squadron 2.0.0

Multithreading and worker pools in Dart to offload CPU-bound or long running tasks and give your mobile and Web apps some air.

Multithreading and worker pools in Dart to offload CPU-bound or long running tasks and give your mobile and Web apps some air.

Features

Worker class: a base worker class managing a platform thread (Isolate or Web Worker) and the communication between clients and workers.

WorkerPool<W> class: a worker pool for W workers. The number of workers is configurable as well as the degree of concurrent workloads distributed to workers in the pool.

Getting Started

Import squadron from your pubspec.yaml file:

dependencies:
   squadron: ^2.0.0

Usage

First implement a service with sync or async methods you want to expose from workers. This approach enables reusing the code in different scenarios: unit tests, direct call from your app, or wrapped in a native Isolate or in a Web Worker.

The example below implements a SampleService with a synchronous cpu() method and an asynchronous io() method.

class SampleService {
  Future io({required int milliseconds}) =>
      Future.delayed(Duration(milliseconds: milliseconds));

  void cpu({required int milliseconds}) {
    final sw = Stopwatch()..start();
    while (sw.elapsedMilliseconds < milliseconds) {/* cpu */}
  }

  // command ids
  static const ioCommand = 1;
  static const cpuCommand = 2;

  // map of command ids to implementatons
  Map<int, CommandHandler> get operations => {
    ioCommand: (WorkerRequest r) => io(milliseconds: r.args[0]),
    cpuCommand: (WorkerRequest r) => cpu(milliseconds: r.args[0]),
  };
}

The operations getter implemented in the service will be used by platform workers to dispatch incoming requests to the right method. The command handlers provided in this map are also responsible for retrieving arguments from the WorkerRequest message and providing them to the service methods.

This SampleService can easily be used as a contract to implemented the Worker. This worker can then be used for Dart Isolates as well as Web Workers.

class SampleWorker extends Worker implements SampleService {
  SampleWorker(dynamic entryPoint, {String? id, List args = const []})
      : super(entryPoint, id: id, args: args);

  @override
  Future io({required int milliseconds}) =>
      send(SampleService.ioCommand, [milliseconds]);

  @override
  Future cpu({required int milliseconds}) =>
      send(SampleService.cpuCommand, [milliseconds]);

  // not used in Workers, just return an empty map
  @override
  final Map<int, CommandHandler> operations = const {};
}

If the requirements above are met, the Isolate's main program can be implemented like this:

SampleWorker createVmSampleWorker() => SampleWorker(_main);

void _main(Map command) {
  final operations = <int, CommandHandler>{};

  final workerPort = ReceivePort();
  workerPort.listen((command) {
    final req = WorkerRequest.deserialize(command);
    if (req.terminate) {
      Isolate.current.kill(priority: Isolate.immediate);
    } else {
      Worker.process(operations, req);
    }
  });

  final startRequest = WorkerRequest.deserialize(command);
  assert(startRequest.connect == true);
  Worker.connect(startRequest.client, workerPort,
      operations: operations, serviceOperations: SampleService().operations);
}

And the Web Worker version is just as easy:

SampleWorker createSampleWorker() => SampleWorker('sample_worker_js.dart.js');

void main() {
  final scope = DedicatedWorkerGlobalScope.instance;
  final operations = <int, CommandHandler>{};

  final com = MessageChannel();
  com.port1.onMessage.listen((MessageEvent e) {
    final req = WorkerRequest.deserialize(e.data);
    if (req.terminate) {
      scope.close();
    } else {
      Worker.process(operations, req);
    }
  });

  scope.onMessage.listen((MessageEvent e) {
    final startRequest = WorkerRequest.deserialize(e.data);
    assert(startRequest.connect == true);
    Worker.connect(startRequest.client, com.port2,
        operations: operations, serviceOperations: SampleService().operations);
  });
}

Using a WorkerPool, you are now able to distribute your workloads:

    var pool = WorkerPool(() => createSampleWorker(), maxWorkers: 4, maxParallel: 2);
    await pool.start();

    var n = 42;
    var cpuResult = await pool.compute((w) => w.cpu(n));
    var ioResult = await pool.compute((w) => w.io(n));

Remarks on Isolates / Web Workers

While Isolates enable multithreading in Dart applications, several aspects must be taken into account:

• Creating an Isolate can be an expensive process.

• Isolate threads are still based on an event loop, processing asynchronous tasks/callbacks one by one.

• Isolates do not share memory; in particular, global contexts are not shared across Isolates and may have to be initialized multiple times thus increasing the application's memory footprint and startup time.

• Communicating with an Isolate invoves marshalling data in and out; theoretically, only primitive types (num, String, bool, null...) and List/Map of primitive types are supported. However, object instances may be sent across Isolates on some platforms, e.g. the Dart Native platform (instances will still be copied).

Web Workers have similar characteristics. Only primitive types and objects implementing Transferable can be sent across Web Worker boundaries.

Scaling Options

Squadron pools manage a collection of workers to avoid the cost of creating a new platform worker each time. Squadron also implements a simple load-balancing mechanism and posts new tasks to workers that are most available (i.e. those with the minimum workload, provided workload < maxParallel). Tasks in Squadron do not have a priority and will be handled in the order they were received.

Depending on the type of processing, scaling tasks with Squadron can be achieved horizontally (adding new workers) or vertically (distributing more tasks to workers).

• For pure CPU-bound tasks, e.g. image/video or other heavy data processing, increasing the maxParallel pool option is likely to NOT yield any benefit for performance. If the event loop is already busy executing a task, subsequent task requests posted to the Worker will not be handled before the executing task is complete. As a result, CPU-bound tasks will be queued and scaling CPU workloads can only be achieved by adding more workers to the pool (horizontal scaling).

• For I/O bound tasks, performance benefits are less obvious. However, if processing requires many I/O operations, offloading such tasks to a worker pool is likely to be beneficial because I/O callbacks will be registered with the platform worker's event loop. This scenario can be interesting in Web application back-ends to make the main event loop more available for other incoming requests -- simple requests e.g. CRUD operations can be fully handled by the main event loop while more complex, long-running, I/O-bound tasks will be processed off the Web app's main event loop. Scaling such tasks can be achieved by increasing the maxParallel (vertical scaling) or maxWorkers (horizontal scaling) pool options. In I/O scenarios, vertical scaling should be preferred.

Worker Cooperation

It is possible to implement some kind of worker cooperation and support more complex scenarios.

For instance, the fact that Isolates and Web Workers do not share memory means it may be cumbersome to implement a local, in-memory cache at worker level. Each worker would have their own copy of the cache, making expiration and update propagation difficult to implement.

As a workaround, it is possible to implement a cache worker as a Singleton (no pooling) and to share the worker's Channel across other workers (Channel objects can be sent across platform workers via the serialize() method).

An example is provided in cache_worker.dart. To access the cache API seamlessly, an abstract class is first defined:

abstract class Cache {
  FutureOr<dynamic> get(dynamic key);
  FutureOr<bool> containsKey(dynamic key);
  FutureOr set(dynamic key, dynamic value, {Duration? timeToLive});
  FutureOr<CacheStat> getStats();
}

A cache service is then implemented (internal details skipped, please refer to the example source code). Note that the CacheService implementation can be synchronous.

class CacheService implements Cache {
  @override
  dynamic get(dynamic key) {
    // retrieve the value associated for the specified key
    // return null if key is not in cache or if the key has expired
  }

  @override
  bool containsKey(dynamic key) {
    // use get() as it implements the expiration logic
    final data = get(key);
    return data != null;
  }

  @override
  void set(dynamic key, dynamic value, {Duration? timeToLive}) {
    // cache the value with the specified key
  }

  @override
  CacheStat getStats() {
    // return cache stats
  }

  // command ids
  static const getOperation = 1;
  static const containsOperation = 2;
  static const setOperation = 3;
  static const statsOperation = 4;

  // command handlers
  Map<int, CommandHandler> get operations => {
        getOperation: (WorkerRequest r) => get(r.args[0]),
        containsOperation: (WorkerRequest r) => containsKey(r.args[0]),
        setOperation: (WorkerRequest r) => set(r.args[0], r.args[1],
            timeToLive: (r.args[2] == null) ? null : Duration(microseconds: r.args[2])),
        statsOperation: (WorkerRequest r) => getStats().serialize()
      };
}

The CacheWorker is easy:

class CacheWorker extends Worker implements Cache {
  CacheWorker(dynamic entryPoint, {String? id, List args = const []})
      : super(entryPoint, id: id, args: args);

  @override
  Future<dynamic> get(dynamic key) => send(CacheService.getOperation, [key]);

  @override
  Future<bool> containsKey(dynamic key) =>
      send(CacheService.containsOperation, [key]);

  @override
  Future set(dynamic key, dynamic value, {Duration? timeToLive}) {
    assert(value != null); // null means not in cache; cannot store null
    return send(CacheService.setOperation, [key, value, timeToLive?.inMicroseconds]);
  }

  @override
  Future<CacheStat> getStats() async =>
      CacheStat.deserialize(await send(CacheService.statsOperation));
}

Finally, a cache client is implemented to proxy calls from other workers to the CacheWorker. This CacheClient can be constructed with a Channel that will be obtained from the CacheWorker.

class CacheClient implements Cache {
  CacheClient(this._remote);

  final Channel _remote;

  @override
  Future<dynamic> get(dynamic key) =>
      _remote.sendRequest(CacheService.getOperation, [key]);

  @override
  Future<bool> containsKey(dynamic key) =>
      _remote.sendRequest(CacheService.containsOperation, [key]);

  @override
  Future set(dynamic key, dynamic value, {Duration? timeToLive}) {
    assert(value != null); // null means not in cache; cannot store null
    return _remote.sendRequest(
        CacheService.setOperation, [key, value, timeToLive?.inMicroseconds]);
  }

  @override
  Future<CacheStat> getStats() async {
      => CacheStat.deserialize(
        await _remote.sendRequest(CacheService.statsOperation, []));
}

Note how getStats() implementations require serialization/deserialization of the CacheStat object. This is necessary to cross platform worker boundaries.

class OtherService {
  OtherService([this._cache]);

  final Cache? _cache;

  // some service method
  Future<int> compute(int n) async {
    // check cache
    int? result = await _cache?.get(n);
    if (result != null) {
      // cache hit
      return n;
    }
    // otherwise compute
    // ...
    // finally, cache (dont await) and return
    cache?.set(n, result);
    return result!;
  }

  // some command ids
  static const computeCommand = 1;

  Map<int, CommandHandler> get operations => {
    // the comand handlers for the command ids
    computeCommand: (r) => compute(r.args[0])
  };
}

The OtherWorker implementation is straightforward:

class OtherWorker extends Worker implements OtherService {
  OtherWorker(dynamic entryPoint, {String? id, List args = const []})
      : super(entryPoint, id: id, args: args);

  @override
  Future<int> compute(int n) => send<int>(OtherService.computeCommand, [n]);

  // other proxy service methods...

  @override
  final Cache? _cache = null;

  @override
  final Map<int, CommandHandler> operations = const {};
}

The platform worker assembles everything. It is essentially the same as above, with some extra initialization code to create a CacheClient.

To create this CacheClient inside the platform worker, the CacheWorker's Channel must be somehow passed to the OtherWorker upon construction. This is done using the share() and the serialize() methods provided by Channel. These methods will return an opaque object that can be safely sent across workers and deserialized to recreate a Channel, thereby bridging the gap between the OtherService and the CacheService.

OtherWorker createOtherWorker([CacheWorker? cache]) =>
    OtherWorker(_main, args: [cache?.channel?.share().serialize()]);

void _main(Map command) {
  final operations = <int, CommandHandler>{};

  final workerPort = ReceivePort();
  workerPort.listen((command) {
    final req = WorkerRequest.deserialize(command);
    if (req.terminate) {
      Isolate.current.kill(priority: Isolate.immediate);
    } else {
      Worker.process(operations, req);
    }
  });

  final startRequest = WorkerRequest.deserialize(command);
  assert(startRequest.connect == true);
  final cacheEndPoint = startRequest.args.isEmpty
      ? null
      : Channel.deserialize(startRequest.args[0]);
  Cache? cache = (cacheEndPoint == null) ? null : CacheClient(cacheEndPoint);
  Worker.connect(startRequest.client, workerPort,
      operations: operations,
      serviceOperations: OtherService(cache).operations);
}

The application's main program is responsible for:

• setting up the CacheWorker Singleton
• setting up a pool of OtherWorkers and making sure the worker factory function receives the CacheWorker Singleton.

final cacheWorker = CacheWorker();
await cacheWorker.start();

final pool = WorkerPool(() => createOtherWorker(cacheWorker), minWorkers: 2, maxWorkers: 5);
await pool.start();

Architecture Diagram

                                   +-----------------------+
                              +--> | CacheWorker singleton | <--+
                              |    +-----------------------+    |
                              |                                 |
 +-------------------+        |           +----------------+   (4)
 |  main program     | --(1)--+    +----> | otherWorker #1 |    |
 |  ---------------- |             |      | -------------- |    |
(2) OtherWorker pool | --(3)--+    |      | cacheClient #1 | ---|
 +-------------------+        |    |      +----------------+    |
                              +----|                            |
                                   |      +----------------+    |
                                   +----> | otherWorker #2 |    |
                                          | -------------- | ---+
                                          | cacheClient #2 |
                                          +----------------+

1: the main program first spawns a CacheWorker
2: the main program creates a pool of OtherWorkers, making sure the CacheWorker Singleton is advertized to OtherWorkers when they are created
3: the pool creates new OtherWorkers as workload builds up, instantiating cache clients bound to the CacheWorker's Channel
4: OtherWorkers query the CacheWorker via their local CacheClient to avoid expensive computations that have been done already 

Worker Monitoring

Monitoring workers in a pool can be done with a simple timer. For instance, to stop workers after a given idle period:

  // install worker monitor
  final timer = Timer.periodic(refreshDuration, (timer) {
    pool.stop((w) => w.idleTime > maxIdle);
  });

Please note that some idle workers may remain alive, depending on the minWorker pool option.

To kill all workers, simply call the pool's stop() method with no predicate.

  // kills all workers
  pool.stop();