What an Interface Decision Costs
How does an interface choice become a cost on every later feature?
Two components are connascent when a change in one forces a change in the other. Visibility, domain scope, direction, synchronicity, contract formality, communication pattern, and coordination each raise or lower a connascence type. That level feeds the Sync Tax Multiplier through connascence degree and coordination overhead, the failure risk multiplier through regressions from divergent values, and the strangler extraction cost through the magnitude of the sync layer. Connascence Degree Cost is connascence degree times the interdependence factor times vacuum effort times the blended hourly rate.
Why does runtime coupling cost more than build-time coupling?
Build-time agreements about names, types, order, algorithms, and meaning are caught by the compiler or tests before anyone ships. Runtime agreements about execution order, timing, shared values, and shared references show up when a customer hits them. Even the weakest runtime coupling is stronger than the strongest build-time coupling, because build-time problems never reach a customer. The goal is not zero coupling. Coupling that serves demand, such as a payment check that must finish before the charge, is worth paying. Accidental coupling is not.
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