Observable Webworker

Simple API for using web workers with RxJS observables

Features

• Simple fromWorker function from main thread side
• Fully RxJS interfaces allowing both main thread and worker thread streaming
• Error handling across the thread boundaries is propagated
  • Under the hood materialize and dematerialize is used as a robust transport of streaming errors
• Automatic handling of worker termination on main thread unsubscription of observable
• Framework agnostic - while the demo uses Angular, the only dependency is rxjs, so React, Vue or plain old JS is completely compatible
• Fully compatible with Webpack worker-plugin
  • Therefore compatible with Angular webworker bundling which uses this
• Class interface based worker creation (should be familiar API for Angular developers)
• Unopinionated on stream switching behavior, feel free to use mergeMap, switchMap or exhaustMap in your worker if the input stream contains multiple items that generate their own stream of results
• Built in interfaces for handling Transferable parts of message payloads so large binaries can transferred efficiently without copying - See Transferable section for usage
• Automatic destruction of worker on unsubscription of output stream, this allows for smart cancelling of computation using switchMap operator, or parallelisation of computation with mergeMap
• Worker Pool strategy - maximise the throughput of units of work by utilising all cores on the host machine

Demo

https://cloudnc.github.io/observable-webworker

Tutorial

https://dev.to/zakhenry/observable-webworkers-with-angular-8-4k6

Articles

• Observable Web Workers, a deep dive into a realistic use case
• Parallel computation in the browser with observable webworkers

Install

Install the npm package: observable-webworker

# with npm
npm install observable-webworker
# or with yarn
yarn add observable-webworker

Usage

Quickstart

Main Thread

💡 Take note! The webworker construction syntax differs for different version of webpack:

Webpack < 5 (deprecated)

// src/readme/hello-legacy-webpack.ts

import { fromWorker } from 'observable-webworker';
import { of } from 'rxjs';

const input$ = of('Hello from main thread');

fromWorker<string, string>(() => new Worker('./hello.worker', { type: 'module' }), input$).subscribe(message => {
  console.log(message); // Outputs 'Hello from webworker'
});

Webpack 5

// src/readme/hello.ts#L2-L12

import { fromWorker } from 'observable-webworker';
import { of } from 'rxjs';

const input$ = of('Hello from main thread');

fromWorker<string, string>(
  () => new Worker(new URL('./hello.worker', import.meta.url), { type: 'module' }),
  input$,
).subscribe(message => {
  console.log(message); // Outputs 'Hello from webworker'
});

Worker Thread

// src/readme/hello.worker.ts

import { DoWork, runWorker } from 'observable-webworker';
import { Observable } from 'rxjs';
import { map } from 'rxjs/operators';

export class HelloWorker implements DoWork<string, string> {
  public work(input$: Observable<string>): Observable<string> {
    return input$.pipe(
      map(message => {
        console.log(message); // outputs 'Hello from main thread'
        return `Hello from webworker`;
      }),
    );
  }
}

runWorker(HelloWorker);

Decorator deprecation notice

Future versions of webpack (Webpack 5) minify webworkers overly aggressively, causing the @ObservableWorker() decorator pattern to no longer function. This decorator has now been deprecated, and will be removed in the next major version of this library.

To migrate from decorators, simply remove the decorator, and invoke the runWorker with your class passed as argument (see example above).

Make sure you don't forget to remove the decorator when you add the runWorker(...) function, otherwise the worker will be run twice, each acting on any message sent.

Transferable

If either your input or output (or both!) streams are passing large messages to or from the worker, it is highly recommended to use message types that implement the Transferable interface (ArrayBuffer, MessagePort, ImageBitmap).

Bear in mind that when transferring a message to a webworker that the main thread relinquishes ownership of the data.

Recommended reading:

• https://developer.mozilla.org/en-US/docs/Web/API/Transferable
• https://developer.mozilla.org/en-US/docs/Web/API/Worker/postMessage

To use Transferables with observable-worker, a slightly more complex interface is provided for both sides of the main/worker thread.

If the main thread is transferring Transferables to the worker, simply add a callback to the fromWorker function call to select which elements of the input stream are transferable.

// src/readme/transferable.main.ts#L7-L11

return fromWorker<ArrayBuffer, string>(
  () => new Worker(new URL('./transferable.worker', import.meta.url), { type: 'module' }),
  input$,
  input => [input],
);

If the worker is transferring Transferables to the main thread simply implement DoTransferableWork, which will require you to add an additional method selectTransferables?(output: O): Transferable[]; which you implement to select which elements of the output object are Transferable.

Both strategies are compatible with each other, so if for example you're computing the hash of a large ArrayBuffer in a worker, you would only need to use add the transferable selector callback in the main thread in order to mark the ArrayBuffer as being transferable in the input. The library will handle the rest, and you can just use DoWork in the worker thread, as the return type string is not Transferable.

Worker Pool Strategy

If you have a large amount of work that needs to be done, you can use the fromWorkerPool function to automatically manage a pool of workers to allow true concurrency of work, distributed evenly across all available cores.

The worker pool strategy has the following features

• Work can be provided as either Observable, Array, or Iterable
• Concurrency is limited to navigation.hardwareConcurrency - 1 to keep the main core free.
  • This is a configurable option if you know you already have other workers running
• Workers are only created when there is need for them (work is available)
• Workers are terminated when there is no more work, freeing up threads for other processes
  • for Observable, work is considered remaining while the observable is not completed
  • for Array, work remains while there are items in the array
  • for Iterable, work remains while the iterator is not result.done
• Workers are kept running while work remains, preventing unnecessary downloading of the worker script
• Custom observable flattening operator can be passed, allowing for custom behaviour such as correlating the output order with input order

  • default operator is mergeAll(), which means the output from the webworker(s) is output as soon as available

Example

In this simple example, we have a function that receives an array of files and returns an observable of the MD5 sum hashes of those files. For simplicity, we're passing the primitives back and forth, however in reality you are likely to want to construct your own interface to define the messages being passed to and from the worker.

Main Thread

// src/readme/worker-pool.main.ts

import { Observable } from 'rxjs';
import { fromWorkerPool } from 'observable-webworker';

export function computeHashes(files: File[]): Observable<string> {
  return fromWorkerPool<File, string>(
    () => new Worker(new URL('./worker-pool-hash.worker', import.meta.url), { type: 'module' }),
    files,
  );
}

Worker Thread

// src/readme/worker-pool-hash.worker.ts

import * as md5 from 'js-md5';
import { DoWorkUnit, runWorker } from 'observable-webworker';
import { Observable } from 'rxjs';
import { map } from 'rxjs/operators';

export class WorkerPoolHashWorker implements DoWorkUnit<File, string> {
  public workUnit(input: File): Observable<string> {
    return this.readFileAsArrayBuffer(input).pipe(map(arrayBuffer => md5(arrayBuffer)));
  }

  private readFileAsArrayBuffer(blob: Blob): Observable<ArrayBuffer> {
    return new Observable(observer => {
      if (!(blob instanceof Blob)) {
        observer.error(new Error('`blob` must be an instance of File or Blob.'));
        return;
      }

      const reader = new FileReader();

      reader.onerror = err => observer.error(err);
      reader.onload = () => observer.next(reader.result as ArrayBuffer);
      reader.onloadend = () => observer.complete();

      reader.readAsArrayBuffer(blob);

      return () => reader.abort();
    });
  }
}

runWorker(WorkerPoolHashWorker);

Note here that the worker class implements DoWorkUnit<File, string>. This is different to before where we implemented DoWork which had the slightly more complex signature of inputting an observable and outputting one.

If using the fromWorkerPool strategy, you must only implement DoWorkUnit as it relies on the completion of the returned observable to indicate that the unit of work is finished processing, and the next unit of work can be transferred to the worker.

Commonly, a worker that implements DoWorkUnit only needs to return a single value, so you may instead return a Promise from the workUnit method.

// src/app/doc/async-work.worker.ts#L7-L14

export class FactorizationWorker implements DoWorkUnit<number, number[]> {
  public async workUnit(input: number): Promise<number[]> {
    return factorize(input);
  }
}

runWorker(FactorizationWorker);

Are you using observable-webworker? Tell us about it! We'd love to hear about the weird and wonderful ways developers are working with streaming workers.