Fitness Function-Driven Development

Test-driven development, or TDD, involves writing tests first then developing the minimal code needed to pass the tests. TDD is an established practice for feature development that can improve code quality and test coverage. What about other, non-functional requirements such as scalability, reliability, observability, and other architectural “-ilities”?

How do we ensure operability and resiliency of features when they go to production? How can we encourage teams to build in these architectural standards, just as test-driven development builds in code quality and test coverage? What are fitness functions?

How do we enable evolution? Architectural goals and constraints may all change independently of functional expectations. Examining these themes often uncovers potential conflicts or architectural tradeoffs.

For example, goals for agility and flexibility may conflict with goals for stability and resiliency. Stability and agility are often diametrically opposed because of the way they are pursued. We attempt to achieve stability by establishing control over change and creating gatekeepers, while innovation and agility are enabled by reducing barriers to change in order to minimize the chance of failure.

Shifting the mindset to value ‘mean time to recovery’ (MTTR) over ‘mean time between failure’ (MTBF) and embracing constant change is a challenge. Evaluating stakeholder motivation and holistically prioritizing the qualities that are important to the organization is the first step in defining architectural fitness functions. This initial exercise is similar to a cross-functional requirements session with the outcome of producing the desired fitness functions.

Source: thoughtworks.com