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Multithreading and worker thread pool in Dart to offload CPU-bound and heavy I/O tasks to Isolate or Web Worker threads.

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

Summary #

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. Tasks posted to the worker pool may be cancelled.

Flutter Demo #

A demo is available from GitHub: squadron_sample.

It provides a Flutter App running on native and browser platforms, showcasing Squadron integration with Flutter.

Getting Started #

Import squadron from your pubspec.yaml file:

dependencies:
   squadron: ^3.1.0

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. The service inherits from WorkerService and must implement the operations map. This collection is essentially a dispatcher used to map service command ids with actual command handlers. Squadron uses this map in platform workers to serve worker requests. The command handlers provided in this map are responsible for retrieving arguments from the WorkerRequest message and providing them to the service method.

class SampleService implements WorkerService {
  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
  @override
  Map<int, CommandHandler> get operations => {
    ioCommand: (WorkerRequest r) => io(milliseconds: r.args[0]),
    cpuCommand: (WorkerRequest r) => cpu(milliseconds: r.args[0]),
  };
}

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]);
}

The platform worker's main program can be implemented using the run() function provided by Squadron 3. The first argument passed to this function is a WorkerService initializer responsible for creating the service to be used by the platform worker. This function will be passed the first WorkerRequest to set up the service. The second argument passed to run() is optional for Web applications, but required in native scenarios where it must be set to the data passed to the Isolate's main program.

  • native implementation:
SampleWorker createVmSampleWorker() => SampleWorker(_main);

void _main(Map command) => run((startRequest) => SampleService(), command);
  • browser implementation:
SampleWorker createJsSampleWorker() => SampleWorker('sample_worker_js.dart.js');

void main() => run((startRequest) => SampleService());

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

    var pool = WorkerPool(createVmSampleWorker, maxWorkers: 4, maxParallel: 2); /* native version */
    // var pool = WorkerPool(createJsSampleWorker, maxWorkers: 4, maxParallel: 2); /* browser version */
    await pool.start();

    var n = 42;
    var cpuResult = await pool.execute((w) => w.cpu(milliseconds: n));
    var ioResult = await pool.execute((w) => w.io(milliseconds: 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...), List/Map of primitive types and some specific types like SendPort 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 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, WorkerService {
  @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 and TTL
  }

  @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 implements WorkerService {
  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;

  @override
  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;
}

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

To create a CacheClient from within the platform worker, the CacheWorker's Channel must be somehow passed to the OtherWorker. 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 instances and the CacheService Singleton.

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

void _main(Map command) {
  run((startRequest) {
    final cacheEndPoint = startRequest.args.isEmpty
        ? null
        : Channel.deserialize(startRequest.args[0]);
    Cache? cache = (cacheEndPoint == null) ? null : CacheClient(cacheEndPoint);
    return OtherService(cache);
  }, command);
}

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

Task Cancellation #

Tasks registered with the worker pool may be cancelled by calling pool.cancel(). A CancelledException will be raised (for value tasks: the future completes with an error) or emitted (for streaming tasks: the stream will emit an error) for each cancelled task. Tasks still pending will fail immediately; tasks already executing when the cancel() method is called will either complete (value task) or emit an exception (streaming tasks).

It should be noted that implementations relying on pool.cancel() will not notify platform workers about the cancellation. Tasks that have been assigned to a platform worker will continue executing until they complete. As a result, a value task already executing cannot be cancelled this way: it will complete and return a value. The situation is slightly different for a streaming task: while it will report cancellation in the main event loop, streaming will continue in the platform worker's event loop.

  final future = pool.execute((w) => w.computeData());
  final stream = pool.stream((w) => w.streamData());

  // no async suspension means Squadron could not schedule any task
  // as a result this cancellation request will cancel both tasks
  pool.cancel();

  stream.listen(
    (value) => print('received value: $value'),   // will not be called
    onError: (e) => print('received error: $e')); // receives a CancelledException

  final result = await future; // throws a CancelledException

  final future = pool.execute((w) => w.computeData());
  final stream = pool.stream((w) => w.streamData());

  // asynchronous suspension gives Squadron a chance to schedule some tasks
  Future(() => null);

  // depending on concurrency settings, this cancellation request should cancel the stream task
  // however the value task should have been scheduled and should complete
  pool.cancel(); 

  stream.listen(
    (value) => print('received value: $value'),   // may be called for a few values
    onError: (e) => print('received error: $e')); // will receive a CancelledException

  final result = await future; // should get the task's result

It is also possible to schedule and cancel individual tasks, eg.:

  final valueTask = pool.scheduleTask((w) => w.computeData());
  final streamTask = pool.scheduleStream((w) => w.streamData());

  // no async suspension means Squadron could not schedule any task

  streamTask.cancel(); // or pool.cancel(streamTask)
  streamTask.stream.listen(
    (value) => print('received value: $value'),   // will not be called
    onError: (e) => print('received error: $e')); // receives a CancelledException

  valueTask.cancel(); // or pool.cancel(valueTask)
  final result = await valueTask.value; // throws a CancelledException

  final valueTask = pool.scheduleTask((w) => w.computeData());
  final streamTask = pool.scheduleStream((w) => w.streamData());

  // asynchronous suspension gives Squadron a chance to schedule some tasks
  Future(() => null);

  streamTask.cancel(); // or pool.cancel(streamTask)
  streamTask.stream.listen(
    (value) => print('received value: $value'),   // may be called for a few values
    onError: (e) => print('received error: $e')); // will receive a CancelledException

  valueTask.cancel(); // or pool.cancel(valueTask)
  final result = await valueTask.value;  // should get the task's result

Cancellation Tokens #

To notify workers of the cancellation, Squadron 3 provides the generic CancellationToken class implementing the base functionality for cancellation tokens. To ensure workers are notified of a token's cancellation, the token must be provided to the worker. Cancellation notifications are posted to workers regardless of the maxParallel concurrency settings. It is the responsibility of the worker service (your code) to verify the token status and abort processing when requested. To ensure the cancellation can be processed, you code must therefore be asynchronous. If the service is essentially CPU-bound, this can be achieved by awaiting a simple Future(() {}).

When a token is cancelled, all tasks associated with the token will be cancelled. A CancelledException will be thrown and the code that started the worker task will receive it.

Squadron provides 3 kinds of cancellation tokens:

  • CancellableToken: a cancellation token that can be triggered programmatically by calling CancellableToken.cancel().
  • TimeOutToken: a cancellation token that will be cancelled automatically after a given duration. Timeout countdown starts then the task is effectively posted to a worker.
  • CompositeToken: a cancellation token monitoring several other tokens. A CompositeToken will be cancelled automatically upon cancellation of one of the tokens (CompositeMode.any) or all of them (CompositeMode.all).

Example:

class SampleService implements WorkerService {
  Future io({required int milliseconds, CancellationToken? token}) async {
    if (token?.cancelled ?? false) throw CancelledException();
    await Future.delayed(Duration(milliseconds: milliseconds));
  }

  Future cpu({required int milliseconds, CancellationToken? token}) async {
    final sw = Stopwatch()..start();
    var count = 0;
    while (sw.elapsedMilliseconds < milliseconds) {
      if (count % 1000 == 0) {
        // avoid flooding the event loop with noop futures, check every 1000 iterations only
        if (token?.cancelled ?? false) throw CancelledException();
        await Future(() {});
      }
      count++;
    }
  }

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

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

  // create a token
  final token = CancellableToken();
  // trigger cancellation after 500 ms
  // in real world, token.cancel() would be called in reaction to an event such as a user action for instance
  // this is similar to a timeout token except that countdown starts immediately
  Timer(Duration(milliseconds: 500), token.cancel);

  // start a computation lasting 1000 ms
  // pass the token to the service + the pool
  // a CancelledException will be thrown after 500 ms
  await pool.execute((w) => w.cpu(milliseconds: 1000, token));

Notes:

  • the token received by workers will not have the same runtime type as the token passed to the worker. The service code should not make any assumption on the token's runtime type and should only manipulate generic CancellationToken objects.
  • the same cancellation token may be used to control cancellation of several tasks.
  • when using a TimeOutToken, the timeout countdown will only start when the task is effectively posted to the worker for execution. If the token is shared across several tasks, the countdown starts with the first worker and applies to all workers.

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 concurrency setting.

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

  // stops all workers
  pool.stop();

All workers will be stopped as soon as all tasks registered with the pool have been processed. Of course, it is possible to cancel pending tasks before stopping the worker pool.

  // cancels pending tasks and stops all workers
  pool.cancel();
  pool.stop();

The pool will not accept new tasks unless it is restarted with pool.start().

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Multithreading and worker thread pool in Dart to offload CPU-bound and heavy I/O tasks to Isolate or Web Worker threads.

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