AP-006 — Cost effective

Infrastructure costs are controlled by design, not by policy. The system is built to be economical at every scale.

Consumption-based infrastructure is preferred where workloads are variable. Right-sized compute. Caching used deliberately. Over-engineering for volume that does not yet exist violates both this principle and KISS.

The cost curve test: What does this cost per thousand transactions at launch? What at a million customers? If the curve is exponential, redesign before you build.

Cost tagging on all cloud resources. Monthly cost reviews in the engineering cadence. Nobody discovers a large unexpected bill at end of month.

Relationship to other principles

Principle	Relationship
AP-001 KISS	Over-engineering for scale that does not yet exist violates both AP-001 and AP-006.
AP-007 Evolution by design	Right-sized now with a tested cost curve is more valuable than over-built for hypothetical future scale.
AP-008 Real time where it matters	Real-time infrastructure has a significant cost premium — it must be justified by a specific use case.
AP-009 Robust and serviceable	Operational complexity has a direct cost in engineering time — simpler systems are cheaper to run.

See the full architectural principles index.