headroom: capacity decisions with an expiry date

Purpose and scope

Use this skill to decide what a system needs now, what evidence should trigger the next change, and what belongs in a deliberately deferred scale design. It produces a decision record, not invented load-test results or an architecture assembled from fashionable components.

Capacity belongs to a workload and an objective: traffic alone is not a model. Consider request shape, concurrency, data growth, dependency limits, failure behavior, recovery objectives, operational ownership, and cost where they can change a decision. State every unknown, estimate, source, and confidence level.

When to use

• "will this architecture handle 100k users"
• "50 users today, plan for maybe 500, keep it boring"
• "should we split this into microservices"