Test parallelization in CI in Selenium Java - Test Execution Trace

Test Overview

This test runs two simple Selenium tests in parallel using TestNG in a Continuous Integration (CI) environment. It verifies that both tests execute independently and pass without interference.

Test Code - TestNG

Selenium Java

import org.openqa.selenium.By;
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.chrome.ChromeDriver;
import org.testng.Assert;
import org.testng.annotations.AfterMethod;
import org.testng.annotations.BeforeMethod;
import org.testng.annotations.Test;

public class ParallelTest {
    private WebDriver driver;

    @BeforeMethod
    public void setUp() {
        driver = new ChromeDriver();
    }

    @Test(timeOut = 10000)
    public void testGoogleTitle() {
        driver.get("https://www.google.com");
        String title = driver.getTitle();
        Assert.assertEquals(title, "Google");
    }

    @Test(timeOut = 10000)
    public void testBingTitle() {
        driver.get("https://www.bing.com");
        String title = driver.getTitle();
        Assert.assertEquals(title, "Bing");
    }

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

Execution Trace - 10 Steps

Step	Action	System State	Assertion	Result
1	TestNG starts test suite with parallel execution enabled	TestNG initializes two test methods to run in parallel threads	—	PASS
2	First thread runs testGoogleTitle: opens Chrome browser	Chrome browser instance 1 opens at blank page	—	PASS
3	Second thread runs testBingTitle: opens Chrome browser	Chrome browser instance 2 opens at blank page	—	PASS
4	First thread navigates to https://www.google.com	Browser instance 1 shows Google homepage	Page title is 'Google'	PASS
5	Second thread navigates to https://www.bing.com	Browser instance 2 shows Bing homepage	Page title is 'Bing'	PASS
6	First thread asserts page title equals 'Google'	Title retrieved from browser instance 1	Assert.assertEquals(title, "Google")	PASS
7	Second thread asserts page title equals 'Bing'	Title retrieved from browser instance 2	Assert.assertEquals(title, "Bing")	PASS
8	First thread closes browser instance 1	Browser instance 1 closed	—	PASS
9	Second thread closes browser instance 2	Browser instance 2 closed	—	PASS
10	TestNG completes all parallel test executions	All tests finished with no failures	All assertions passed	PASS

Failure Scenario

Failing Condition: If the browser driver is not thread-safe or shared incorrectly, tests interfere causing wrong page titles or exceptions

Execution Trace Quiz - 3 Questions

What ensures that the two tests run at the same time in this TestNG setup?

A Calling driver.get() twice in the same test

B Enabling parallel="methods" with thread-count="2" in TestNG suite configuration

C Using @BeforeMethod and @AfterMethod annotations

D Running tests sequentially in one thread

Key Result

Always create and quit a separate WebDriver instance per test method when running tests in parallel to avoid conflicts and ensure reliable results.

Practice

1. What is the main benefit of running tests in parallel in a CI environment?

easy

A. It increases the number of test cases by duplicating them.

B. It ensures tests run in a specific order to avoid failures.

C. It reduces the total test execution time by running tests simultaneously.

D. It disables tests that are flaky to improve stability.

Solution

1. Step 1: Understand parallel test execution

   Parallel execution means running multiple tests at the same time instead of one after another.

2. Step 2: Identify the benefit in CI

   Running tests simultaneously saves time, making the feedback faster in continuous integration.

3. Final Answer:

   It reduces the total test execution time by running tests simultaneously. -> Option C

4. Quick Check:

   Parallel tests = faster execution [OK]

Hint: Parallel tests run together to save time [OK]

Common Mistakes:

• Thinking parallel means tests run in order
• Believing parallel increases test count
• Assuming parallel disables flaky tests

2. Which TestNG XML attribute correctly enables parallel test execution?

easy

A.

B.

C.

D.

Solution

1. Step 1: Recall TestNG XML syntax for parallelism

   The correct attribute to enable parallel execution is 'parallel' with values like 'methods', 'tests', or 'classes'.

2. Step 2: Identify correct thread count attribute

   The attribute 'thread-count' sets how many threads run in parallel.

3. Final Answer:

   <suite parallel="methods" thread-count="4"> -> Option B

4. Quick Check:

   parallel + thread-count = correct syntax [OK]

Hint: Use parallel and thread-count attributes in TestNG XML [OK]

Common Mistakes:

• Using non-existent attributes like runParallel
• Confusing thread-count with threads
• Using order="sequential" disables parallelism

3. Given this TestNG XML snippet, what is the maximum number of tests running at the same time?

<suite name="Suite" parallel="tests" thread-count="3">
  <test name="Test1">...</test>
  <test name="Test2">...</test>
  <test name="Test3">...</test>
  <test name="Test4">...</test>
</suite>

medium

A. 3 tests run simultaneously

B. 4 tests run simultaneously

C. 1 test runs at a time

D. 2 tests run simultaneously

Solution

1. Step 1: Understand parallel="tests" meaning

   Setting parallel="tests" means TestNG runs multiple <test> tags at the same time.

2. Step 2: Check thread-count value

   thread-count="3" limits the number of simultaneous threads to 3, so max 3 tests run together.

3. Final Answer:

   3 tests run simultaneously -> Option A

4. Quick Check:

   parallel=tests + thread-count=3 means 3 parallel tests [OK]

Hint: thread-count limits max parallel tests [OK]

Common Mistakes:

• Assuming all tests run at once ignoring thread-count
• Confusing parallel="methods" with parallel="tests"
• Thinking thread-count is ignored

4. You configured TestNG with parallel="methods" and thread-count="2". Your tests share a static WebDriver instance. What problem will most likely occur?

medium

A. Tests will fail due to WebDriver conflicts from shared instance.

B. Tests will run faster without any issues.

C. Tests will run sequentially ignoring parallel settings.

D. Tests will create multiple WebDriver instances automatically.

Solution

1. Step 1: Understand parallel methods with shared static WebDriver

   Parallel methods run at the same time, but sharing a static WebDriver means all tests use the same browser instance.

2. Step 2: Identify concurrency issues

   Sharing one WebDriver causes conflicts like commands overlapping, leading to test failures.

3. Final Answer:

   Tests will fail due to WebDriver conflicts from shared instance. -> Option A

4. Quick Check:

   Shared static WebDriver + parallel = conflicts [OK]

Hint: Avoid shared static WebDriver in parallel tests [OK]

Common Mistakes:

• Assuming shared WebDriver works fine in parallel
• Thinking parallel disables shared resource conflicts
• Believing TestNG creates separate WebDriver automatically

5. You want to run your Selenium tests in parallel in CI using TestNG. Your suite has 5 test classes, each with 3 test methods. You want to run all methods in parallel but limit total threads to 6. Which TestNG XML configuration is correct?

hard

A.

B.

C.

D.

Solution

1. Step 1: Identify parallel="methods" for running all test methods in parallel

   To run all methods in parallel, use parallel="methods" in the suite tag.

2. Step 2: Set thread-count="6" to limit total threads

   thread-count="6" limits the number of simultaneous threads to 6, matching the requirement.

3. Step 3: Confirm classes are inside a single <test> tag

   All classes are grouped under one <test> tag, so methods from all classes run in parallel within the thread limit.

4. Final Answer:

   <suite parallel="methods" thread-count="6"> -> Option D

5. Quick Check:

   parallel="methods" + thread-count="6" + single <test> [OK]

Hint: Use parallel="methods" with thread-count to control concurrency [OK]

Common Mistakes:

• Using parallel="classes" runs classes in parallel, not methods
• Using thread-count too high ignores limit
• Splitting classes into multiple tests changes parallel behavior