6 Must-Know Factors for QA Engineers Deciding Between TDD and BDD

Test-Driven Development (TDD) and Behavior-Driven Development (BDD) are the most commonly used development methods in the software development industry.

Throughout my career, I have often observed that people struggle to find the best approach for their projects. So, my goal with this article is to help quality assurance engineer make the best decision in selecting the most suitable methodology for their specific project needs.

Test-Driven Development vs Behavior-Driven Development

Test-Driven Development (TDD) is a software development methodology where the tests will be implemented first before developing the actual functionality, while Behavior-Driven Development (BDD) is a methodology where the application will be designed and developed according to the behaviours a user expects to experience from the application.

The core differences between TDD and BDD are in their focus, technologies, role and audience.

TDD focuses on writing tests that define the system's behavior from a technical standpoint. Developers write unit tests, and quality assurance engineers write test cases that validate specific functions against specified requirements.

In contrast, BDD focuses on defining the system's behavior from the user’s or stakeholder’s point of view. It emphasises collaboration between technical and non-technical team members to clarify requirements through solid examples of the system's behaviour.

These differing focuses are reflected in the technologies typically employed for each approach. TDD uses unit testing frameworks such as JUnit for Java or pytest for Python, focusing on checking the accuracy of individual functions or modules. While in BDD, tests are developed using more human-readable language like Gherkin, which uses a "Given, When, Then" format. These BDD scenarios describe the expected behavior of the system according to user stories or business requirements.

In terms of roles, TDD serves as specifications for the behavior of the code. They govern the design and implementation process by specifying the expected results of individual units of the code. BDD, on the other hand, acts as executable specifications for the behavior of the system as a whole and ensures that the developed system meets business requirements.

As for who uses it, TDD is mainly aimed at developers and quality assurance engineers who are directly involved in coding and testing software components, while BDD involves various stakeholders, which could include developers, quality assurance engineers, and business analysts, to collaborate on defining and verifying the system behaviors.

Now that we've explored the key differences between TDD and BDD, you may be wondering which approach is best suited for your specific project. To help you make this decision, let's examine several crucial factors to consider when choosing between TDD and BDD

Factors to evaluate before choosing a methodology

1. Project Requirements and Complexity

TDD is suitable for projects where the main focus is on code accuracy and robustness. It is effective for projects with complex algorithms or complex business logic. BDD, on the other hand, is suitable for projects when validating the system behavior from the users' perspective is more important.

For instance, if you were working on a finance app that performs complex calculations, TDD would be more effective as it focuses on testing the core logic to ensure that the calculations are accurate. On the other hand, if you were working on a customer support system where user interaction with the system is the priority you can leverage the BDD approach for that.

2. Team Composition and Collaboration

TDD will be a good choice if the project team comprises developers and quality assurance engineers who are comfortable writing and maintaining tests using programming languages. On the other hand, BDD will be ideal when non-technical team members are needed to collaborate on test writing.

In one of my past project experiences, we used BDD for an e-commerce project because the business analyst was also involved in creating tests. Since the business analyst was more familiar with writing scenarios using Gherkin, we used Cucumber as the test tool, and the quality assurance engineers developed the test code for those scenarios. This helped us maintain a shared understanding of the business requirements.

If the team comprised only developers and testers who were comfortable writing code-based tests, then we would have chosen the TDD, as the whole team could write unit tests and manage test code more effectively.

3. Communication and Clarity of Requirements

The TDD approach focuses on the accuracy of code functions, which may require additional effort to ensure that the tests are aligned with business goals and user expectations. Whereas, in BDD, tests, are human-readable, facilitating clear communication across the whole team.

If you were working on a fairly common project, such as a library management system, where the requirements are well known to the team, you could choose to use TDD. On the other hand, if it was something a bit more nuanced, like a project management tool where requirements involve multiple project stakeholders, then the BDD approach would be ideal, as you can use Gherkin syntax to maintain clear and understandable scenarios for all stakeholders.

The use of Gherkin syntax in BDD helps maintain clear and understandable scenarios for all stakeholders, making it particularly useful when requirements involve multiple project stakeholders.

4. Development Speed and Iterative Processes

Both TDD and BDD approaches contribute to maintaining development speed, but they do so in different ways. TDD provides fast feedback through automated unit and API tests, allowing quick validation of changes and helping the development team maintain efficiency.

BDD, on the other hand, particularly excels in encouraging an iterative approach. It supports refining and validating system behavior through collaboration, making it well-suited for projects with ongoing user feedback and iterative development cycles. BDD's flexibility allows for continuous updates to user stories and scenarios based on evolving requirements and feedback.

5. Knowledge of Tools & Technologies

It's important to evaluate your team members' competency with relevant TDD and BDD tools, as this is an important criterion in deciding between both methodologies. Selecting an approach that aligns with the team's existing skills can reduce the learning curve and additional training costs.

However, it is also important to continue providing proper training to team members about the chosen approach and its associated tools, which will accelerate development activities.

6. Organizational Culture

TDD usually fits well with project-based companies, where the focus is on delivering specific, well-defined outcomes. The BDD approach is typically more suitable for product companies, where continuous interaction and stakeholder communication are priorities.

By choosing an approach that matches your organization's culture and priorities, you can enhance team collaboration and ensure that development efforts remain closely aligned with business objectives.

Summary

Choosing between TDD and BDD for your project needs to consider several factors. TDD will be ideal for projects that need thorough testing at the code level focusing on technical design and architecture. On the other hand, BDD suits projects that emphasize collaboration, communication, and clarity of requirements. It uses human-readable language to bridge the technical gap between the diverse team roles such as developers, quality assurance engineers, and product owners. Ultimately, it aligns development with the business goals, ensuring the project meets user expectations.