What is Selenium Grid? How does it work?

┌──────────────────────────────────┐
│           HUB (Central Server)    │
│  localhost:4444                   │
│  Receives test commands           │
│  Manages node registry           │
│  Routes tests to matching node    │
└──────────────────────────────────┘
          │            │
    ┌─────┴──┐   ┌─────┴──┐
    │ Node 1  │   │ Node 2  │
    │ Chrome  │   │ Firefox │
    │ Windows │   │ Linux   │
    └─────────┘   └─────────┘

# Selenium 4 Grid modes:

# 1. Standalone (Hub + Node in ONE process)
java -jar selenium-server-4.x.jar standalone

# 2. Distributed (separate components)
java -jar selenium-server-4.x.jar router   --port 4444
java -jar selenium-server-4.x.jar distributor
java -jar selenium-server-4.x.jar node     --port 5555

# Terminal 1 — Start Grid
java -jar selenium-server-4.27.0.jar standalone --port 4444

# Open Grid UI at: http://localhost:4444/ui

// Instead of new ChromeDriver(), use RemoteWebDriver
URL gridUrl = new URL("http://localhost:4444/wd/hub");

// Specify browser capabilities
ChromeOptions options = new ChromeOptions();
options.addArguments("--headless=new");

// Connect to Grid
WebDriver driver = new RemoteWebDriver(gridUrl, options);

driver.get("https://example.com");
System.out.println(driver.getTitle());
driver.quit();

// BaseTest.java — reads browser from TestNG parameter
@Parameters("browser")
@BeforeMethod
public void setUp(@Optional("chrome") String browser) throws MalformedURLException {
    URL gridUrl = new URL("http://localhost:4444/wd/hub");
    MutableCapabilities options;

    switch (browser.toLowerCase()) {
        case "firefox": options = new FirefoxOptions(); break;
        case "edge":    options = new EdgeOptions();    break;
        default:        options = new ChromeOptions();  break;
    }

    WebDriver driver = new RemoteWebDriver(gridUrl, options);
    driver.manage().window().maximize();
    DriverManager.setDriver(driver);
}

<!-- testng.xml — run same tests on 3 browsers simultaneously -->
<suite name="Cross-Browser Grid" parallel="tests" thread-count="3">

    <test name="Chrome">
        <parameter name="browser" value="chrome"/>
        <classes><class name="com.automateqa.tests.LoginTest"/></classes>
    </test>

    <test name="Firefox">
        <parameter name="browser" value="firefox"/>
        <classes><class name="com.automateqa.tests.LoginTest"/></classes>
    </test>

    <test name="Edge">
        <parameter name="browser" value="edge"/>
        <classes><class name="com.automateqa.tests.LoginTest"/></classes>
    </test>

</suite>

# docker-compose.yml — spin up Grid with Chrome + Firefox nodes
version: "3"
services:
  selenium-hub:
    image: selenium/hub:4.27.0
    ports:
      - "4442:4442"
      - "4443:4443"
      - "4444:4444"

  chrome-node:
    image: selenium/node-chrome:4.27.0
    shm_size: 2g
    depends_on: [selenium-hub]
    environment:
      - SE_EVENT_BUS_HOST=selenium-hub
      - SE_NODE_MAX_SESSIONS=3
    deploy:
      replicas: 3   # 3 Chrome nodes

  firefox-node:
    image: selenium/node-firefox:4.27.0
    shm_size: 2g
    depends_on: [selenium-hub]
    environment:
      - SE_EVENT_BUS_HOST=selenium-hub

docker-compose up -d
# Grid UI: http://localhost:4444/ui