Selenium Grid

Distribute tests across multiple machines and browsers using Selenium Grid for scalable test execution.

Selenium 4 Stable

Selenium Grid allows you to run tests on multiple machines with different browsers simultaneously, dramatically reducing test execution time for large test suites.

Grid Architecture

                    ┌─────────────────┐
                    │   Hub (Router)  │
                    │   Port 4444     │
                    └────────┬────────┘
                             │
        ┌────────────────────┼────────────────────┐
        │                    │                    │
        ▼                    ▼                    ▼
┌───────────────┐   ┌───────────────┐   ┌───────────────┐
│   Node 1      │   │   Node 2      │   │   Node 3      │
│   Chrome      │   │   Firefox     │   │   Edge        │
│   Windows     │   │   Linux       │   │   Windows     │
└───────────────┘   └───────────────┘   └───────────────┘

Quick Start with Docker

Docker Compose Setup

version: '3.8'

services:
  selenium-hub:
    image: selenium/hub:latest
    container_name: selenium-hub
    ports:
      - "4442:4442"
      - "4443:4443"
      - "4444:4444"

  chrome:
    image: selenium/node-chrome:latest
    shm_size: 2gb
    depends_on:
      - selenium-hub
    environment:
      - SE_EVENT_BUS_HOST=selenium-hub
      - SE_EVENT_BUS_PUBLISH_PORT=4442
      - SE_EVENT_BUS_SUBSCRIBE_PORT=4443
      - SE_NODE_MAX_SESSIONS=4

  firefox:
    image: selenium/node-firefox:latest
    shm_size: 2gb
    depends_on:
      - selenium-hub
    environment:
      - SE_EVENT_BUS_HOST=selenium-hub
      - SE_EVENT_BUS_PUBLISH_PORT=4442
      - SE_EVENT_BUS_SUBSCRIBE_PORT=4443
      - SE_NODE_MAX_SESSIONS=4

  edge:
    image: selenium/node-edge:latest
    shm_size: 2gb
    depends_on:
      - selenium-hub
    environment:
      - SE_EVENT_BUS_HOST=selenium-hub
      - SE_EVENT_BUS_PUBLISH_PORT=4442
      - SE_EVENT_BUS_SUBSCRIBE_PORT=4443
      - SE_NODE_MAX_SESSIONS=4

Start the grid:

docker-compose up -d

# Check status at http://localhost:4444

Connecting to Grid

Connect to Selenium Grid

Selenium 4 Stable

import org.openqa.selenium.WebDriver;
import org.openqa.selenium.remote.RemoteWebDriver;
import org.openqa.selenium.chrome.ChromeOptions;
import java.net.URL;

// Connect to Grid Hub
URL gridUrl = new URL("http://localhost:4444");

// Chrome on Grid
ChromeOptions chromeOptions = new ChromeOptions();
WebDriver chromeDriver = new RemoteWebDriver(gridUrl, chromeOptions);

// Firefox on Grid
FirefoxOptions firefoxOptions = new FirefoxOptions();
WebDriver firefoxDriver = new RemoteWebDriver(gridUrl, firefoxOptions);

// Edge on Grid
EdgeOptions edgeOptions = new EdgeOptions();
WebDriver edgeDriver = new RemoteWebDriver(gridUrl, edgeOptions);

// Use like a regular driver
chromeDriver.get("https://example.com");
System.out.println(chromeDriver.getTitle());

// Always quit when done
chromeDriver.quit();

from selenium import webdriver
from selenium.webdriver.common.options import ArgOptions

# Connect to Grid Hub
grid_url = "http://localhost:4444"

# Chrome on Grid
chrome_options = webdriver.ChromeOptions()
chrome_driver = webdriver.Remote(
  command_executor=grid_url,
  options=chrome_options
)

# Firefox on Grid
firefox_options = webdriver.FirefoxOptions()
firefox_driver = webdriver.Remote(
  command_executor=grid_url,
  options=firefox_options
)

# Edge on Grid
edge_options = webdriver.EdgeOptions()
edge_driver = webdriver.Remote(
  command_executor=grid_url,
  options=edge_options
)

# Use like a regular driver
chrome_driver.get("https://example.com")
print(chrome_driver.title)

# Always quit when done
chrome_driver.quit()

const { Builder } = require('selenium-webdriver');
const chrome = require('selenium-webdriver/chrome');

// Connect to Grid Hub
const gridUrl = 'http://localhost:4444';

// Chrome on Grid
const chromeDriver = await new Builder()
  .usingServer(gridUrl)
  .forBrowser('chrome')
  .build();

// Firefox on Grid
const firefoxDriver = await new Builder()
  .usingServer(gridUrl)
  .forBrowser('firefox')
  .build();

// Use like a regular driver
await chromeDriver.get('https://example.com');
console.log(await chromeDriver.getTitle());

// Always quit when done
await chromeDriver.quit();

using OpenQA.Selenium;
using OpenQA.Selenium.Remote;
using OpenQA.Selenium.Chrome;

// Connect to Grid Hub
Uri gridUrl = new Uri("http://localhost:4444");

// Chrome on Grid
var chromeOptions = new ChromeOptions();
IWebDriver chromeDriver = new RemoteWebDriver(gridUrl, chromeOptions);

// Firefox on Grid
var firefoxOptions = new FirefoxOptions();
IWebDriver firefoxDriver = new RemoteWebDriver(gridUrl, firefoxOptions);

// Use like a regular driver
chromeDriver.Navigate().GoToUrl("https://example.com");
Console.WriteLine(chromeDriver.Title);

// Always quit when done
chromeDriver.Quit();

Grid Configuration Options

Environment Variables

# docker-compose.yml node configuration
chrome:
  image: selenium/node-chrome:latest
  environment:
    # Connection
    - SE_EVENT_BUS_HOST=selenium-hub
    - SE_EVENT_BUS_PUBLISH_PORT=4442
    - SE_EVENT_BUS_SUBSCRIBE_PORT=4443

    # Capacity
    - SE_NODE_MAX_SESSIONS=5     # Max concurrent sessions
    - SE_NODE_SESSION_TIMEOUT=300  # Session timeout in seconds

    # Node info
    - SE_NODE_OVERRIDE_MAX_SESSIONS=true

Scaling Nodes

# Scale Chrome nodes to 3 instances
docker-compose up -d --scale chrome=3

# Check node status
curl http://localhost:4444/status

Cross-Browser Testing

Run Test Across Browsers

Selenium 4 Stable

import org.testng.annotations.*;
import org.openqa.selenium.remote.RemoteWebDriver;
import java.net.URL;

public class CrossBrowserTest {
  private WebDriver driver;
  private String browser;

  @Parameters({"browser"})
  @BeforeMethod
  public void setup(String browser) throws Exception {
      this.browser = browser;
      URL gridUrl = new URL("http://localhost:4444");

      switch (browser.toLowerCase()) {
          case "chrome":
              driver = new RemoteWebDriver(gridUrl, new ChromeOptions());
              break;
          case "firefox":
              driver = new RemoteWebDriver(gridUrl, new FirefoxOptions());
              break;
          case "edge":
              driver = new RemoteWebDriver(gridUrl, new EdgeOptions());
              break;
      }
  }

  @Test
  public void testHomepage() {
      driver.get("https://example.com");
      Assert.assertTrue(driver.getTitle().contains("Example"));
  }

  @AfterMethod
  public void teardown() {
      if (driver != null) {
          driver.quit();
      }
  }
}

// testng.xml
// <suite name="Cross Browser" parallel="tests" thread-count="3">
//   <test name="Chrome"><parameter name="browser" value="chrome"/>...</test>
//   <test name="Firefox"><parameter name="browser" value="firefox"/>...</test>
//   <test name="Edge"><parameter name="browser" value="edge"/>...</test>
// </suite>

import pytest
from selenium import webdriver

def pytest_addoption(parser):
  parser.addoption("--browser", action="store", default="chrome")


@pytest.fixture
def driver(request):
  browser = request.config.getoption("--browser")
  grid_url = "http://localhost:4444"

  if browser == "chrome":
      options = webdriver.ChromeOptions()
  elif browser == "firefox":
      options = webdriver.FirefoxOptions()
  elif browser == "edge":
      options = webdriver.EdgeOptions()

  driver = webdriver.Remote(command_executor=grid_url, options=options)
  yield driver
  driver.quit()


# Run with different browsers:
# pytest --browser=chrome
# pytest --browser=firefox
# pytest --browser=edge

# Or run all browsers in parallel with pytest-xdist:
# pytest -n 3 --browser=chrome &
# pytest -n 3 --browser=firefox &
# pytest -n 3 --browser=edge &

const { Builder } = require('selenium-webdriver');

const browsers = ['chrome', 'firefox', 'edge'];

describe('Cross Browser Tests', function() {
  browsers.forEach((browserName) => {
      describe(`${browserName} tests`, function() {
          let driver;

          beforeEach(async function() {
              driver = await new Builder()
                  .usingServer('http://localhost:4444')
                  .forBrowser(browserName)
                  .build();
          });

          afterEach(async function() {
              await driver.quit();
          });

          it('should load homepage', async function() {
              await driver.get('https://example.com');
              const title = await driver.getTitle();
              expect(title).to.include('Example');
          });
      });
  });
});

[TestFixture("chrome")]
[TestFixture("firefox")]
[TestFixture("edge")]
public class CrossBrowserTest
{
  private IWebDriver driver;
  private string browser;

  public CrossBrowserTest(string browser)
  {
      this.browser = browser;
  }

  [SetUp]
  public void Setup()
  {
      Uri gridUrl = new Uri("http://localhost:4444");

      switch (browser)
      {
          case "chrome":
              driver = new RemoteWebDriver(gridUrl, new ChromeOptions());
              break;
          case "firefox":
              driver = new RemoteWebDriver(gridUrl, new FirefoxOptions());
              break;
          case "edge":
              driver = new RemoteWebDriver(gridUrl, new EdgeOptions());
              break;
      }
  }

  [Test]
  public void TestHomepage()
  {
      driver.Navigate().GoToUrl("https://example.com");
      Assert.That(driver.Title, Does.Contain("Example"));
  }

  [TearDown]
  public void Teardown()
  {
      driver?.Quit();
  }
}

Grid with Video Recording

# docker-compose with video recording
version: '3.8'

services:
  selenium-hub:
    image: selenium/hub:latest
    ports:
      - "4444:4444"

  chrome-video:
    image: selenium/node-chrome:latest
    shm_size: 2gb
    depends_on:
      - selenium-hub
    environment:
      - SE_EVENT_BUS_HOST=selenium-hub
      - SE_EVENT_BUS_PUBLISH_PORT=4442
      - SE_EVENT_BUS_SUBSCRIBE_PORT=4443
      - SE_START_VNC=true
      - SE_START_NO_VNC=true
    ports:
      - "7900:7900"  # VNC port

  video:
    image: selenium/video:latest
    depends_on:
      - chrome-video
    environment:
      - DISPLAY_CONTAINER_NAME=chrome-video
      - FILE_NAME=chrome_video.mp4
    volumes:
      - ./videos:/videos

Access VNC at http://localhost:7900 (password: secret)

Cloud Grid Services

Service	Pros	Cons
Sauce Labs	Huge browser selection	Expensive
BrowserStack	Real devices	Expensive
LambdaTest	Good pricing	Fewer features
Self-hosted	Full control	Maintenance overhead

Best Practices

Set session timeouts - Prevent hung sessions from blocking nodes
Use Docker volumes for test artifacts (screenshots, videos)
Monitor Grid status - Check /status endpoint
Scale based on load - Add nodes as needed
Clean up sessions - Always call driver.quit()

Next Steps

Parallel Execution - Local parallelization
Headless Browsers - Faster grid nodes
pytest Integration - Python CI/CD setup
TestNG and JUnit - Java CI/CD setup
Browser Options - Configure remote browsers