Editing Test-driven development (section)

== TDD for complex systems ==
Exercising TDD on large, challenging systems requires a modular architecture, well-defined components with published interfaces, and disciplined system layering with maximization of platform independence. These proven practices yield increased testability and facilitate the application of build and test automation.<ref name="Pathfinder Solutions" />

=== Designing for testability ===
Complex systems require an architecture that meets a range of requirements.  A key subset of these requirements includes support for the complete and effective testing of the system. Effective modular design yields components that share traits essential for effective TDD.

* [[Cohesion (computer science)|High Cohesion]] ensures each unit provides a set of related capabilities and makes the tests of those capabilities easier to maintain.
* [[Coupling (computer programming)|Low Coupling]] allows each unit to be effectively tested in isolation.
* Published Interfaces restrict Component access and serve as contact points for tests, facilitating test creation and ensuring the highest fidelity between test and production unit configuration.

A key technique for building effective modular architecture is Scenario Modeling where a set of sequence charts is constructed, each one focusing on a single system-level execution scenario. The Scenario Model provides an excellent vehicle for creating the strategy of interactions between components in response to a specific stimulus. Each of these Scenario Models serves as a rich set of requirements for the services or functions that a component must provide, and it also dictates the order in which these components and services interact together. Scenario modeling can greatly facilitate the construction of TDD tests for a complex system.<ref name="Pathfinder Solutions" />

=== Managing tests for large teams ===
In a larger system, the impact of poor component quality is magnified by the complexity of interactions. This magnification makes the benefits of TDD accrue even faster in the context of larger projects. However, the complexity of the total population of tests can become a problem in itself, eroding potential gains. It sounds simple, but a key initial step is to recognize that test code is also important software and should be produced and maintained with the same rigor as the production code.

Creating and managing the [[Software architecture|architecture]] of test software within a complex system is just as important as the core product architecture. Test drivers interact with the UUT, [[test double]]s and the unit test framework.<ref name="Pathfinder Solutions" />