Fitness Functions

Fitness functions form the protective substrate of an evolutionary architecture. If architects design a system around particular characteristics, those features may be orthogonal to testability. Fortunately, modern engineering practices have vastly improved around testability, making formerly difficult architectural characteristics automatically verifiable. This is the area of evolutionary architecture that requires the most work, but fitness functions allows equal treatment for formerly disparate concerns.

We encourage architects to start thinking of all kinds of architectural verification mechanisms as fitness functions, including things they have previously considered ad hocly. For example, many architectures have a service-level agreement around scalability and corresponding tests. They also have rules around security requirements, with accompanying verification mechanisms. Architects often think of these as separate categories, but both intents are the same: verify some feature of the architecture. By thinking of all architectural verification as fitness functions, there is more consistency when automation and other beneficial synergistic interactions are defined.


Developers sometimes co-opt terms that sound cool and make them into broader synonyms, as is the case for refactoring. As defined by Martin Fowler, refactoring is the process of restructuring existing computer code without changing its external behavior. For many developers, refactoring has become synonymous with change, but there are key differences.

It is exceedingly rare that a team refactors an architecture; rather, they restructure it, making substantive changes to both structure and behavior. Architecture patterns exist in part to make certain architectural characteristics primary in an application. Switching patterns entails switching priorities, which isn’t refactoring. For example, architects might choose an EDA for scalability. If the team switches to a different architectural pattern, it likely won’t support the same level of scalability.

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