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parallel-webpack - Building multi-configs in parallel

parallel-webpack allows you to run multiple webpack builds in parallel, spreading the work across your processors and thus helping to significantly speed up your build. For us at trivago it has reduced the build from 16 minutes to just 2 minutes - for 32 variants. That performance improvement naturally comes at the expense of utilizing all available CPU cores.


npm install parallel-webpack --save-dev

You can choose whether to install parallel-webpack globally or locally. At trivago, we keep our build tools locally to the project so that we have full control over its versions.

Basic example

Given a webpack.config.js like this:

var path = require('path');
module.exports = [{
    entry: './pageA.js',
    output: {
        path: path.resolve(__dirname, './dist'),
        filename: 'pageA.bundle.js'
}, {
    entry: './pageB.js',
    output: {
        path: path.resolve(__dirname, './dist'),
        filename: 'pageB.bundle.js'

parallel-webpack will run both specified builds in parallel.

Variants example

Sometimes, just using different configurations like above won't be enough and what you really want or need is the same configuration with some adjustments. parallel-webpack can help you with generating those configuration variants as well.

var createVariants = require('parallel-webpack').createVariants;

// Those options will be mixed into every variant
// and passed to the `createConfig` callback.
var baseOptions = {
    preferredDevTool: process.env.DEVTOOL || 'eval'

// This object defines the potential option variants
// the key of the object is used as the option name, its value must be an array
// which contains all potential values of your build.
var variants = {
    minified: [true, false],
    debug: [true, false],
    target: ['commonjs2', 'var', 'umd', 'amd']

function createConfig(options) {
    var plugins = [
        new webpack.optimize.DedupePlugin(),
        new webpack.optimize.OccurenceOrderPlugin(),
        new webpack.DefinePlugin({
            DEBUG: JSON.stringify(JSON.parse(options.debug))
    if(options.minified) {
        plugins.push(new webpack.optimize.UglifyJsPlugin({
            sourceMap: false,
            compress: {
                warnings: false
    return {
        entry: './index.js',
        devtool: options.preferredDevTool,
        output: {
            path: './dist/',
            filename: 'MyLib.' +
                (options.minified ? '.min' : '') +
                (options.debug ? '.debug' : '')
                + '.js',
        plugins: plugins

module.exports = createVariants(baseOptions, variants, createConfig);

The above configuration will create 16 variations of the build for you, which parallel-webpack will distribute among your processors for building.

[WEBPACK] Building 16 targets in parallel
[WEBPACK] Started building MyLib.umd.js
[WEBPACK] Started building MyLib.umd.min.js
[WEBPACK] Started building MyLib.umd.debug.js
[WEBPACK] Started building MyLib.umd.min.debug.js

[WEBPACK] Started building MyLib.amd.js
[WEBPACK] Started building MyLib.amd.min.js
[WEBPACK] Started building MyLib.amd.debug.js
[WEBPACK] Started building MyLib.amd.min.debug.js

[WEBPACK] Started building MyLib.commonjs2.js
[WEBPACK] Started building MyLib.commonjs2.min.js
[WEBPACK] Started building MyLib.commonjs2.debug.js
[WEBPACK] Started building MyLib.commonjs2.min.debug.js

[WEBPACK] Started building MyLib.var.js
[WEBPACK] Started building MyLib.var.min.js
[WEBPACK] Started building MyLib.var.debug.js
[WEBPACK] Started building MyLib.var.min.debug.js

Running the watcher

One of the features that made webpack so popular is certainly its watcher which continously rebuilds your application.

When using parallel-webpack, you can easily use the same feature as well by specifying the --watch option on the command line:

parallel-webpack --watch

Specifying retry limits

As a side-effect of using parallel-webpack, an error will no longer lead to you having to restart webpack. Instead, parallel-webpack will keep retrying to build your application until you've fixed the problem.

While that is highly useful for development it can be a nightmare for CI builds. Thus, when building with parallel-webpack in a CI context, you should consider to use the --max-retries (or -m option) to force parallel-webpack to give up on your build after a certain amount of retries:

parallel-webpack --max-retries=3

Specifying the configuration file

When you need to use a configuration file that is not webpack.config.js, you can specify its name using the --config parameter:

parallel-webpack --config=myapp.webpack.config.js

Switch off statistics (improves performance)

While the statistics generated by Webpack are very usually very useful, they also take time to generate and print and create a lot of visual overload if you don't actually need them.

Since version 1.3.0, generating them can be turned off:

parallel-webpack --no-stats

Limiting parallelism

Under certain circumstances you might not want parallel-webpack to use all of your available CPUs for building your assets. In those cases, you can specify the parallel, or p for short, option to tell parallel-webpack how many CPUs it may use.

parallel-webpack -p=2

Configurable configuration

Sometimes, you might want to access command line arguments within your webpack.config.js in order to create a more specific configuration.

parallel-webpack will forward every parameter specified after -- to the configuration as is:

parallel-webpack -- --app=trivago

Within webpack.config.js:

// => [ 'node', 'parallel-webpack', '--app=trivago' ]

parallel-webpack adds the first two values to process.argv to ensure that there are no differences between various ways of invoking the webpack.config.js.

Node.js API

Just like webpack, you can also use parallel-webpack as an API from node.js (You can specify any other option used in worker-farm):

var run = require('parallel-webpack').run,
    configPath = require.resolve('./webpack.config.js');

run(configPath, {
    watch: false,
    maxRetries: 1,
    stats: true, // defaults to false
    maxConcurrentWorkers: 2 // use 2 workers

You can pass a notify callback as well.

var run = require('parallel-webpack').run,
    configPath = require.resolve('./webpack.config.js'),
    options = {/*...*/};

function notify() {
// do things

run(configPath, options, notify);

NOTE: In watch mode notify callback provided with Node.js API will run only once when all of the builds are finished.


createVariants(baseConfig: Object, variants: Object, configCallback: Function): Object[]

Alters the given baseConfig with all possible variants and maps the result into a valid webpack configuration using the given configCallback.

createVariants(variants: Object, configCallback: Function): Object[]

Creates all possible variations as specified in the variants object and maps the result into a valid webpack configuration using the given configCallback.

createVariants(baseConfig: Object, variants: Object): Object[]

Alters the given baseConfig with all possible variants and returns it.

createVariants(variants: Object): Object[]

Creates all possible variations from the given variants and returns them as a flat array.