Fitness functions from evolutionary architecture are automated checks that the system preserves its important characteristics over time. The 2×2 matrix lays them out along two axes.
Horizontally — atomic (a single service) vs holistic (the whole system). Vertically — triggered (on event, in CI) vs continuous (always-on in production via SLOs and budgets).
The matrix helps you answer where you already have automated guarantees and where architectural properties only survive on tribal memory — and which cell to strengthen next.
List the architectural qualities the system must preserve.
Place each check into one of the four matrix cells.
Pick the empty or weak cell that should be strengthened next.