Continuous testing is essential for achieving product delivery goals in the current paradigm. Collaboration supports quality processes between QE and development teams. Automation testing has largely replaced the traditional manual testing approach. Parallel testing is a common automation testing strategy used by QE teams to save time and maximize coverage. This requires transitioning to parallel execution with TestNG or Selenium.

Continuous testing demands that Quality Engineers increase the test execution time.

“Efficiency is not just about doing things quickly; it’s about finding smarter ways to accomplish tasks and achieve desired results.”

Table of contents:

  • Introduction to parallel execution
  • Pros of Parallel testing over sequential testing
  • Challenges in the Parallel Execution
  • To achieve the Parallel Execution through the config.yml file

Parallel execution introduction:

Dependent test cases are executed one after another, independent test cases are executed at the same time. Ultimately QE can use parallel execution to run the test cases in two or more versions of software in parallel with/ without the same input. Parallel execution is used to execute the test cases parallel in different systems, same/different devices.

Parallel testing refers to the simultaneous execution of multiple tests in separate thread processes. When using Selenium, this means that multiple tests can be run at the same time on different browsers, devices, or environments, rather than being executed one after the other. 

Sequential Execution:

Sequential Execution

Parallel Execution:

Parallel Execution

Pros of Parallel testing over sequential testing:

Testers should consider adopting parallel testing in Selenium for several interesting reasons. Here are some of the most significant benefits it offers:

Accelerated test execution speed:

Parallel testing reduces the time required to perform automation testing. Running tests at the same time on multiple devices or input combinations saves time. This allows for faster feedback and quicker identification of issues.

Increased test coverage and wider compatibility:

Parallel testing enables testing across various devices, browsers, and operating systems simultaneously. The test coverage ensures a high-quality product by reducing risks, bugs, and increasing compatibility.

Overcoming the testing bottleneck:

Testing is often a bottleneck in the DevOps process, causing delays in the release cycle. Parallel testing accelerates test execution and keeps the release process on track.

Early bug detection and faster bug fixing:

Parallel testing allows for earlier detection of bugs. By executing more tests in less time, testers can uncover issues sooner, facilitating faster bug fixing. This results in a more efficient development cycle and a shorter time to market.


Flexibility to switch between testing methods is available. Testers can switch to sequential testing when necessary, ensuring adaptability.

Selenium parallel testing offers faster test execution, more comprehensive coverage, and bug detection.

Challenges may arise during TestNG parallel test execution. Some of the common challenges include:

Test dependencies:

If tests have dependencies on each other, parallel execution can cause issues. For example, if one test modifies data that another test relies on, running them concurrently may lead to unexpected failures. Testers need to take careful management of dependencies to ensure proper execution.

Resource contention:

Parallel execution requires distributing tests across multiple threads or machines. This can lead to resource contention, such as conflicts for database connections, file access, or network resources. Testers must handle resource allocation and synchronization effectively to avoid conflicts.

Test data management:

Incorporating parallel execution via the YML file, along with the capability to adapt or eliminate data and xpath files in response to job execution, introduces a complexity in data management aligned with the jobs. This approach requires testers to gain a deeper understanding to proficiently oversee data, ensuring more adept management.

Parallel test execution can make test reporting and analysis more challenging. Consolidating and interpreting test results from multiple parallel threads or machines requires careful consideration. Testers need to use appropriate reporting tools and techniques to gather meaningful insights from parallel test runs.

Debugging and troubleshooting:

Identifying and resolving issues in parallel test execution can be more difficult compared to sequential execution. Test failures may occur concurrently, making it harder to pinpoint the root causes. Testers should have effective debugging and troubleshooting strategies in place to address failures in parallel test runs.

Test stability and reliability:

Parallel execution can expose test stability and reliability issues. Tests that are not designed to be thread-safe or have race conditions may fail when executed in parallel. Testers should ensure that their tests are robust and able to handle concurrent execution.

To overcome these challenges, testers can employ best practices, such as proper test design, effective synchronization mechanisms, separate test data environments, and comprehensive error-handling strategies. Regular monitoring, analysis, and optimization of parallel test execution can also help ensure successful implementation.

Executing Selenium Automation Testing in Parallel across Multiple Browsers using TestNG:

In this section, we will focus on writing test cases using the TestNG framework and understanding how to configure the TestNG YML file to enable parallel execution. We will execute our test cases on different browsers – Chrome, Firefox, and Safari– within the CircleCi platform.

Changes need to be made when the test cases are executed in CircleCi:

In the config.yml, make the below changes:

  1. Mention the file name which needs to be replaced by another new file.


cp /home/circleci/project/src/main/resources/ /home/circleci/project/src/main/resources/

cp /home/circleci/project/src/main/resources/ — the file which needs to be replaced.
/home/circleci/project/src/main/resources/ — will replace the file

   2. Mention the filename along with the path which needs to be removed once the circle-ci job execution is completed.


rm /home/circleci/project/src/main/resources/testData/DataEngine.xlsx

Note: The two lines of code can be used to copy or remove any kind of file during the job execution.

Cons of Parallel Testing:

The device-dependent test cases can’t be executed parallelly.


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