josdejong
workerpool
workerpool offers an easy way to create a pool of workers for both dynamically offloading computations as well as managing a pool of dedicated workers. workerpool basically implements a thread pool pattern. There is a pool of workers to execute tasks. New tasks are put in a queue. A worker executes one task at a time, and once finished, picks a new task from the queue. Workers can be accessed via a natural, promise based proxy, as if they are available straight in the main application.
workerpool runs on Node.js and in the browser.
Features
- Easy to use
- Runs in the browser and on node.js
- Dynamically offload functions to a worker
- Access workers via a proxy
- Cancel running tasks
- Set a timeout on tasks
- Handles crashed workers
- Small: 7 kB minified and gzipped
- Supports transferable objects (only for web workers and worker_threads)
Why
JavaScript is based upon a single event loop which handles one event at a time. Jeremy Epstein explains this clearly:
In Node.js everything runs in parallel, except your code. What this means is that all I/O code that you write in Node.js is non-blocking, while (conversely) all non-I/O code that you write in Node.js is blocking.
This means that CPU heavy tasks will block other tasks from being executed. In case of a browser environment, the browser will not react to user events like a mouse click while executing a CPU intensive task (the browser "hangs"). In case of a node.js server, the server will not respond to any new request while executing a single, heavy request.
For front-end processes, this is not a desired situation. Therefore, CPU intensive tasks should be offloaded from the main event loop onto dedicated workers. In a browser environment, Web Workers can be used. In node.js, child processes and worker_threads are available. An application should be split in separate, decoupled parts, which can run independent of each other in a parallelized way. Effectively, this results in an architecture which achieves concurrency by means of isolated processes and message passing.
Install
Install via npm:
npm install workerpoolLoad
To load workerpool in a node.js application (both main application as well as workers):
const workerpool = require('workerpool');To load workerpool in the browser:
<script src="workerpool.js"></script>To load workerpool in a web worker in the browser:
importScripts('workerpool.js');Setting up the workerpool with React or webpack5 requires additional configuration steps, as outlined in the webpack5 section.
Use
Offload functions dynamically
In the following example there is a function add, which is offloaded dynamically to a worker to be executed for a given set of arguments.
myApp.js
const workerpool = require('workerpool');
const pool = workerpool.pool();
function add(a, b) {
return a + b;
}
pool
.exec(add, [3, 4])
.then(function (result) {
console.log('result', result); // outputs 7
})
.catch(function (err) {
console.error(err);
})
.then(function () {
pool.terminate(); // terminate all workers when done
});Note that both function and arguments must be static and stringifiable, as they need to be sent to the worker in a serialized form. In case of large functions or function arguments, the overhead of sending the data to the worker can be significant.
Dedicated workers
A dedicated worker can be created in a separate script, and then used via a worker pool.
myWorker.js
const workerpool = require('workerpool');
// a deliberately inefficient implementation of the fibonacci sequence
function fibonacci(n) {
if (n < 2) return n;
return fibonacci(n - 2) + fibonacci(n - 1);
}
// create a worker and register public functions
workerpool.worker({
fibonacci: fibonacci,
});This worker can be used by a worker pool:
myApp.js
const workerpool = require('workerpool');
// create a worker pool using an external worker script
const pool = workerpool.pool(__dirname + '/myWorker.js');
// run registered functions on the worker via exec
pool
.exec('fibonacci', [10])
.then(function (result) {
console.log('Result: ' + result); // outputs 55
})
.catch(function (err) {
console.error(err);
})
.then(function () {
pool.terminate(); // terminate all workers when done
});
// or run registered functions on the worker via a proxy:
pool
.proxy()
.then(function (worker) {
return worker.fibonacci(10);
})
.then(function (result) {
console.log('Result: ' + result); // outputs 55
})
.catch(function (err) {
console.error(err);
})
.then(function () {
pool.terminate(); // terminate all workers when done
});Worker can also initialize asynchronously:
myAsyncWorker.js
define(['workerpool/dist/workerpool'], function (workerpool) {
// a deliberately inefficient implementation of the fibonacci sequence
function fibonacci(n) {
if (n < 2) return n;
return fibonacci(n - 2) + fibonacci(n - 1);
}
// create a worker and register public functions
workerpool.worker({
fibonacci: fibonacci,
});
});Examples
Examples are available in the examples directory:
https://github.com/josdejong/workerpool/tree/master/examples
API
The API of workerpool consists of two parts: a function workerpool.pool to create a worker pool, and a function workerpool.worker to create a worker.
pool
A workerpool can be created using the function workerpool.pool:
workerpool.pool([script: string] [, options: Object]) : Pool
When a script argument is provided, the provided script will be started as a dedicated worker. When no script argument is provided, a default worker is started which can be used to offload functions dynamically via Pool.exec. Note that on node.js, script must be an absolute file path like __dirname + '/myWorker.js'. In a browser environment, script can also be a data URL like 'data:application/javascript;base64,...'. This allows embedding the bundled code of a worker in your main application. See examples/embeddedWorker for a demo.
The following options are available:
minWorkers: number | 'max'. The minimum number of workers that must be initialized and kept available. Setting this to'max'will createmaxWorkersdefault workers (see below).maxWorkers: number. The default number of maxWorkers is the number of CPU's minus one. When the number of CPU's could not be determined (for example in older browsers),maxWorkersis set to 3.maxQueueSize: number. The maximum number of tasks allowed to be queued. Can be used to prevent running out of memory. If the maximum is exceeded, adding a new task will throw an error. The default value isInfinity.workerType: 'auto' | 'web' | 'process' | 'thread'.- In case of
'auto'(default), workerpool will automatically pick a suitable type of worker: when in a browser environment,'web'will be used. When in a node.js environment,worker_threadswill be used if available (Node.js >= 11.7.0), elsechild_processwill be used. - In case of
'web', a Web Worker will be used. Only available in a browser environment. - In case of
'process',child_processwill be used. Only available in a node.js environment. - In case of
'thread',worker_threadswill be used. Ifworker_threadsare not available, an error is thrown. Only available in a node.js environment.
- In case of
workerTerminateTimeout: number. The timeout in milliseconds to wait for a worker to cleanup it's resources on termination before stopping it forcefully. Default value is1000.forkArgs: String[]. Forprocessworker type. An array passed asargsto child_process.forkforkOpts: Object. Forprocessworker type. An object passed asoptionsto child_process.fork. See nodejs documentation for available options.workerOpts: Object. Forwebworker type. An object passed to the constructor of the web worker. See WorkerOptions specification for available options.workerThreadOpts: Object. Forworkerworker type. An object passed to worker_threads.options. See nodejs documentation for available options.onCreateWorker: Function. A callback that is called whenever a worker is being created. It can be used to allocate resources for each worker for example. The callback is passed as argument an object with the following properties:forkArgs: String[]: theforkArgsoption of this poolforkOpts: Object: theforkOptsoption of this poolworkerOpts: Object: theworkerOptsoption of this poolscript: string: thescriptoption of this pool Optionally, this callback can return an object containing one or more of the above properties. The provided properties will be used to override the Pool properties for the worker being created.
onTerminateWorker: Function. A callback that is called whenever a worker is being terminated. It can be used to release resources that might have been allocated for this specific worker. The callback is passed as argument an object as described foronCreateWorker, with each property sets with the value for the worker being terminated.emitStdStreams: boolean. Forprocessorthreadworker type. Iftrue, the worker will emitstdoutandstderrevents instead of passing it through to the parent streams. Default value isfalse.
Important note on
'workerType': when sending and receiving primitive data types (plain JSON) from and to a worker, the different worker types ('web','process','thread') can be used interchangeably. However, when using more advanced data types like buffers, the API and returned results can vary. In these cases, it is best not to use the'auto'setting but have a fixed'workerType'and good unit testing in place.
A worker pool contains the following functions:
Pool.exec(method: Function | string, params: Array | null [, options: Object]) : Promise<any, Error>
Execute a function on a worker with given arguments.- When
methodis a string, a method with this name must exist at the worker and must be registered to make it accessible via the pool. The function will be executed on the worker with given parameters. - When
methodis a function, the provided functionfnwill be stringified, send to the worker, and executed there with the provided parameters. The provided function must be static, it must not depend on variables in a surrounding scope. - The following options are available:
- When
Pool.proxy() : Promise<Object, Error>
Create a proxy for the worker pool. The proxy contains a proxy for all methods available on the worker. All methods return promises resolving the methods result.Pool.stats() : Object
Retrieve statistics on workers, and active and pending tasks.Returns an object containing the following properties:
{ totalWorkers: 0, busyWorkers: 0, idleWorkers: 0, pendingTasks: 0, activeTasks: 0 }Pool.terminate([force: boolean [, timeout: number]]) : Promise<void, Error>If parameter
forceis false (default), workers will finish the tasks they are working on before terminating themselves. Any pending tasks will be rejected with an error 'Pool terminated'. Whenforceis true, all workers are terminated immediately without finishing running tasks. Iftimeoutis provided, worker will be forced to terminate when the timeout expires and the worker has not finished.
The function Pool.exec and the proxy functions all return a Promise. The promise has the following functions available:
Promise.then(fn: Function<result: any>) : Promise<any, Error>
Get the result of the promise once resolve.Promise.catch(fn: Function<error: Error>) : Promise<any, Error>
Get the error of the promise when rejected.Promise.finally(fn: Function<void>)
Logic to run when the Promise eitherresolvesorrejectsPromise.cancel() : Promise<any, Error>
A running task can be cancelled. The worker executing the task is enforced to terminate immediately. The promise will be rejected with aPromise.CancellationError.Promise.timeout(delay: number) : Promise<any, Error>
Cancel a running task when it is not resolved or rejected within given delay in milliseconds. The timer will start when the task is actually started, not when the task is created and queued. The worker executing the task is enforced to terminate immediately. The promise will be rejected with aPromise.TimeoutError.
Example usage:
const workerpool = require('workerpool');
function add(a, b) {
return a + b;
}
const pool1 = workerpool.pool();
// offload a function to a worker
pool1
.exec(add, [2, 4])
.then(function (result) {
console.log(result); // will output 6
})
.catch(function (err) {
console.error(err);
});
// create a dedicated worker
const pool2 = workerpool.pool(__dirname + '/myWorker.js');
// supposed myWorker.js contains a function 'fibonacci'
pool2
.exec('fibonacci', [10])
.then(function (result) {
console.log(result); // will output 55
})
.catch(function (err) {
console.error(err);
});
// send a transferable object to the worker
// supposed myWorker.js contains a function 'sum'
const toTransfer = new Uint8Array(2).map((_v, i) => i)
pool2
.exec('sum', [toTransfer], { transfer: [toTransfer.buffer] })
.then(function (result) {
console.log(result); // will output 3
})
.catch(function (err) {
console.error(err);
});
// create a proxy to myWorker.js
pool2
.proxy()
.then(function (myWorker) {
return myWorker.fibonacci(10);
})
.then(function (result) {
console.log(result); // will output 55
})
.catch(function (err) {
console.error(err);
});
// create a pool with a specified maximum number of workers
const pool3 = workerpool.pool({ maxWorkers: 7 });worker
A worker is constructed as:
workerpool.worker([methods: Object<String, Function>] [, options: Object]) : void
Argument methods is optional and can be an object with functions available in the worker. Registered functions will be available via the worker pool.
The following options are available:
onTerminate: ([code: number]) => Promise<void> | void. A callback that is called whenever a worker is being terminated. It can be used to release resources that might have been allocated for this specific worker. The difference with pool'sonTerminateWorkeris that this callback runs in the worker context, whileonTerminateWorkeris executed on the main thread.
Example usage:
// file myWorker.js
const workerpool = require('workerpool');
function add(a, b) {
return a + b;
}
function multiply(a, b) {
return a * b;
}
// create a worker and register functions
workerpool.worker({
add: add,
multiply: multiply,
});Asynchronous results can be handled by returning a Promise from a function in the worker:
// file myWorker.js
const workerpool = require('workerpool');
function timeout(delay) {
return new Promise(function (resolve, reject) {
setTimeout(resolve, delay);
});
}
// create a worker and register functions
workerpool.worker({
timeout: timeout,
});Transferable objects can be sent back to the pool using Transfer helper class:
// file myWorker.js
const workerpool = require('workerpool');
function array(size) {
var array = new Uint8Array(size).map((_v, i) => i);
return new workerpool.Transfer(array, [array.buffer]);
}
// create a worker and register functions
workerpool.worker({
array: array,
});Tasks may configure an abort handler to perform cleanup operations when timeout or cancel is called on a task. the abortListenerTimeout option can be configured to control when cleanup should be aborted in the case an abortHandler never resolves. This timeout trigger will cause the given worker to be cleaned up. Allowing a new worker to be created if need be.
function asyncTimeout() {
var me = this;
return new Promise(function (resolve) {
let timeout = setTimeout(() => {
resolve();
}, 5000);
// An abort listener allows for cleanup for a given worker
// such that it may be resused for future tasks
// if an execption is thrown within scope of the handler
// the worker instance will be destroyed.
me.worker.addAbortListener(async function () {
clearTimeout(timeout);
resolve();
});
});
}
// create a worker and register public functions
workerpool.worker(
{
asyncTimeout: asyncTimeout,
},
{
abortListenerTimeout: 1000
}
);Events
You can send data back from workers to the pool while the task is being executed using the workerEmit function:
workerEmit(payload: any) : unknown
This function only works inside a worker and during a task.
Example:
// file myWorker.js
const workerpool = require('workerpool');
function eventExample(delay) {
workerpool.workerEmit({
status: 'in_progress',
});
workerpool.workerEmit({
status: 'complete',
});
return true;
}
// create a worker and register functions
workerpool.worker({
eventExample: eventExample,
});To receive those events, you can use the on option of the pool exec method:
pool.exec('eventExample', [], {
on: function (payload) {
if (payload.status === 'in_progress') {
console.log('In progress...');
} else if (payload.status === 'complete') {
console.log('Done!');
}
},
});Utilities
Following properties are available for convenience:
- platform: The Javascript platform. Either node or browser
- isMainThread: Whether the code is running in main thread or not (Workers)
- cpus: The number of CPUs/cores available
Roadmap
- Implement functions for parallel processing:
map,reduce,forEach,filter,some,every, ... - Implement graceful degradation on old browsers not supporting webworkers: fallback to processing tasks in the main application.
- Implement session support: be able to handle a series of related tasks by a single worker, which can keep a state for the session.
Related libraries
- https://github.com/andywer/threads.js
- https://github.com/piscinajs/piscina
- https://github.com/learnboost/cluster
- https://github.com/adambom/parallel.js
- https://github.com/padolsey/operative
- https://github.com/calvinmetcalf/catiline
- https://github.com/Unitech/pm2
- https://github.com/godaddy/node-cluster-service
- https://github.com/ramesaliyev/EasyWebWorker
- https://github.com/rvagg/node-worker-farm
Build
First clone the project from github:
git clone git://github.com/josdejong/workerpool.git
cd workerpoolInstall the project dependencies:
npm installThen, the project can be build by executing the build script via npm:
npm run buildThis will build the library workerpool.js and workerpool.min.js from the source files and put them in the folder dist.
Test
To execute tests for the library, install the project dependencies once:
npm installThen, the tests can be executed:
npm testTo test code coverage of the tests:
npm run coverageTo see the coverage results, open the generated report in your browser:
./coverage/index.htmlPublish
- Describe changes in HISTORY.md.
- Update version in package.json, run
npm installto update it inpackage-lock.jsontoo. - Push to GitHub.
- Deploy to npm via
npm publish. - Add a git tag with the version number like:
git tag v1.2.3 git push --tags
License
Copyright (C) 2014-2024 Jos de Jong wjosdejong@gmail.com
Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.