workerpool
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 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.
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, Chrome, Firefox, Opera, Safari, and IE10+.
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, less than 4 kB minified and gzipped
Install
Install via npm:
npm install workerpool
Load
To load workerpool in a node.js application (both main application as well as workers):
var 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');
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
var workerpool = require('workerpool');
var pool = workerpool.pool();
function add(a, b) {
return a + b;
}
pool.exec(add, [3, 4])
.then(function (result) {
console.log('result', result);
pool.clear();
});
Note that both function and arguments must be static and stringifiable, as they need to be send 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
var workerpool = require('workerpool');
function fibonacci(n) {
if (n < 2) return n;
return fibonacci(n - 2) + fibonacci(n - 1);
}
workerpool.worker({
fibonacci: fibonacci
});
This worker can be used by a worker pool:
myApp.js
var workerpool = require('workerpool');
var pool = workerpool.pool(__dirname + '/myWorker.js');
pool.exec('fibonacci', [10])
.then(function (result) {
console.log('Result: ' + result);
pool.clear();
});
pool.proxy()
.then(function (worker) {
worker.fibonacci(10)
.then(function (result) {
console.log('Result: ' + result);
});
});
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'
.
The following options are available:
maxWorkers: number
. The default number of workers number of CPU's minus one. When the number of CPU's could not be determined (for example in older browsers), maxWorkers
is 3 by default.
A worker pool contains the following functions:
Pool.exec(method: Function | string, params: Array | null) : Promise.<*, Error>
Execute a function on a worker with given arguments.
- When
method
is 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
method
is a function, the provided function fn
will 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.
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.clear([force: boolean])
Clear all workers from the pool. If parameter force
is false (default), workers will finish the tasks they are working on before terminating themselves. When force
is true, all workers are terminated immediately without finishing running tasks.
The function Pool.exec
and the proxy functions all return a Promise
. The promise has the following functions available:
Promise.then(fn: Function.<result: *>)
Get the result of the promise once resolve.Promise.catch(fn: Function.<error: Error>)
Get the error of the promise when rejected.Promise.cancel()
A running task can be cancelled. The worker executing the task is enforced to terminate immediately.
The promise will be rejected with a Promise.CancellationError
.Promise.timeout(delay: number)
Cancel a running task when it is not resolved or rejected withing given delay in milliseconds.
The worker executing the task is enforced to terminate immediately.
The promise will be rejected with a Promise.TimeoutError
.
Example usage:
var workerpool = require('workerpool');
function add(a, b) {
return a + b;
}
var pool1 = workerpool.pool();
pool1.exec(add, [2, 4])
.then(function (result) {
console.log(result);
});
var pool2 = workerpool.pool(__dirname + '/myWorker.js');
pool2.exec('fibonacci', [10])
.then(function (result) {
console.log(result);
});
pool2.proxy()
.then(function (myWorker) {
myWorker.fibonacci(10)
.then(function (result) {
console.log(result);
});
});
var pool3 = workerpool.pool({maxWorkers: 7});
worker
A worker is constructed as:
workerpool.worker([methods: Object.<String, Function>])
Argument methods
is optional can can be an object with functions available in the worker. Registered functions will be available via the worker pool.
Example usage:
var workerpool = require('workerpool');
function add(a, b) {
return a + b;
}
function multiply(a, b) {
return a * b;
}
workerpool.worker({
add: add,
multiply: multiply
});
Asynchronous results can be handled by returning a Promise from a function in the worker:
var workerpool = require('workerpool');
function timeout(delay) {
return new Promise(function (resolve, reject) {
setTimeout(resolve, delay)
});
}
workerpool.worker({
timeout: timeout
});
Roadmap
- Implement a property
minWorkers
, to ensure a minimum number of workers
always up and running. - 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.
Sources of inspiration
Build
First clone the project from github:
git clone git://github.com/josdejong/workerpool.git
cd workerpool
Install the project dependencies:
npm install
Then, the project can be build by executing the build script via npm:
npm run build
This 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 install
Then, the tests can be executed:
npm test
To test code coverage of the tests:
npm run coverage
To see the coverage results, open the generated report in your browser:
./coverage/lcov-report/index.html
License
Copyright (C) 2014-2016 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.