Test-driven development (TDD) explained

Test-driven development (TDD) is a software development process that involves writing tests for your code before you write the code. This approach has transformed the development methodology around testing. While the traditional waterfall model of software development was linear, with testing occurring near the end of one long timeline, TDD makes testing an ongoing, iterative process.

TDD follows a simple cycle: Write a test for a desired feature, ensure the test fails (because the feature does not exist yet), and then write just enough code to pass the test. This cycle repeats with further improvements and new features until the product is complete.

The TDD approach is rooted in Agile development, emphasizing iterative development, collaborative efforts based on customer feedback, and a flexible response to change. With TDD, each development round starts with a clear, testable goal, embedding quality assurance in every step of the process.

The TDD process explained

TDD’s iterative process is a simple cycle: test, code, and refactor. These three steps repeat for each new feature until development is complete.

Consider this process for building a hypothetical online scheduling system:

• Step 1: You start by writing a unit test for one core element of the proposed functionality, like searching for available bookings. You expect the test to fail, which is why this is sometimes called the red stage.
• Step 2: You write just enough code to pass the test: implementing the search functionality for the booking system. This phase is known as the green stage because the aim is to pass the test.
• Step 3: Once your code passes, you move to the refactoring phase. Here, you refine newly implemented code for greater efficiency and to ensure its alignment with the design requirements.

You repeat this process with each remaining feature, from booking a reservation to processing cancellations, until the application is complete.

Benefits of test-driven development for software delivery teams

TDD enhances collaboration by fostering a shared understanding of product requirements and goals. In conjunction with behavior-driven development (BDD), it bridges the gap between technical and non-technical stakeholders and helps align development with user expectations and business objectives. BDD’s user-centric scenarios complement TDD’s test cases, making the technical aspects more accessible and relatable to non-technical team members.

With tests defining clear goals, stakeholders know from the outset what the code should accomplish — applications made following TDD processes tend to adhere closely to the specified requirements.

These applications also tend to be robust. TDD’s test-code-refactor cycle encourages a quick feedback loop. By integrating testing into the development cycle from the outset, you can detect bugs early in the development process. This strategy reduces the risk of encountering complex issues later on when they are much harder to fix.

Here are some additional benefits of TDD:

• Improved design and architecture: Because the design evolves iteratively as you add new tests, TDD often leads to a more thoughtful, clean, and maintainable code structure.
• Lower long-term costs: Although TDD may take more effort up front, it typically reduces the cost of bug fixes and maintenance in the long run.
• Increased confidence in code changes: A comprehensive testing suite lets developers make changes or refactor confidently, knowing that tests already protect existing features.
• Documentation and specification: The tests in TDD serve as effective documentation and specification for the code so that new team members can easily understand the software’s functionality and intent.

In short, TDD streamlines the development process, minimizing the frequency and severity of bugs, and leading to a more efficient and productive software development lifecycle.

Best practices for TDD

Effectively implementing TDD requires following some best practices. Writing clear, targeted tests and ensuring continuous code improvement will allow you to realize the full potential of TDD. TDD best practices include:

• Start simple
• Be expressive and comprehensive
• Structure and organize
• Refactor regularly
• Build a comprehensive test suite

Start simple

Each test should assess only one aspect of the code. Avoid creating tests that cover multiple functionalities, as this breadth can make it hard to pinpoint the cause of failures.

Start with the most fundamental features of your application. For example, if you are developing an API, test the individual endpoints for correct response codes and basic input handling before proceeding to more complex interaction scenarios. Also, keep test setups as simple as possible. Excessive setup can make the tests hard to read and maintain.

Be expressive and comprehensive

Use your testing framework’s assertion library effectively and use specific assertions in your tests. Assert specific conditions like equality, exceptions, or null values instead of general conditions.

For example, when testing a sorting function, instead of just checking if the output is sorted, assert the expected order of elements. Frameworks like JUnit for Java or PyTest for Python, which offer a range of assertion methods, can help make your tests more expressive and precise.

Structure and organize

Structure your tests following the Arrange-Act-Assert pattern:

• Arrange: Set up the test data.
• Act: Execute the function to be tested.
• Assert: Check the results.

Choose test names that clearly describe what the test is verifying. For example, if your test validates an addition function, name it testAdditionReturnsCorrectSum() rather than test1().

Similarly, use clearly defined constants, not hard-coded values, to improve understandability and maintainability. When necessary, use comments to explain why a test exists or why you used certain data — especially for complex scenarios.

Refactor regularly

Use static code analysis tools like SonarQube to identify code smells and other issues. Incorporate practices like code reviews and pair programming to ensure refactoring improves code clarity and maintainability without introducing new bugs.

Build a comprehensive test suite

You need a comprehensive test suite, including unit tests and integration tests, as well as end-to-end tests. Do not just do happy path testing — include negative tests (testing for failure conditions), equivalence partitioning (reducing the number of test cases by dividing input data into equivalent partitions), and boundary value tests (testing the edge-case values). You are looking for total coverage, from the most common scenarios to the less frequent — but potentially critical — edge cases.

Implementing TDD with CI/CD

The core principles of TDD align perfectly with the objectives of continuous integration/continuous delivery (CI/CD): assured code quality, rapid and reliable software releases, and a consistent feedback loop throughout the development process.

CI/CD pipelines need a clean, efficient, and easily maintainable codebase to function effectively. With TDD, every new feature or functionality is underpinned by a comprehensive suite of tests from the outset. Code is continuously refactored, improving structure and readability.

With CI/CD, you need to be able to detect and resolve issues in a flash. TDD’s early bug detection abilities enable the identification and resolution of issues at the earliest possible opportunity, preventing bugs from proliferating within the main codebase.

Furthermore, both TDD and CI/CD prescribe the automation of the testing process. When you integrate automated TDD-driven testing into the CI/CD pipeline, testing becomes an integral part of the development and deployment process, rather than a separate stage.

Finally, rapid and frequent changes are the norm in a CI/CD environment, so developers must be confident in their code. With TDD, devs proceed with the knowledge that their recent code changes have passed a battery of testing.

TDD is not merely compatible with CI/CD practices — it enables CI/CD pipelines. For further insight into managing test environments within CI/CD processes, read The path to production: how and where to segregate test environments.

Conclusion

The test-first methodology of TDD brings code reliability, efficient bug detection, and reduced long-term maintenance costs. Structured testing with a comprehensive range of expressive test cases optimizes the software development process and improves code quality.

The TDD framework ensures rigorously tested and reliable code, a core requirement for the rapid deployment cycles of CI/CD. It is difficult to imagine how CI/CD could function without it. To get started delivering reliable and high-quality software with the TDD framework as part of your CI/CD process, sign up for a free CircleCI account today.