Behaviour-Driven Development
Behavior-Driven Development (BDD) is an agile software development technique that extends the principles of Test-Driven Development (TDD) by emphasising collaboration between developers, testers, and non-technical stakeholders to define the desired behavior of an application. In BDD, development begins by defining the expected behaviors of a system in a shared, human-readable format that both technical and non-technical team members can understand. This approach bridges the gap between business requirements and technical implementation, ensuring that the software delivers real value and meets user needs.
BDD typically employs a “Given-When-Then” format to describe scenarios, using language that expresses system behavior from the user’s perspective. For example, “Given a user has an active account, when they log in with correct credentials, then they should be redirected to their dashboard”. These specifications are often written in a language like Gherkin, which allows business stakeholders to read and understand the behavior-driven scenarios alongside developers.
In BDD, tests are derived directly from these scenarios, making them accessible and meaningful to everyone involved in the project. This shared understanding reduces miscommunication, aligns the team with business objectives, and fosters collaboration. The focus is on specifying what the system should do rather than how it should do it. BDD test scenarios become living documentation for the system’s behavior, clarifying functionality and expectations as the product evolves.
BDD and TDD compared
While BDD builds upon the principles of TDD, there are key distinctions in focus, scope, and process.
| Aspect | TDD | BDD |
|---|---|---|
| Focus | TDD focuses on verifying specific units of functionality through unit tests, which test the internal mechanics of a feature. | BDD is concerned with the overall behavior of the system as it aligns with user stories and business requirements. TDD emphasises making small units of code work correctly, whereas BDD emphasises ensuring that the system’s behavior matches user expectations. |
| Scope | In TDD, developers write unit tests for individual methods and functions, which ensures isolated parts of the system work correctly. | BDD focuses on end-to-end scenarios that describe user interactions with the system. While TDD tests verify isolated pieces of logic, BDD scenarios test complete behaviors, making it easier to see the impact on the overall user experience. |
| Process | TDD is primarily a developer-centered approach, with test cases typically written in code. | BDD encourages collaboration between developers, QA, and business stakeholders by creating test cases in a common language that everyone can understand. This collaboration promotes shared ownership and ensures that development aligns with business goals. |
While TDD typically uses unit testing frameworks like JUnit or NUnit, BDD often uses tools specifically designed for behavior-driven development, such as Cucumber for Java or SpecFlow for .NET. These tools support the Given-When-Then structure and can integrate with Gherkin syntax to create human-readable specifications that double as executable tests.
Example of BDD in C# with SpecFlow
Consider a login feature for a user portal. Using SpecFlow and the Given-When-Then structure, we
can write a BDD scenario for this behavior in a Gherkin file, such as Login.feature.
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Feature: User Login
As a user of the portal
I want to be able to log in to my account
So that I can access my personal dashboard
Scenario: Successful login
Given a user with username "Alice" and password "password123"
When the user attempts to log in with correct credentials
Then the user should be redirected to the dashboard
This scenario describes the behavior in a language that both developers and stakeholders can understand. The developer then implements step definitions in C# to link this specification with the application logic:
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[Binding]
public class LoginSteps
{
private readonly UserPortal _portal;
private bool _loginSuccessful;
public LoginSteps(UserPortal portal)
{
_portal = portal;
}
[Given(@"a user with username ""(.*)"" and password ""(.*)""")]
public void GivenAUserWithUsernameAndPassword(string username, string password)
{
_portal.CreateUser(username, password);
}
[When(@"the user attempts to log in with correct credentials")]
public void WhenTheUserAttemptsToLogInWithCorrectCredentials()
{
_loginSuccessful = _portal.Login("Alice", "password123");
}
[Then(@"the user should be redirected to the dashboard")]
public void ThenTheUserShouldBeRedirectedToTheDashboard()
{
Assert.True(_loginSuccessful);
Assert.True(_portal.IsUserOnDashboard());
}
}
In this example, each step of the scenario links directly to a method in C#, where behavior can be tested. The test steps directly reflect the initial requirements, ensuring that the application behaves as stakeholders expect. This clarity fosters communication across the team, as the behavior scenarios are easy to read and understand, creating a shared language for discussing requirements.
Critique of BDD
BDD provides numerous advantages, particularly in complex or customer-focused projects. The collaborative approach helps prevent misunderstandings and clarifies requirements early in the development process, making it less likely that developers build the wrong functionality. BDD also emphasies the user experience, as test scenarios are written from the user’s perspective, leading to more user-centric designs. Finally, BDD scenarios become living documentation that can be easily maintained and updated, ensuring that everyone has a clear view of the system’s behavior and requirements.
One of the main critiques of BDD is that it requires a high level of discipline and collaboration between stakeholders, developers, and QA teams. For BDD to be effective, all parties need to be actively involved in defining scenarios, which can be difficult to achieve in teams where communication is limited or stakeholders are less engaged. BDD’s collaborative process is ideal for companies with a culture of open communication, but in more siloed environments, it may be difficult to bring everyone together consistently for behavior definition and refinement.
Another limitation of BDD is its dependency on clear, stable requirements. BDD scenarios are based on behaviors derived from user stories, so if requirements are not well-defined or are subject to frequent change, the process can become cumbersome. When requirements are unstable, teams may end up spending significant time rewriting scenarios to keep them in sync with the latest expectations. This is particularly challenging in fast-paced or highly exploratory projects, where new insights and requirements evolve frequently. In such cases, BDD can feel like an overhead rather than an asset, as the behavior definitions and test scenarios require constant maintenance.
Additionally, BDD scenarios are often written in a human-readable syntax like Gherkin, which aims to make tests understandable to both technical and non-technical team members. However, in practice, it can be difficult to write scenarios that are genuinely meaningful to non-technical stakeholders. If scenarios become too technical or involve implementation details, they lose their accessibility, which diminishes the primary benefit of BDD. Writing high-quality, business-focused scenarios in Gherkin requires practice, and even then, it can be challenging to keep them simple enough for everyone while detailed enough to be valuable for development and testing.
BDD can also lead to an over-focus on scenario coverage rather than holistic testing. Because BDD scenarios aim to cover specific behaviors, there is a risk of missing edge cases or unusual paths that may not be considered in initial scenarios. This narrow focus on “expected” behaviors can leave gaps in test coverage, especially in complex systems where real-world usage can involve unexpected combinations of user actions or inputs. To address this, BDD typically needs to be supplemented with other forms of testing, which can increase the testing load.
Finally, tooling complexity can be a hurdle in BDD. While tools like Cucumber and SpecFlow facilitate BDD by supporting behavior specification and test automation, these tools come with their own learning curves and maintenance requirements. The technical overhead involved in managing BDD scenarios and ensuring they are consistently up-to-date can add complexity, especially in large teams or projects. New team members or stakeholders unfamiliar with BDD may struggle to understand the process, requiring additional training and time investments.
In summary, while BDD provides value by aligning development with business needs and fostering communication, it is not universally applicable or easy to adopt. BDD works best in projects with clear requirements, stable goals, and a high level of stakeholder involvement. When these conditions aren’t met, BDD can add complexity and maintenance overhead without delivering its intended benefits. For organisations considering BDD, it’s essential to weigh the collaboration and scenario-writing costs against the value it provides to ensure that BDD is a good fit for the project’s context and team structure.