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Fitness function-driven development

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By Paula Paul and Rosemary Wang

Published: Jan 11, 2019

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?

Architecture standards evolve constantly. For example, compliance standards today will likely be different a year from now; ongoing changes in those standards should be reflected in gates to production deployment. The idea that architecture can support change was described by Neal Ford, Rebecca Parsons, and Pat Kua in 2017 as evolutionary architecture.

“An evolutionary architecture supports guided, incremental change as the first principle across multiple dimensions.”

What are fitness functions?

How do we enable evolution? Architectural goals and constraints may all change independently of functional expectations. Fitness functions describe how close an architecture is to achieving an architectural aim. During test-driven development we write tests to verify that features conform to desired business outcomes; with fitness function-driven development we can also write tests that measure a system’s alignment to architectural goals.

Regardless of the application architecture (monolith, microservices, or other), fitness function-driven development can introduce continuous feedback for architectural conformance and inform the development process as it happens, rather than after the fact. For example, logging is sometimes added as an afterthought and may not contain consistent or useful debugging information. Leveraging a fitness function to ensure that new code has structured, sensible logging during the development process ensures the operability and debuggability required for a production application.

Where to start

Think of architecture as a product having user journeys, and start by gathering input from stakeholders in business, compliance, operations, security, infrastructure, and application development to understand the architectural attributes they consider most important to the success of the business. These attributes often align with the architectural “-ilities.” After team members jot down their top five or six attributes, group the results into common themes such as resilience, operability, or stability.

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.

After collecting fitness functions, draft them in a testing framework. Ideally, fitness functions should describe the intent of the “-ility.” It addresses it in terms of an objective metric that is meaningful to product teams or stakeholders. Monitoring critical qualities as fitness functions can help product teams avoid architectural drift and objectively measure technical debt. But, where do these fitness functions reside, and how do they signal potential architectural drift? Creating the desired fitness functions — and including them in appropriate delivery pipelines — communicates these metrics as an important aspect of enterprise architecture. Regular fitness function reviews can focus architectural efforts on meaningful and quantifiable outcomes. These reviews also help maintain important system quality attributes for production readiness.

As a result, every new service or piece of software is developed in a way that passes the fitness functions and supports the architectural qualities we value. Fitness function-driven development is a natural extension of continuous integration; gatekeepers are automated, so they don’t block the flow to production.