Use Selenium Grid for Cross-Browser Compatibility Testing

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What is Selenium Grid?

Selenium is a browser automation library with bindings for most common programming languages. It is most often used for testing web applications, but can also be used to automate any task that a web browser can perform. In contrast to similar tools such as Nightmare.js, Selenium can run tasks or tests on any version of any major browser. This can make it more complicated to get running, but allows you to test your application’s behavior on exactly the platforms that users are likely to need.

For many applications, the Selenium standalone server is sufficient. However, Selenium can also be configured as a grid, with multiple nodes communicating with a central hub. The nodes and hub can be run on the same computer or server, or can be located on separate Linodes. Selenium Grid offers two main benefits:

  • Selenium runs on Java and is compatible with all major operating systems. This makes it possible to set up a grid consisting of servers (or virtual machines) running Linux, OSX, and Windows. A test suite can then be run against the grid, with each test run on multiple browsers and operating systems. The hub will delegate each test to a node that has the requested capability (e.g. sending a Safari test to a node with OSX and the Safari webdriver).

  • For larger projects, running a lengthy test suite in series can be time consuming. By running the test suite across a grid consisting of multiple servers, it is possible to distribute the tests across multiple nodes and significantly increase the performance of the testing process.

This guide shows how to set up a simple Selenium grid consisting of a hub and two nodes, all on separate Linodes. A simple test script will then be used to demonstrate running tests against different versions of Firefox.

Prepare Grid Linodes

Install Java and other dependencies on each Linode that will be part of the Selenium grid. This guide uses three Linodes for this purpose, but you can also run all of the nodes from the same Linode if you prefer. Throughout this guide, these Linodes will be referred to as hub, node-1, and node-2.

Install Java

The steps in this section will install the Java 8 JDK on Ubuntu 16.04. For other distributions, see the official docs.

  1. Install software-properties-common to easily add new repositories:

    sudo apt-get install software-properties-common
  2. Add the Java PPA:

    sudo add-apt-repository ppa:webupd8team/java
  3. Update the source list:

    sudo apt-get update
  4. Install the Java JDK 8:

    sudo apt-get install oracle-java8-installer

Install Dependencies

When running tests with Selenium, each grid node can only run tests on browsers that have been installed on that node. Each browser also requires a separate executable webdriver. For this example, install Geckodriver and different versions of Firefox on node-1 and node-2.

  1. Check the latest release of Geckodriver on the releases page and download it to node-1 and node-2:

  2. Extract the archive and move the executable to a location in your PATH:

    tar -xvf geckodriver-v0.19.1-linux64.tar.gz
    sudo mv geckodriver /usr/local/bin/
  3. On node-1, install the latest stable version of Firefox:

    sudo apt install firefox
  4. On node-2, install the Firefox developer edition:

    sudo add-apt-repository ppa:ubuntu-mozilla-daily/firefox-aurora
    sudo apt update && sudo apt install firefox
  5. Check and record the version numbers to use when running tests:


    firefox --version
    Mozilla Firefox 58.0.2


    firefox --version
    Mozilla Firefox 59.0

Download Selenium

Selenium provides a single .jar file that can be used to run a standalone server, hub, or node. Check the latest release at the Selenium downloads page and download the file to each Linode using wget:


Start Grid Hub

  1. On the hub Linode, start the hub by running the Selenium server with -role set to hub:

    java -jar selenium-server-standalone-3.10.0.jar -role hub
  2. The resulting output will give you URLs to use for registering nodes and connecting to the hub. Copy these URLs for later use:

    21:27:51.470 INFO [GridLauncherV3.launch] - Selenium build info: version: '3.10.0', revision: '176b4a9'
    21:27:51.475 INFO [GridLauncherV3$2.launch] - Launching Selenium Grid hub on port 4444
    2018-03-06 21:27:52.248:INFO::main: Logging initialized @1166ms to org.seleniumhq.jetty9.util.log.StdErrLog
    21:27:52.446 INFO [Hub.start] - Selenium Grid hub is up and running
    21:27:52.447 INFO [Hub.start] - Nodes should register to
    21:27:52.448 INFO [Hub.start] - Clients should connect to

Configure Grid Nodes

  1. On node-1 and node-2, create a node configuration file config.json and add the following content. Replace the hub address with the public IP of the hub Linode, and replace the version with the version of Firefox installed on the respective nodes. If you are putting the grid and nodes on the same Linode, replace the IP address with http://localhost:4444:

    File: config.json
          "browserName": "firefox",
          "marionette": true,
          "maxInstances": 5,
          "seleniumProtocol": "WebDriver",
          "version": "58.0.2"
      "proxy": "org.openqa.grid.selenium.proxy.DefaultRemoteProxy",
      "maxSession": 5,
      "port": 5555,
      "register": true,
      "registerCycle": 5000,
      "hub": "",
      "nodeStatusCheckTimeout": 5000,
      "nodePolling": 5000,
      "role": "node",
      "unregisterIfStillDownAfter": 60000,
      "downPollingLimit": 2,
      "debug": false,
      "servlets" : [],
      "withoutServlets": [],
      "custom": {}
  2. Connect each node to the hub:

    java -jar selenium-server-standalone-3.10.0.jar -role node -nodeConfig config.json
  3. You should see output similar to the following, indicating that your nodes have been successfully registered to the hub:

    21:33:22.856 INFO - Selenium Server is up and running on port 5555
    21:33:22.857 INFO - Selenium Grid node is up and ready to register to the hub
    21:33:22.895 INFO - Starting auto registration thread. Will try to register every 5000 ms.
    21:33:22.896 INFO - Registering the node to the hub:
    21:33:23.064 INFO - Updating the node configuration from the hub
    21:33:23.178 INFO - The node is registered to the hub and ready to use

You can also check the output from the hub itself:


21:27:53.849 INFO [DefaultGridRegistry.add] - Registered a node
21:27:56.445 WARN [BaseRemoteProxy.<init>] - Max instance not specified. Using default = 1 instance
21:27:56.450 INFO [DefaultGridRegistry.add] - Registered a node
21:27:56.743 WARN [BaseRemoteProxy.<init>] - Max instance not specified. Using default = 1 instance
  1. Navigate to in a web browser (replace with the public IP address of your hub Linode) to see a console listing your available nodes.

    The console should show that each node is configured to use a different version of Firefox.

Prepare Local Test Environment

In this example, the test script will be run from your local development machine. It will connect to the remote grid and execute the tests from there. If you do not have an available development machine, use a separate Linode.

Install Node.js and NPM

This guide uses the NPM selenium-webdriver package, which contains Node.js bindings for Selenium.

  1. Use curl to download the setup script provided by NodeSource. Replace the Node version in the curl command with the version you would like to install:

    curl -sL -o
  2. Run the script:

    sudo bash
  3. The setup script will run an apt-get update automatically, so you can install Node.js right away:

    sudo apt install nodejs npm

The Node Package Manager (NPM) will be installed alongside Node.js.

Create an Example Test Script

This script tests the Linode Docs home page.

  1. Create a directory for the test suite:

    mkdir test-selenium && cd test-selenium
  2. Initialize a Node.js app within the directory:

    npm init

    Accept the default values when prompted.

  3. Install NPM packages:

    npm install --save selenium-webdriver
  4. Create test.js and add the following script. Replace on Line 11 with the IP address of hub:

    File: ~/test-selenium/test.js
    const {Builder, By, Capabilities, Key, until} = require('selenium-webdriver');
    let firefox = require('selenium-webdriver/firefox');
    const VERSIONS = ['58.0.2','59.0'];
    function buildDrivers(versions) {
        let drivers = [];
        for (let version of versions) {
            driver = new Builder().forBrowser('firefox')
                                                new firefox.Options().headless())
        console.log('built drivers for ' + versions);
        return drivers;
    async function example(driver) {
           try {
            await driver.get('');
            await driver.findElement('q')).sendKeys('nginx', Key.RETURN);
            let el = driver.findElement(By.linkText('How to Configure nginx'));
            await driver.wait(until.elementIsVisible(el), 1000);
            title = await driver.getTitle();
          } finally {
            await driver.quit();
    async function main() {
        const drivers = await buildDrivers(VERSIONS);
        for (let driver of drivers) {
  5. Save the test script, then run it:

    node test.js

    If successful, the script will search for NGINX in the Linode docs library, visit one of the results pages, and check that the page title matches the link text. It will print out the page title as well:

    built drivers for 58.0.2,59.0
    How to Configure nginx
    How to Configure nginx
  6. The driver requested two versions of Firefox to run on the two nodes. test.js runs the driver asynchronously across two nodes in order to run the getTitle method across two browser versions.

By creating different drivers for each combination of platform, browser, and version you want to test, you can specify which node or nodes should be used to run each test. If more than one node has the requested capabilities, Selenium will choose one of the nodes at random. In this way it is possible to run a large, cross-browser test suite in much less time than it would take to run the tests one at a time.

More Information

You may wish to consult the following resources for additional information on this topic. While these are provided in the hope that they will be useful, please note that we cannot vouch for the accuracy or timeliness of externally hosted materials.

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