Software development methodologies are constantly evolving to help developers write better code faster. Three methodologies that have gained popularity in recent years are Test Driven Development (TDD), Behavior Driven Development (BDD), and Domain Driven Design (DDD). Although the names are similar, these methodologies have distinct differences. This post will compare TDD, BDD, and DDD - their origins, principles, practices, and use cases.
Kent Beck conceptualized TDD in the late 1990s as part of the Extreme Programming (XP) methodology. It emerged out of the need for rapid software development that produced high-quality, bug-free code.
TDD is based on a short development cycle where requirements are converted to specific test cases, then code is improved to pass the new tests. This keeps the code focused on its intended purpose throughout development. TDD relies on the following principles:
- Write tests before writing any functional code
- Refactor code frequently to improve design
- Only write enough code to pass the test; no over-engineering
- Keep tests and code as simple as possible
A TDD workflow generally follows these practices:
- Add a test case for the next bit of functionality
- Run tests and see the new test fail (red)
- Write implementation code to pass the test (green)
- Refactor if needed
The cycle is red-green-refactor. Tests drive the code design. Developers only write enough code for the test to pass, avoiding over-engineering. Frequent refactoring keeps the code clean and maintainable.
TDD shines when building features iteratively. Writing tests first forces developers to think through requirements before coding. It results in a comprehensive test suite that encourages less buggy code. TDD is commonly used for:
- Web application back-ends (APIs, microservices)
- Application logic/business rules
- Anything developed iteratively
- More upfront time investment to write tests
- Dependence on the ability to write good tests
- Not as useful when requirements are unclear ## Behavior Driven Development (BDD)
BDD emerged in the early 2000s out of the need to build software that actually meets business needs. It extends TDD by writing tests in a natural language style, understood by both technical and non-technical teams. Dan North coined the methodology as a response to TDD shortcomings.
BDD focuses on defining application behavior in a way that both business stakeholders and developers understand. The principles of BDD include:
- Tests written in plain English using a standard defined language
- Tests driven by business outcomes, not technical details
- Business and technical teams collaborate on requirements
- Requirements become automated regression tests
BDD brings business and technical stakeholders together to define application behavior. They collaborate to write tests that describe the expected outcomes of the system. BDD tests follow a well-defined format:
- Given [context]
- When [event occurs]
- Then [expected outcome]
These plain English tests describe the complete behavior of a piece of functionality instead of just the technical details. Tests are automated into a regression suite using frameworks like Cucumber or JBehave.
BDD is useful when business participation is critical to building the right software. It ensures everyone understands the requirements upfront. BDD is commonly used for:
- Customer-facing web/mobile applications
- Complex business systems with intricate rules
- Projects with significant non-technical stakeholder participation
- More upfront time needed for collaboration and test writing
- Harder to apply BDD practices on internal back-end systems
Eric Evans introduced DDD in his 2004 book Domain-Driven Design: Tackling Complexity in the Heart of Software. It emerged as the industry focused more on complex business domains and logic.
DDD aims to help developers write software that accurately models the business domain. The core premise is that complexity comes from a software model that does not align with the business domain. Its principles include:
- Focus on the core domain and domain logic
- Constantly collaborate with domain experts
- Build an evolving model of the domain
- Speak a “ubiquitous language” understood by all team members
- Isolate complexity through boundaries and layers
DDD relies heavily on modeling the domain into entities, value objects, aggregates, and bounded contexts. Developers must constantly revisit the domain model to improve understanding. Practices include:
- Event storming sessions to identify domain events
- Refining a shared ubiquitous language
- Developing a context map to define boundaries
- Exploring complex subdomains by prototyping
DDD is most useful for complex business domains that call for intricate logic. DDD gives structure to domain complexity. It is commonly used for:
- Financial systems
- Insurance applications
- Healthcare software
- Other domains with ever-evolving business rules
- Steep learning curve around tactical patterns and principles
- Hard to correctly identify contexts and layers
- Modeling requires significant upfront analysis and rework
While TDD, BDD, and DDD have some overlap, they serve distinct purposes:
- TDD specifies how to write tests and code, focused on quality and design
- BDD specifies how to elicit and implement requirements by capturing behavior
- DDD specifies how to structure complex domains by intelligently modeling them
TDD is a development practice. BDD is a team approach to requirements and testing. DDD is a way of modeling complex business domains. All three complement each other. TDD gives you the tests. BDD gives you the requirements as tests. DDD structures the code well to handle complexity.
You can certainly practice TDD and BDD outside of DDD. However, DDD greatly benefits from both TDD and BDD practices. TDD forces you to write tests first, helping you better understand the domain as you model it. BDD ensures you capture the right domain requirements from the start.
- TDD focuses on clean code and design
- BDD focuses on defining the right software behavior
- DDD focuses on modeling complex business domains
All three methodologies aim to produce higher-quality software that meets user needs. They encourage iterating quickly while keeping code clean and maintainable. TDD, BDD, and DDD are all valuable practices that can be combined for great results on complex projects with evolving business domains.