What is Behavior-Driven Development (BDD)
Behavior-Driven Development (BDD) is an Agile software development methodology that emphasizes collaboration, communication, and shared understanding between different stakeholders in a project. It was introduced in the early 2000s as an evolution of Test-Driven Development (TDD). BDD aims to provide a common language for developers, testers, and business representatives to define the expected behavior of an application in a clear, concise, and unambiguous manner.
The primary goal of BDD is to ensure that all team members understand the requirements and the desired behavior of the application. It does this by focusing on user stories and scenarios, which are written in a structured, human-readable format that is easy to understand by both technical and non-technical stakeholders. BDD encourages the use of examples to describe the behavior of the application and to drive the development process.
BDD Process
Essentially, day-to-day BDD activity is a three-step, iterative process:
- Discovery
- Formulation
- Automation
Discovery: Collaborative Conversations
The discovery phase is the starting point of the BDD process, where team members come together to discuss and define the application's expected behavior. This phase encourages open communication and collaboration between developers, testers, and business representatives, ensuring that everyone has a shared understanding of the project requirements.
During discovery, the team engages in collaborative conversations, often called "Three Amigos" sessions, where they discuss user stories and scenarios. The user stories provide a high-level description of the desired functionality, while scenarios dive deeper into specific examples of how the application should behave in different situations. The goal of these conversations is to identify any ambiguities or misunderstandings and to reach a consensus on the expected behavior.
Formulation: Writing Scenarios
Once the team has a clear understanding of the desired behavior, they move on to the formulation phase, where they write scenarios in a structured, human-readable format. Scenarios are a critical aspect of BDD, as they serve as both documentation and executable specifications for the application.
Scenarios are written using the Given-When-Then syntax, which provides a clear and consistent structure for defining the preconditions, actions, and expected outcomes:
- Given: Describes the initial state or context of the system.
- When: Specifies the action or event that occurs.
- Then: Outlines the expected outcome or result.
The use of this syntax helps ensure that scenarios are easy to understand for both technical and non-technical stakeholders. Additionally, the scenarios should be written in a way that focuses on the behavior of the application, rather than its implementation details.
Automation: Implementing Test Code
The final phase of the BDD process is automation, where the scenarios are translated into test code that can be executed to verify the application's behavior. This phase involves writing step definitions, which are code snippets that map each step in the scenario to the corresponding code in the application.
By implementing test code based on the scenarios, the team ensures that the application's behavior aligns with the agreed-upon requirements. The test code also serves as a form of living documentation that stays up-to-date as the application evolves, providing a clear and accurate representation of the expected behavior at any given time.
As the application is developed and updated, the automated tests are run continuously to ensure that any changes do not break existing functionality. This process of continuous testing helps to identify and resolve issues early in the development cycle, ultimately leading to higher-quality software and a more efficient development process.
BDD Tools and Frameworks
To support the implementation of BDD, various tools and frameworks have been developed. These tools facilitate the writing, execution, and reporting of scenarios and tests, making it easier for teams to follow BDD practices.
Cucumber
Cucumber is one of the most popular BDD frameworks, supporting multiple programming languages, including Ruby, Java, JavaScript, and Python. It provides a simple way to define scenarios using the Gherkin language and to execute them as automated tests by mapping them to step definitions. Cucumber also integrates with various testing libraries and supports plugins to extend its functionality.
SpecFlow
SpecFlow is a BDD framework for .NET developers, closely modeled after Cucumber. It allows the definition of scenarios using Gherkin and the creation of step definitions in C#. SpecFlow integrates with popular testing libraries like NUnit and xUnit, and it can be used alongside continuous integration tools like Jenkins and TeamCity.
Behat
Behat is a BDD framework designed specifically for PHP developers. It supports defining scenarios in Gherkin and writing step definitions in PHP. Behat integrates with PHPUnit and other testing libraries, making it a popular choice for PHP projects. It also includes a variety of extensions to enhance its capabilities.
JBehave
JBehave is a BDD framework for Java developers, developed by Dan North, the creator of BDD. Like Cucumber, it supports defining scenarios using Gherkin and writing step definitions in Java. JBehave integrates with popular Java testing libraries, such as JUnit, and can be used with continuous integration tools like Jenkins and Bamboo.
Karate
Karate is a BDD framework for Java developers, designed specifically for testing APIs. Unlike other BDD frameworks, Karate combines the ability to define scenarios using Gherkin with a built-in DSL (Domain-Specific Language) for writing tests, eliminating the need for separate step definitions. This approach simplifies the test-writing process and makes it more accessible to team members with various technical backgrounds.
BDD Example
To illustrate the BDD process in practice, let's walk through a simple example of implementing BDD for a fictional online banking application. The goal of this example is to demonstrate how BDD concepts and practices are applied in a real-world scenario.
User Story
The product owner provides the development team with a user story that describes the desired functionality:
As a bank customer,
I want to transfer funds between my accounts,
So that I can manage my finances more effectively.
Discovery: Collaborative Conversation
The development team, along with the product owner and a quality assurance representative, engage in a "Three Amigos" session to discuss the user story and identify scenarios that represent the expected behavior.
Formulation: Writing Scenarios
The team collaboratively writes the following scenarios to describe the expected behavior:
Feature: Fund Transfer
Scenario: Transfer funds between accounts with sufficient balance
Given I have a checking account with a balance of 500
And I have a savings account with a balance of 1000
When I transfer 200 from my checking account to my savings account
Then my checking account balance should be 300
And my savings account balance should be 1200
Scenario: Transfer funds between accounts with insufficient balance
Given I have a checking account with a balance of 100
And I have a savings account with a balance of 1000
When I attempt to transfer 200 from my checking account to my savings account
Then the transfer should be declined
And my checking account balance should remain 100
And my savings account balance should remain 1000
Automation: Implementing Test Code
The development team implements step definitions for the scenarios using their chosen BDD framework. In this example, we'll use Behave with Python:
from behave import given, when, then
from bank_account import Account, InsufficientFundsException
@given("I have a checking account with a balance of {balance:d}")
def step_given_checking_account_balance(context, balance):
context.checking_account = Account(balance)
@given("I have a savings account with a balance of {balance:d}")
def step_given_savings_account_balance(context, balance):
context.savings_account = Account(balance)
@when("I transfer {amount:d} from my checking account to my savings account")
def step_when_transfer_from_checking_to_savings(context, amount):
context.checking_account.transfer_to(context.savings_account, amount)
@when("I attempt to transfer {amount:d} from my checking account to my savings account")
def step_when_attempt_transfer_from_checking_to_savings(context, amount):
try:
context.checking_account.transfer_to(context.savings_account, amount)
except InsufficientFundsException:
# Transfer declined
@then("my checking account balance should be {expected_balance:d}")
def step_then_checking_account_balance(context, expected_balance):
assert context.checking_account.get_balance() == expected_balance
@then("my savings account balance should be {expected_balance:d}")
def step_then_savings_account_balance(context, expected_balance):
assert context.savings_account.get_balance() == expected_balance
@then("the transfer should be declined")
def step_then_transfer_declined(context):
# Handled by the try-except block in the step definition above
References
Ryusei Kakujo
Focusing on data science for mobility
Bench Press 100kg!