructor() {
    super(async () => new ImageProcess());
  }

  begin() {
    this.process(this.pendingImages);
  }

  convert(...images: string[]) {
    for (const img of images) {
      this.pendingImages.enqueue(img);
    }
  }
}

Once a pool is constructed, it can be shutdown by calling the .shutdown() method, and awaiting the result.

IPC Support

Within the comm package, there is support for two primary communication elements: child and parent. Usually parent indicates it is the owner of the sub process. Child indicates that it has been created/spawned/forked by the parent and will communicate back to it's parent. This generally means that a parent channel can be destroyed (i.e. killing the subprocess) where a child channel can only exit the process, but the channel cannot be destroyed.

IPC as a Worker

A common pattern is to want to model a sub process as a worker, to be a valid candidate in a WorkPool. The WorkUtil class provides a utility to facilitate this desire.

Code: Spawned Worker Signature

class WorkUtil {
 static spawnedWorker<X>(
    config: SpawnConfig & {
      execute: (channel: ParentCommChannel, input: X) => any,
      destroy?: (channel: ParentCommChannel) => any,
      init?: (channel: ParentCommChannel) => any,
    }
  )

When creating your work, via process spawning, you will need to provide the script (and any other features you would like in SpawnConfig). Additionally you must, at a minimum, provide functionality to run whenever an input element is up for grabs in the input source. This method will be provided the communication channel (parent) and the input value. A simple example could look like:

Code: Simple Spawned Worker

    const pool = new WorkPool(() =>
      WorkUtil.spawnedWorker<string>({
        command: FsUtil.resolveUnix(__dirname, 'simple.child-launcher.js'),
        fork: true,
        async init(channel) {
          return channel.listenOnce('ready'); // Wait for child to indicate it is ready
        },
        async execute(channel, inp) {
          const res = channel.listenOnce('response'); //  Register response listener
          channel.send('request', { data: inp }); // Send request

          const { data } = await res; // Get answer
          console.log('Sent', inp, 'Received', data);

          assert(inp + inp === data); // Ensure the answer is double the input
        }
      })
    );

Code: Spawned Worker Target

const exec = new ChildCommChannel<{ data: string }>();

exec.listenFor('request', data => {  
  exec.send('response', { data: (data.data + data.data) }); // When data is received, return double
});

exec.send('ready'); // Indicate the child is ready to receive requests

const heartbeat = () => setTimeout(heartbeat, 5000); // Keep-alive
heartbeat();