Editing Test-driven development (section)

=== 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" />