Parallel Test Execution with Selenium Grid allows you to run multiple Selenium tests at the same time across different browsers, browser versions, and machines. This significantly reduces test execution time and improves test coverage.
Below is a clear, step-by-step explanation.
1. What Is Selenium Grid?
Selenium Grid is a tool that:
Distributes test execution across multiple machines (nodes)
Supports parallel execution
Enables cross-browser and cross-platform testing
Grid Components
Hub: Central point that receives test requests
Nodes: Machines/browsers where tests actually run
2. Why Use Parallel Execution?
Benefits
✅ Faster test execution
✅ Better CI/CD pipeline performance
✅ Cross-browser testing at scale
✅ Efficient use of infrastructure
Example:
100 tests × 5 seconds each
Sequential: ~8 minutes
Parallel (10 nodes): ~50 seconds
3. Selenium Grid Architecture (Modern Grid 4)
Selenium Grid 4 uses a distributed architecture with:
Router
Distributor
Session Map
Event Bus
👉 For users, this complexity is hidden—you mostly interact with a single Grid URL.
4. Setting Up Selenium Grid (Quick Start)
Start Selenium Grid (Standalone Mode)
java -jar selenium-server-4.x.x.jar standalone
Grid URL:
http://localhost:4444
5. Writing Selenium Tests for Grid
You must use RemoteWebDriver instead of local drivers.
Example (Java + Selenium)
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.remote.RemoteWebDriver;
import org.openqa.selenium.chrome.ChromeOptions;
import java.net.URL;
public class GridTest {
public static void main(String[] args) throws Exception {
ChromeOptions options = new ChromeOptions();
WebDriver driver = new RemoteWebDriver(
new URL("http://localhost:4444/wd/hub"),
options
);
driver.get("https://example.com");
System.out.println(driver.getTitle());
driver.quit();
}
}
6. Enabling Parallel Execution with Test Frameworks
Using TestNG (Most Common)
testng.xml
<!DOCTYPE suite SYSTEM "https://testng.org/testng-1.0.dtd">
<suite name="Parallel Suite" parallel="tests" thread-count="3">
<test name="Chrome Test">
<classes>
<class name="tests.ChromeTest"/>
</classes>
</test>
<test name="Firefox Test">
<classes>
<class name="tests.FirefoxTest"/>
</classes>
</test>
</suite>
Test Class Example
@Test
public void openSite() {
WebDriver driver = new RemoteWebDriver(gridUrl, options);
driver.get("https://example.com");
driver.quit();
}
Parallel options:
parallel="methods"
parallel="classes"
parallel="tests"
7. Running Tests on Multiple Browsers
FirefoxOptions options = new FirefoxOptions();
options.setPlatformName("WINDOWS");
WebDriver driver = new RemoteWebDriver(gridUrl, options);
Selenium Grid automatically matches the request to an available node.
8. Using Selenium Grid with Docker (Recommended)
Start Grid with Docker
docker run -d -p 4444:4444 selenium/standalone-chrome
Or full Grid:
docker-compose up
Advantages
Easy scaling
Consistent environments
Ideal for CI/CD
9. Best Practices for Parallel Execution
✅ Make tests independent (no shared state)
✅ Avoid static WebDriver instances
✅ Use ThreadLocal<WebDriver> for thread safety
✅ Close drivers properly
✅ Keep test data isolated
ThreadLocal Example
private static ThreadLocal<WebDriver> driver = new ThreadLocal<>();
10. Common Issues & Solutions
Issue Cause Solution
Tests fail randomly Shared WebDriver Use ThreadLocal
Sessions not created Node unavailable Check Grid UI
Slow execution Too few nodes Scale nodes
Browser mismatch Wrong capabilities Verify options
11. Grid UI & Monitoring
Grid Console:
http://localhost:4444/ui
Shows:
Active sessions
Registered nodes
Browser availability
Key Takeaway
Parallel execution with Selenium Grid dramatically improves test speed and scalability by distributing tests across multiple browsers and machines.
Learn Selenium with JAVA Training in Hyderabad
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