Selenium Grid

Yuri Kan

Selenium Grid

Master Selenium Grid: set up distributed test execution, configure Hub/Node architecture, run parallel cross-browser tests with Docker and cloud providers.

Quick Answer

Selenium Grid covers essential QA skills — after this lesson you can set up Selenium Grid for distributed and parallel test execution.

— Yuri Kan, Senior QA Lead

What You Will Learn

Set up Selenium Grid for distributed and parallel test execution
Configure Hub and Node architecture for cross-browser testing at scale
Integrate Selenium Grid with Docker and cloud providers like BrowserStack

Table of Contents

What Is Selenium Grid?

Selenium Grid allows you to run tests on multiple machines (nodes) in parallel, across different browsers and operating systems. Instead of running 100 tests sequentially on one machine (taking 2 hours), you can run them across 10 nodes in parallel (taking 12 minutes).

Why You Need Grid

Without Grid	With Grid
1 browser at a time	Multiple browsers simultaneously
Sequential execution	Parallel execution
Single OS	Multiple OS (Windows, Linux, macOS)
100 tests × 2 min = 200 min	100 tests ÷ 10 nodes = 20 min
One machine	Distributed across machines

Selenium Grid 4 Architecture

Selenium Grid 4 introduced a modernized architecture:

┌─────────────────────────────────────┐
│           Selenium Grid             │
│                                     │
│  ┌──────────┐  ┌──────────────┐    │
│  │  Router   │  │  Distributor │    │
│  └────┬─────┘  └──────┬───────┘    │
│       │               │            │
│  ┌────┴─────┐  ┌──────┴───────┐    │
│  │  Session  │  │   Node       │    │
│  │  Map      │  │   Manager    │    │
│  └──────────┘  └──────────────┘    │
└─────────────────────────────────────┘
         │              │
    ┌────┴────┐   ┌────┴────┐
    │  Node 1 │   │  Node 2 │
    │ Chrome  │   │ Firefox │
    └─────────┘   └─────────┘

Components:

Router — entry point, receives WebDriver requests
Distributor — assigns sessions to available Nodes
Session Map — tracks which Node owns which session
Node — runs browsers and executes tests

Setting Up Selenium Grid

Standalone Mode (Single Machine)

# Download selenium-server
wget https://github.com/SeleniumHQ/selenium/releases/download/selenium-4.18.0/selenium-server-4.18.0.jar

# Run standalone (hub + node on same machine)
java -jar selenium-server-4.18.0.jar standalone

Grid is now available at http://localhost:4444.

Hub and Node Mode (Multiple Machines)

# On the hub machine
java -jar selenium-server-4.18.0.jar hub

# On node machine 1 (Chrome)
java -jar selenium-server-4.18.0.jar node --hub http://hub-ip:4444

# On node machine 2 (Firefox)
java -jar selenium-server-4.18.0.jar node --hub http://hub-ip:4444

Connecting Tests to Grid

from selenium import webdriver
from selenium.webdriver.chrome.options import Options

options = Options()
options.browser_version = "latest"
options.platform_name = "linux"

driver = webdriver.Remote(
    command_executor="http://localhost:4444",
    options=options
)

driver.get("https://app.example.com")
# ... run your tests
driver.quit()

ChromeOptions options = new ChromeOptions();
WebDriver driver = new RemoteWebDriver(
    new URL("http://localhost:4444"),
    options
);

Docker-Based Selenium Grid

Docker is the recommended way to run Selenium Grid. It provides consistent, reproducible environments.

Docker Compose Setup

# docker-compose.yml
version: "3"
services:
  selenium-hub:
    image: selenium/hub:4.18.0
    container_name: selenium-hub
    ports:
      - "4442:4442"
      - "4443:4443"
      - "4444:4444"

  chrome-node:
    image: selenium/node-chrome:4.18.0
    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
    deploy:
      replicas: 2

  firefox-node:
    image: selenium/node-firefox:4.18.0
    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-node:
    image: selenium/node-edge:4.18.0
    depends_on:
      - selenium-hub
    environment:
      - SE_EVENT_BUS_HOST=selenium-hub
      - SE_EVENT_BUS_PUBLISH_PORT=4442
      - SE_EVENT_BUS_SUBSCRIBE_PORT=4443

# Start the grid
docker compose up -d

# Scale chrome nodes
docker compose up -d --scale chrome-node=5

# View grid status
open http://localhost:4444/ui

Dynamic Grid with Docker

Selenium Grid 4 supports dynamic Docker-based node creation:

# grid-config.toml
[docker]
url = "http://localhost:2375"
video-image = "selenium/video:latest"
assets-path = "/opt/selenium/assets"

[[docker.configs]]
image = "selenium/standalone-chrome:latest"
stereotype = '{"browserName": "chrome"}'
max-sessions = 4

[[docker.configs]]
image = "selenium/standalone-firefox:latest"
stereotype = '{"browserName": "firefox"}'
max-sessions = 4

Parallel Test Execution

TestNG (Java)

<!-- testng.xml -->
<suite name="Parallel Suite" parallel="tests" thread-count="3">
  <test name="Chrome Tests">
    <parameter name="browser" value="chrome"/>
    <classes>
      <class name="tests.LoginTest"/>
      <class name="tests.CheckoutTest"/>
    </classes>
  </test>
  <test name="Firefox Tests">
    <parameter name="browser" value="firefox"/>
    <classes>
      <class name="tests.LoginTest"/>
      <class name="tests.CheckoutTest"/>
    </classes>
  </test>
  <test name="Edge Tests">
    <parameter name="browser" value="edge"/>
    <classes>
      <class name="tests.LoginTest"/>
      <class name="tests.CheckoutTest"/>
    </classes>
  </test>
</suite>

pytest (Python)

# Run tests in parallel using pytest-xdist
pip install pytest-xdist
pytest tests/ -n 4  # 4 parallel workers

WebdriverIO (JavaScript)

// wdio.conf.js
exports.config = {
  maxInstances: 10,
  capabilities: [
    { browserName: 'chrome', maxInstances: 5 },
    { browserName: 'firefox', maxInstances: 3 },
    { browserName: 'MicrosoftEdge', maxInstances: 2 }
  ],
  services: ['selenium-standalone'],
};

Cloud Grid Providers

For enterprise-scale testing, cloud providers offer managed Selenium Grids:

Provider	Browsers	Real Devices	Pricing
BrowserStack	3,000+	Yes	From $29/mo
Sauce Labs	2,000+	Yes	From $39/mo
LambdaTest	3,000+	Yes	From $15/mo

BrowserStack Integration

from selenium import webdriver

options = webdriver.ChromeOptions()
options.set_capability('bstack:options', {
    'os': 'Windows',
    'osVersion': '11',
    'browserVersion': 'latest',
    'projectName': 'E-Commerce Tests',
    'buildName': 'Build 1.0',
})

driver = webdriver.Remote(
    command_executor=f'https://{USERNAME}:{ACCESS_KEY}@hub-cloud.browserstack.com/wd/hub',
    options=options
)

Grid Monitoring and Debugging

Grid UI Dashboard

Access http://localhost:4444/ui to see:

Connected nodes and their capabilities
Active sessions
Queue length
Node health status

Video Recording

Docker Selenium supports video recording of test sessions:

chrome-node:
  image: selenium/node-chrome:4.18.0
  volumes:
    - ./videos:/opt/selenium/assets
  environment:
    - SE_RECORD_VIDEO=true

Exercise: Set Up a Selenium Grid

Create a Docker Compose file with Hub, 2 Chrome nodes, 1 Firefox node
Start the grid and verify all nodes appear in the dashboard
Modify an existing Selenium test to use RemoteWebDriver pointing to the grid
Run the same test on Chrome and Firefox using capabilities
Run tests in parallel using pytest-xdist or TestNG
Compare execution time: sequential vs parallel

Key Takeaways

Selenium Grid enables parallel, distributed test execution across browsers and machines
Docker Compose is the easiest way to set up and manage a Grid
Grid 4 has modernized architecture with Router, Distributor, Session Map, and Nodes
Cloud providers (BrowserStack, Sauce Labs) offer managed grids for enterprise scale
Parallel execution can reduce test suite time by 80-90%
Always monitor Grid health through the built-in dashboard

Knowledge Check

1. What is the primary purpose of Selenium Grid?

2. In Selenium Grid 4 architecture, what is the role of the Router?

3. What is the advantage of using Docker with Selenium Grid?

Frequently Asked Questions

What is selenium grid?

Selenium Grid is a key concept in Test Automation. This lesson teaches you to set up Selenium Grid for distributed and parallel test execution, providing practical skills you can apply immediately in your testing work.

How do I apply selenium grid in real projects?

Start by practicing the core techniques covered in this lesson. Specifically, you should configure Hub and Node architecture for cross-browser testing at scale. Apply these skills in your current project to see immediate results.

Why is selenium grid important for QA engineers?

Selenium Grid is a core skill that employers look for in QA professionals. It directly impacts test coverage, defect detection, and team efficiency. Mastering it strengthens your Test Automation capabilities and makes you more effective at delivering quality software.

What should I know before learning selenium grid?

You should have a basic understanding of software testing fundamentals. Familiarity with selenium grid will help, but the lesson includes review sections for key prerequisites.

How does selenium grid help my QA career?

Knowledge of selenium grid is frequently listed in QA job descriptions and interview questions. It demonstrates expertise in selenium grid, parallel testing selenium and shows you can contribute to quality assurance at a professional level. Senior roles especially value this competency.

Further Reading Selenium Grid 4: Distributed Test Execution Architecture → Selenium WebDriver in 2026: Still Relevant? →

Selenium Grid

What You Will Learn

What Is Selenium Grid? #

Why You Need Grid #

Selenium Grid 4 Architecture #

Setting Up Selenium Grid #

Standalone Mode (Single Machine) #

Hub and Node Mode (Multiple Machines) #

Connecting Tests to Grid #

Docker-Based Selenium Grid #

Docker Compose Setup #

Dynamic Grid with Docker #

Parallel Test Execution #

TestNG (Java) #

pytest (Python) #

WebdriverIO (JavaScript) #

Cloud Grid Providers #

BrowserStack Integration #

Grid Monitoring and Debugging #

Grid UI Dashboard #

Video Recording #

Exercise: Set Up a Selenium Grid #

Key Takeaways #

Knowledge Check

Frequently Asked Questions

What Is Selenium Grid?

Why You Need Grid

Selenium Grid 4 Architecture

Setting Up Selenium Grid

Standalone Mode (Single Machine)

Hub and Node Mode (Multiple Machines)

Connecting Tests to Grid

Docker-Based Selenium Grid

Docker Compose Setup

Dynamic Grid with Docker

Parallel Test Execution

TestNG (Java)

pytest (Python)

WebdriverIO (JavaScript)

Cloud Grid Providers

BrowserStack Integration

Grid Monitoring and Debugging

Grid UI Dashboard

Video Recording

Exercise: Set Up a Selenium Grid

Key Takeaways