What Is Parallel Testing And Why Is It Important?

Organizations are switching from manual testing to Selenium automation testing more frequently as technology evolves, which increases scalability. Many still need help to do parallel testing and instead turn to labor and resource intensive sequential approaches. The Selenium Grid was created to make parallel testing easier, reinforcing its significance for effective quality assurance procedures.

In this blog, we will learn the significance of parallel testing in streamlining the release cycles. From understanding its fundamentals, we will explore how parallel testing optimizes test execution and dive into practical insights on when and how to execute parallel testing effectively.

What is Parallel Testing?

Parallel testing is a software testing technique that leverages automation testing to execute multiple test cases or test suites concurrently in multiple environments, real devices, and browser configurations. This is contrary to serial testing/sequential testing, where tests are executed one after the other, a time-consuming approach to testing, especially for large test suites.

To learn more about parallel testing, check out our YouTube video: What Is Parallel Testing In Selenium

Why is Parallel Testing Required?

Let’s take a test scenario of sequential execution. Say hypothetically, we want to test a signup form’s functionality and have written the automation test script. If we wanted to perform this test on over 3 different browsers, and let’s say the Chrome test takes 3 minutes, the Firefox test takes 4 minutes, and the Edge test takes 5 minutes. So, in total the sequential test will take 12 minutes.

This is only the sign-up form; what happens if we run automation scripts over other modules such as the pricing page, demo scheduling form, contact form, and more? Even if we automate our Selenium test suites and run them one after the other, it will take forever for your tests to complete.

Now, what would happen if we were to run three parallel tests simultaneously in the above scenario? The total execution time would have been trimmed down from 12 minutes to just 5 minutes.

The performance of the continuous integration and continuous delivery(CI/CD) pipeline can be enhanced by dividing big test suites into smaller, independent jobs and executing them concurrently. In addition to giving us more time to fix errors, this will enable us to obtain thorough test reports on the test data. Afterward, development teams can use these reports to identify bugs in the code and swiftly fix them to maximize CI/CD performance.

Parallel testing is therefore invaluable for testing complex systems and large applications efficiently since it optimizes resource utilization and minimizes testing time. However, this can also introduce additional challenges, such as the need to coordinate tests and prevent tests from “interfering with each other”. Test parts can only interact with each other in the case of distributed testing. In distributed testing, the test cases are executed across multiple machines to decrease testing time and improve test coverage and here test parts can also have interactions with each other.

When to Execute Parallel Testing in Selenium?

Now that we know how parallel testing in Selenium is helpful for the continuous integration and delivery of applications, it’s time to understand when and how to implement parallel testing in the Software Development Life Cycle(SDLC). We can use parallel testing for the following scenarios:

• For financial applications related to the banking sector, where most applications are identical. Parallel testing in Selenium can verify the earlier and newest application versions.
• While importing legacy data from one system to another or a company moving from an old system to a new system.
• When a new update for an application closely resembles its previous version, running parallel tests becomes helpful.
• While performing regression testing, parallel testing can speed up the testing efforts by concurrently running tests across various features or modules.
• When dealing with extensive test suites containing numerous test cases, parallel testing can significantly reduce testing time by executing multiple tests simultaneously.

How to Approach Parallel Test Cases in Selenium?

While performing parallel testing in Selenium, it’s essential to adhere to specific steps for efficient execution. These steps include:

Identify Testable Components

Identifying testable components involves breaking down the software application into distinct units or modules that can be independently tested in parallel. This process helps in determining which parts of the application can be tested concurrently, leading to more efficient testing strategies. These testable components will include individual modules, features, or even separate applications within a larger system. For example, login functionality, user registration, shopping cart management, payment processing, etc.

Design Independent Test Cases

Designing independent test cases is crucial for parallel testing efficiency. Each test case should be self-contained, devoid of dependencies on other test cases or external factors. This ensures that tests can be executed in parallel without the risk of interference or incorrect results. By creating independent test cases, testers can achieve true parallelism, optimizing resource utilization and reducing testing time.

Allocate Test Environments

Allocating test environments involves multiple environments or machines to execute parallel tests concurrently. It’s essential to ensure that each environment is identical in configurations, operating systems, and dependencies to maintain consistency and reliability across parallel test executions. By standardizing the test environments, testers can minimize variations and ensure consistent behavior of the application under test, thereby enhancing the accuracy and reliability of test results obtained from parallel testing.

Manage Test Data

When conducting parallel testing, careful test data management is essential to avoid conflicts and ensure accurate results. It’s crucial to prevent scenarios where multiple tests compete for the same data, which can lead to conflicts or erroneous outcomes. To address this issue, consider using separate databases or creating unique datasets for each parallel test execution. By isolating test data for each test instance, testers can mitigate the risk of data corruption or interference, ensuring reliable and consistent test results across parallel executions.

Implement Test Synchronization

During parallel test execution, synchronization becomes crucial in cases where coordination between tests is necessary. For instance, if a test case depends on the output of a preceding test case, synchronization ensures its execution in the correct sequence.

Techniques such as locks, semaphores, or messaging systems can be employed to coordinate parallel test execution effectively. By synchronizing test execution, testers can maintain the integrity of test results and ensure that dependencies are handled appropriately, thereby enhancing the reliability of parallel testing workflows.

Entry-Level and Exit-Level Criteria

The best approach to execute tests in parallel is to create different projects for browser compatibility testing various parts of a web application and a master project to test those different projects. Generally, there are two-level criteria for parallel testing in Selenium. The first one is Entry-level criteria, and the other one is Exit-Level criteria.

Entry-Level Criteria

In entry-level criteria, specific tasks are defined that should be satisfied before executing tests in parallel successfully, such as :

• A testing environment setup is required before starting parallel testing in Selenium.
• Pre-conditions and test scenarios must be defined before starting the automated browser testing process.
• New data and legacy data must be migrated successfully.
• Choose a parallel testing framework that is suitable for the project.
• Analyze the application architecture and identify the parallelization test suites.

Exit-Level Criteria

The Exit level criteria describe the steps for executing parallel testing successfully, which include:

• Running old systems against newly developed systems.
• Understanding the difference between the old and new systems.
• Going through the complete automated browser testing cycle while using the same input.
• Measuring the output of a newly developed system as compared to the old system.
• Reporting bugs (if any are found) to the development team.
• Verify if the parallel execution provides the expected benefits by validating the parallelization strategy.
• Ensure that shared resources are managed effectively and that conflicts are prevented during parallel testing.
• Conduct an exit review with relevant stakeholders to determine whether parallel tests were successful.

Code Example of Parallel Testing in Python

Let us write test cases to understand how parallel testing will help us to reduce the testing time. We will write test cases to verify the correctness of the web page title and the length of the title, initially employing sequential testing and then transitioning to parallel testing.

To perform parallel testing, we will use the pytest-xdist library. It is an extension of PyTest which will help us to run the tests in parallel mode and hence, reduce the execution time. So let us install both pytest and pytest-xdist library.

Use the following command to install pytest

Use the following command to install pytest-xdist library

To learn more about the pytest and its installation process, check out our blog on Selenium Python Tutorial: Getting Started With Pytest.

Let’s create a basic Python program with get_title() and get_title_length() functions to get the web page title and the length of the web page title, respectively.

Now to test these Python functions using pytest we will create a new test file called test.py and import the parallel_testing file into it.

The above code imports functions get_title() and get_title_length() from the parallel_testing file to retrieve the web page title and its length. The two test functions, test_title() and test_title_length(), are defined to verify the correctness of the title and its length retrieved from the browser, respectively, using the assert statements with custom error messages.

Use the following command to run the test code using pytest.

Output

In this scenario, sequential testing was performed, resulting in a total test execution time of 16.23 seconds. In this case, we can notice that the two browser instances sequentially open one after the other. Now let us perform the same testing using parallel testing.

Run the test again using the following pytest command

where “2” is the number of workers. It specifies the number of threads that will run in parallel to execute testing.

Output

In this case, we have used parallel testing to execute test cases concurrently across multiple browser instances or environments, resulting in a total test execution time of 9.05 seconds. In this case, we can notice that the two browser instances open simultaneously.

Hence we can observe that this approach optimizes testing efficiency and reduces overall testing time by running tests in parallel rather than sequentially.

You can find the GitHub repository for all the above codes here.

During automated parallel testing, situations may arise where verifying software on local infrastructure isn’t feasible. For instance, you might need to test our web app with Firefox browser (version 64.0) on a Windows 10 machine, necessitating an identical setup. However, relying solely on local setups can be costly and impractical for scalability. In these situations, we can leverage cloud-based cross-browser testing tools such as LambdaTest.

To learn how to use LambdaTest cloud selenium grid for parallel automation testing, check our blog, Selenium Grid Tutorial: Parallel Testing Guide with Examples

Note

Run your Parallel Tests on an infrastructure that doesn’t BREAK! Try LambdaTest Today!

Tools for Parallel Testing

We can use various tools for parallel testing to facilitate concurrent execution of test cases across multiple environments or machines, enhancing testing efficiency and scalability. Some of the top tools are as follows:-

LambdaTest

LambdaTest is an AI-powered test orchestration and execution platform that lets you run manual and automated tests at scale with over 3000+ real devices, browsers, and OS combinations.

It is a powerful tool for parallel testing by offering a cloud-based Selenium Grid that enables simultaneous execution of tests across numerous browser and device combinations. Additionally, it provides integrations with popular testing frameworks and CI/CD tools, further enhancing its capabilities for parallel testing workflows.

Features:

• It provides a cross-browser testing solution to test web apps across diverse browsers and devices simultaneously.
• It seamlessly scales test execution with Selenium Grid integration.
• It provides visual UI testing to detect visual discrepancies and ensure consistent UI rendering.
• It conducts live interactive testing on real browsers and devices.
• It captures automated screenshots to detect visual regressions.
• It executes parallel testing concurrently across multiple environments for faster results.
• It provides a Single sign-on(SSO) authentication feature to access multiple applications with one set of login credentials.
• Its scalable infrastructure dynamically allocates resources to handle varying testing demands, ensuring high performance and cost efficiency.
• It provides a facility for Real-Time Testing and Real-Device Testing.
• It allows us to test websites in different locations by providing a cloud of 3,000+ real devices and desktop browsers with geolocation testing.
• It offers various intеgration options with tеam collaboration tools to strеamlinе our softwarе tеsting and dеvеlopmеnt procеssеs.

TestNG

TestNG facilitates parallel testing by providing built-in support for running test cases in parallel across multiple threads, classes, or suites. It offers different parallel execution modes, such as methods, classes, and tests, allowing testers to distribute test execution efficiently. Additionally, TestNG manages thread safety and ensures proper synchronization, making it easier to implement parallel testing without encountering issues related to concurrency.

Features:

• Allows testers to specify parallel execution at different levels, including methods, classes, and test suites.
• Manages thread safety and synchronization automatically.
• Enables customization of the thread pool size.
• Offers various parallel execution strategies to cater to different testing requirements and scenarios.

To perform parallel testing using TestNG, check out our blog: Create TestNG XML File & Execute Parallel Testing,

Appium

Appium facilitates parallel testing by allowing testers to execute automated tests concurrently across multiple devices and platforms. It provides native support for parallel execution through its server architecture, enabling simultaneous test execution on different devices or emulators. Additionally, Appium supports distributed testing by connecting to multiple Appium servers, allowing for efficient utilization of resources and faster test execution. Further, with the help of Xcode9, Appium also supports parallel Real Device and Simulator testing for parallel iOS tests.

Features:

• Allows testing of native, hybrid, and mobile web applications across various platforms such as iOS, Android, and Windows.
• Provides native automation APIs for each platform.
• Supports testing on real devices as well as emulators and simulators.
• Offers integration with cloud-based testing platforms.

Learn to perform parallel testing using Appium with examples with the help of our blog: Complete Tutorial On Appium Parallel Testing [With Examples].

JUnit

JUnit facilitates parallel testing by offering built-in support for concurrent execution of test cases. It provides annotations to enable parameterized testing, which can be run concurrently. Additionally, JUnit 4 introduced the ParallelComputer runner, which allows test classes to be executed concurrently using multiple threads. JUnit 5 further enhances parallel testing capabilities with the @Execution annotation, which allows specifying different parallel execution modes for test methods and test classes.

Features:

• Supports parameterized testing, allowing the same test case to be executed with different input values.
• Offers various execution modes for parallel testing and provides flexibility in distributing test cases.
• Manages the lifecycle of test methods and provides hooks to execute setup and teardown operations.
• Supports various test runners, including IDE integrations, Maven, Gradle, etc.

Consider checking our blog on Parallel Testing With JUnit 5 And Selenium [Tutorial] to learn how to perform parallel testing using JUnit.

Limitations of Parallel Testing

Parallel testing has many benefits, however, some of its limitations should also be considered. Below are some of the limitations of parallel testing:

Inter-dependent Tests

Inter-dependent tests hinder parallel testing as one test depends on the outcome of another, preventing simultaneous execution. To maintain result integrity and prevent inaccuracies, it’s crucial to resolve any failures before proceeding with subsequent tests. This approach conserves time and resources by ensuring tests are only executed when prerequisites are met, improving reliability and accuracy.

Data Dependencies

Data dependencies pose challenges in parallel testing as tests accessing the same data may conflict, risking corruption or inconsistency. To ensure effectiveness, avoid such dependencies by isolating test data or using strategies like data cloning. This optimizes workflows and minimizes conflicts, enhancing parallel testing reliability.

Infrastructure Limitations

Infrastructure limitations can impede parallel testing when concurrent test handling capacity is restricted. Running more tests than infrastructure supports can lead to failures and inconsistencies due to insufficient resources. To mitigate this risk, limit parallel testing to match infrastructure capacity, ensuring stable execution and minimizing resource exhaustion. This approach maintains result integrity and accuracy, particularly in resource-constrained environments.

Multiple-browser Issues

While parallel testing helps us to perform cross-browser compatibility testing, its browser coverage remains limited without distributed testing. Distributed testing, involving setups with multiple machines and browsers, is necessary to fully leverage parallelization for comprehensive browser coverage.

Test Synchronization

Parallel execution may cause synchronization issues, especially when tests have dependencies or require specific sequences. To manage this, deploy synchronization mechanisms like locks, semaphores, or messaging systems. We must design and plan the synchronization strategy meticulously to uphold order and prevent conflicts.

Resource Utilization and Scalability

Optimal resource allocation and scalability are vital for effective parallel testing. Yet, managing these aspects, particularly with numerous tests or distributed environments, can be challenging. To address this challenge, we must assess and allocate resources thoughtfully according to testing needs. By using load balancers, resource management tools, or cloud-based infrastructure to dynamically scale resources in response to test demands we can ensure efficient resource utilization and scalability.

Intermittent Failures

Parallel testing may lead to sporadic failures, arising from intricate interactions among parallel executions. These failures pose challenges in reproduction and debugging. To tackle this, we must employ robust logging and reporting mechanisms to capture comprehensive test execution details. Also implementing strategies like test isolation, randomized test order, and effective synchronization will help us to mitigate intermittent failures and simplify the debugging processes.

Test Environment Consistency

Consistency in test environments across parallel executions is crucial to ensure reliable and accurate results. When running tests concurrently, all environments must have the same configurations, dependencies, and software versions to avoid discrepancies in test outcomes. However, achieving uniformity across all environments can be challenging due to differences in setups, configurations, and dependencies.

To tackle the above-mentioned limitations and to successfully perform parallel testing we must follow some set of practices. Below are some effective approaches that you can use to implement parallel testing in Selenium.

Best Practices for Parallel Testing

Running automated browser testing in parallel is not that easy, even with Selenium Grid, and it happens due to the unstructured automation testing framework that we use for parallel test execution in Selenium. Here are some best practices that can help us execute Selenium in parallel successfully.

Generate Independent Test Cases

If we generate independent tests that can run on their own, then it will be easier for us to execute them in parallel. In simple words, the tests need to be self-contained. So, when we run any test, we don’t have to worry about the order in which we run our test suite.

Sometimes, tests in parallel execution exhibit invalid behavior like failing when it shouldn’t or failing when it shouldn’t, which is known as flakiness. Independent tests can reduce the flakiness in the automated browser testing process by decreasing the number of possible breaking points in a test. Another significant advantage of using independent tests is that if one fails, it won’t stop us from testing other testing functionality.

Parallel Testing Using Cloud-Based Selenium Grid

Performing automated browser testing on the local Selenium Grid can be exhausting, as not only do we have to manage and maintain all the machines, but also to set them as well. Parallel testing on a local Selenium grid will have some scalability issues as we won’t be able to cover all the major browsers, their different versions, and OS.

With a cloud-based Selenium Grid, we don’t have to worry about setup anymore. With LambdaTest Selenium Grid, we can run parallel tests on 3000+ real browsers and their different versions. We would not only save a lot of money on setting up the infrastructure but also we would get ample time to do tasks that matter.

Avoid Hard Code values

Most testers have to run their tests in a particular order due to hard code embedded directly in the source code. This approach doesn’t allow us to run tests in parallel efficiently, as it creates dependencies between test cases.

So, avoid hard code values in the Selenium scripts to ensure that each of the tests is independent and can be executed whenever necessary. We can even opt for a data-driven approach in the testing framework to configure the test cases.

Prevent Dependencies Between Selenium Test Automation Cases

Many testers often find it challenging to execute tests in parallel due to dependencies between various test cases. When tests are dependent on each other, they need to be run in a particular order, which often affects the parallel testing strategy. So, we should focus on creating independent and atomic tests that can be executed independently.

Manage Test Data Efficiently

The primary key to successful parallel Selenium test automation is handling the test data efficiently. But to achieve this, we need an effective strategy that can create test data when required, as well as clean it up when necessary. Below are some basic data management strategies that can be very effective.

• Refreshing data continuously: This approach allows us to reset your source code during test executions.
• Using RESTful API: It is a great way to create and destroy data during runtime.
• Selfish data generation: This approach has a creational strategy but doesn’t offer anything for data cleanup.

These are just a few approaches that can be used to manage test data effectively. You can even combine two approaches to attain the desired output.

Create Tests That Can Run In Parallel Effectively

If we are planning to adopt parallel testing in Selenium, we certainly don’t want to disappoint ourselves by creating multiple tests that cannot run in parallel just before the release of our product. So, develop the Selenium test automation cases with parallelization in mind from the beginning.

Planning from the beginning will not only save us from last-minute disasters but will also help test our application efficiently across all combinations. We know it sounds complicated, but writing test cases to run in parallel is easier, petite, and quicker.

Be Strategic And Prioritize

In parallel testing, consider factors like test dependencies and resource constraints when prioritizing test cases. Aim to cover critical functionalities and high-risk areas first to maximize test coverage within the available resources. Additionally, focus on tests that provide quick feedback on application stability and reliability. It’s also beneficial to periodically reassess and adjust test priorities based on evolving project requirements and feedback from previous test runs.

By strategically prioritizing test cases, we can optimize testing efforts and ensure efficient utilization of resources in parallel testing workflows. Keep in mind, that parallel testing doesn’t grant permission to execute an unlimited number of tests!

Conclusion

In this comprehensive blog, we’ve explored the various aspects of parallel testing in Selenium, starting from its fundamentals to practical approaches, examples, benefits, limitations, important tools, and best practices. By understanding the scenarios for parallelization and its potential benefits, we can make informed decisions to enhance our testing processes and streamline our SDLC.

Hari Sapna Nair

Sapna works as a Content Writer at LambdaTest. An avid reader and passionate developer, she embraces the role of a tech blogger who loves to share her wealth of knowledge. Firmly believing in personal growth through uplifting others, she has chosen blogging as her medium for this purpose. Always eager to learn more and connect with new people, she remains ready to explore new horizons and engage with diverse audience.

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