karma-parallel

karma-parallel

A Karma JS plugin to support sharding tests to run in parallel across multiple browsers.

Overview

This is intended to speed up the time it takes to run unit tests by taking advantage of multiple cores. From a single karma server, multiple instances of a browser are spun up. Each browser downloads all of the spec files, but when a describe block is encountered, the browsers deterministically decide if that block should run in the given browser.

This leads to a way to split up unit tests across multiple browsers without changing any build processes.

Installation

The easiest way is to install karma-parallel as a devDependency.

Using NPM

npm install karma-parallel --save-dev

Using Yarn

yarn add karma-parallel --dev

Examples

Basic

// karma.conf.js
module.exports = function(config) {
  config.set({

    frameworks: ['mocha' /* or 'jasmine' */, 'parallel'], // this will load the framework and beforeMiddleware
    
    parallelOptions: {
      executors: 4,
      shardStrategy: 'round-robin' /* or 'description-length' */
    }
  });
};

Options

parallelOptions [object]: Options for this plugin

parallelOptions.executors [int=cpu_cores-1]: The number of browser instances to use to test. If you test on multiple types of browsers, this spin up the number of executors for each browser type.

parallelOptions.shardStyle [string='round-robin']: This plugin works by overriding the test suite describe() function. When it encounters a describe, it must decide if it will skip the tests inside of it, or not.

• The round-robin style will only take every executors test suite and skip the ones in between.
• The description-length deterministically checks the length of the description for each test suite use a modulo of the number of executors.

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For more information on Karma see the homepage.

See Also

karma-sharding

This similar project works by splitting up the actual spec files across the browser instances.

Pros:

• Reduces memory by only loading some of the spec files in each browser instance

Cons:

• Requires the spec files to reside in separate files, meaning it is not compatible with bundlers such as karma-webpack or karma-browserify